Impact of artificial sweeteners and rare sugars on the gut microbiome

Abstract

Alternative sugars are often used as sugar substitutes because of their low calories and glycemic index. Recently, consumption of these sweeteners in diet foods and beverages has increased dramatically, raising concerns about their health effects. This review examines the types and characteristics of artificial sweeteners and rare sugars and analyzes their impact on the gut microbiome. In the section on artificial sweeteners, we have described the chemical structures of different sweeteners, their digestion and absorption processes, and their effects on the gut microbiota. We have also discussed the biochemical properties and production methods of rare sugars and their positive and negative effects on gut microbial communities. Finally, we have described how artificial sweeteners and rare sugars alter the gut microbiome and how these changes affect the gut environment. Our observations aim to improve our understanding regarding the potential health implications of the consumption of artificial sweeteners and low-calorie sugars.

Introduction

Sweetness is one of the basic characteristics of food caused by sugars, which are mainly found in fruits and vegetables. These sugars are used to balance the flavor of food and provide the energy required for survival (Brooks, 1972; Goldfein and Slavin, 2015; Misra et al., 2016). Sweetness is an important determinant of food quality, and sweeteners are often added to processed food to balance its sweetness (Schiffman and Gatlin, 1993). Sucrose is a classic sweetener that neutralizes other tastes, such as bitterness and salinity, and adds depth and complexity to flavors, in addition to sweetness (Eggleston, 2019; White, 2014). Sucrose softens the dough and adds a crunchy texture because of caramelization. In addition, caramelization of sucrose adds color, stability, and flavor (Sengar and Sharma, 2014). However, overconsumption of sucrose has been attributed to various health problems, including obesity, diabetes, and heart disease, leading to increased interest in and use of alternative sweeteners (Chattopadhyay et al., 2014).

Artificial sweeteners are the most commonly used alternative sweeteners, which not only provide high levels of sweetness in small amounts, but are also economical (Chattopadhyay et al., 2014; Inglett, 1976). As a result, artificial sweeteners are commonly used in soft drinks and various processed food items, often labeled as “zero sugar” food (Kroger et al., 2006; Silva et al., 2021). However, the sweetness of artificial sweeteners differs from that of sucrose, and their unique processing characteristics render their management difficult (Sang et al., 2014). Recently, bioconversion processes have enabled the industrial production of natural rare sugars, such as allulose and tagatose (Cheetham and Wootton, 1993; Li et al., 2021; Oh, 2007; Zhang et al., 2016). Although the sweetness of allulose and tagatose is lower than that of sucrose, they are similar to sucrose, making them easy to process, and their low-calorie properties render them ideal for sweetening low-sugar products (Hu et al., 2021; Oh, 2007).

Artificial sweeteners and rare sugars are not digested or absorbed by the human body and may influence the gut microbiota. Therefore, the aim of this review was to examine the physical, chemical, and biological properties of artificial sweeteners and rare sugars used in food processing. Furthermore, we reviewed their potential effects on the gut microbiota (Figs. 1 and 2).

Fig. 1.

Structure of artificial sweeteners

Fig. 2.

Enzymatic production process of rare sugars. A D-allulose, B Tagatose, C Isomaltulose, D Trehalose

Physicochemical and biological properties of artificial sweeteners

Artificial sweeteners, also known as nonnutritive sweeteners, provide high level of sweetness with little or no caloric contribution. These sweeteners exert their effects by interacting with T1R2 and T1R3 sweet taste receptors in humans, which are recognized as ligand-binding sites. Once bound, these sites are activated, creating a perception of intense sweetness (Romo-Romo et al., 2017). Therefore, artificial sweeteners generally have high sweetness index even in relatively small amounts. They are typically colorless and odorless, and exist as white solids, making them good sugar substitutes (Chattopadhyay et al., 2014). Some, including acesulfame potassium (Ace-K) and saccharin, can have off-flavors, such as bitterness or a metallic taste, and are therefore often combined with other artificial sweeteners to balance their flavors (Horne et al., 2002). Artificial sweeteners are primarily eliminated via the urine and digestive system. In some cases, these compounds are metabolized by other substances. However, some may remain in the body depending on the absorption, distribution, and excretion characteristics of the individual sweeteners (Bornemann et al., 2018; Czarnecka et al., 2021; Mahmood and Al-Juboori, 2020). Despite their use, the relationship between artificial sweeteners and their potential toxicity is not completely understood and remains a topic of ongoing research. Although generally considered safe at the recommended consumption levels, excessive intake of these artificial sweeteners may pose potential health risks. The physicochemical properties of artificial sweeteners and rare sugars (Tables. 1, 2) and their effects on gut microbiota (Tables. 3, 4) are described

Table 1.

Physical and chemical characterization of artificial sweeteners

Alternative sweeteners	Synthesis	Molecular formula	MW	Sweetness	Kcal/g	ADI (mg/kg b.w./day)
Sucrose	Natural	C12H22O11	342.30	1	4	–
Acesulfame potassium	Chemical	C4H4KNO4S	201.24	200	0	15
Aspartame	Chemical	C14H18N2O5	294.31	180–200	4	50
Saccharin	Chemical	C7H5NO3S	183.18	300–500	0	0.3
Sucralose	Chemical	C12H19Cl3O8	397.63	600	0	15
Neotame	Chemical	C20H30N2O5	378.47	8000	0	5

Table 2.

Physical and chemical characterization of rare sugars

Alternative sweeteners	Commercial synthesis	Molecular formula	Sweetness	kcal/g	Derived from	Glycemic Index (GI)	Characteristics
Sucrose	Natural	C12H22O11	1	4	Nature	72	• High calories and GI • Taste sweeteners • Overuse intakes cause the obesity and diabetes
Allulose	Enzymatic	C6H12O6	0.7	0.4	Fructose	N.D	• Reduction of calories and GI • Sucrose-like taste and sweeteners • Additional functional foods (bake, juice etc.) • Few/no adverse effect in vitro/in vivo
Tagatose	Enzymatic	C6H12O6	0.9	1.5	Galactose	3
Isomaltulose	Enzymatic	C12H22O11	0.4–0.5	4	Sucrose	32
Trehalose	Enzymatic	C12H22O11	0.45	4	Lactose, Galactose	N.D

Table 3.

Effect of artificial sweeteners on the gut microbiota

Alternative sweeteners	Species	Dose and exposure	Outcome	Ref
Acesulfame potassium	CD-1 mice	37.5 mg/kg b.w./day for 4 weeks	• In Male - Increased body-weight - Increased the abundance of Bacteroides, Anaerostipes, and Sutterella • In Female - Not change body-weight - Increased the Mucispirillum - Decreased Lactobacillus, Clostridium, an unassigned Ruminococcaceae genus and an unassigned Oxalobacteraceae genus	Bian et al. (2017a)
	Wistar rats	40 or 120 mg/kg b.w./day for 28 days	• In Male - No significant differences • In Female - Variability in the bacterial abundances could be observed Verrucomicrobiaceae	Murali et al. (2022)
	Human	1.7 to 33.2 mg/day for 4 days	- No significant differences	Frankenfeld et al. (2015)
Aspartame	Obese Rats	High-fat diet or standard 5 to 7 mg/kg b.w./day for 8 weeks	• Common - Increased the abundance of Enterobacteriaceae and Clostridium leptum • In high-fat diet - Increased the genus Roseburia	Palmnäs et al. (2014)
	Maternal mice	high-fat/sucrose diet 5 to 7 mg/kg b.w./day for 18 weeks	• Common - Disrupt weight regulation, glucose control, and gut microbiota in both mother and offspring • Maternal mice - Increased the abundance of Clostridium cluster IV - Decreased the abundance of Enterococcaceae, Enterococcus, and Parasutterella • Offspring - Overabundance of Porphyromonadaceae	Jodi et al. (2020)
	Human	0.425 g/day for 2 weeks	- No significant differences	Ahmad et al. (2020)
	Human	62.7 mg/day for 4 days	- No significant differences	Frankenfeld et al. (2015)
	Human	50 mg/kg b.w./day with maltodextrin	- Increased the abundance of Bifidobacterium and B. coccoides group - Decreased the abundance of Bacteroides/Prevotellat	Gerasimidis et al. (2020)
	Human	240 mg/kg b.w./day for 2 weeks	- No significant differences - Induce functional changes in the gut microbiota	Suez et al. (2022b)
Sucralose	Male C57BL/6 J mice	5 mg/kg b.w./day for 3 or 6 months	• 3 months - Increased the abundance of Ruminococcus - Decreased the abundance of Lachnospiraceae, Dehalobacteriaceae, Anaerostipes, Staphylococcus, Peptostreptococcaceae, and Bacillus • 6 months - Increased the abundance of Akkermansia, Turicibacter, Roseburia, Clostridiaceae, and Christensenellaceae - Decreased the abundance of Streptococcus, Lachnospiraceae, Dehalobacteriaceae, and Erysipelotrichaceae	Bian et al. (2017b)
	Male C57BL/6 J mice	0.0003 to 0.3 mg/mL for 16 weeks	- Increased the abundance of Allobaculum (total) - Increased the abundance of Tenacibaculum and Ruegeria in the jejunum (0.0003 mg/mL) - Increased the abundance of Staphylococcus and Corynebacterium in the ileum (0.003 mg/mL) - Decreased the abundance of Lachnoclostridium in the cecum (0.0003 and 0.3 mg/mL)	Zheng et al. (2022)
	Human	0.136 g/day for 2 weeks	- No significant differences	Ahmad et al. (2020)
	Human	780 mg/day for 7 days	- No significant differences	Thomson et al. (2019)
	Human	48 mg/day for 10 weeks	- Increased the abundance of Blautia coccoides - Decreased the abundance of Lactobacillus acidophilus	Méndez-García et al. (2022)
Saccharin	Male C57BL/6 J mice	0.3 mg/mL for 3 or 6 months	• 3 months - Increased the abundance of Sporosarcina, Jeotgalicoccus, Akkermansia, Oscillospira and Corynebacterium - Decreased the abundance of Anaerostipes and Ruminococcus • 6 months - Increased the abundance of Corynebacterium, Roseburia and Turicibacter - Decreased the abundance of Ruminococcus, Adlercreutzia and Dorea	Bian et al. (2017c)
	Male C57BL/6 J mice	250 mg/kg b.w./day for 10 weeks	• C57BL/6 J male mice - No significant differences • T1R2-KO (genetic ablation of sweet taste receptors) - No significant differences	Serrano et al. (2021)
	Human	400 mg/kg b.w./day of sodium saccharin for 2 weeks	- No significant differences	Serrano et al. (2021)
	Human	180 mg/day saccharin + 5,820 mg/day glucose or only 5,000 mg/day glucose for 2 weeks	• Saccharin + glucose - No significant differences • Only glucose - Decreased the abundance of Fusobacterium in the oral microbiota	Suez et al. (2022b)
Neotame	Male CD-1 mice	0.75 mg/kg b.w./day for 4 weeks	- Increased the abundance of Bacteroidetes - Decreased the abundance of Lachnospiraceae and Ruminococcaceae	Chi et al. (2018)
	Human	–	- No report	–

Table 4.

Effect of rare sugar on the gut microbiota

Alternative sweeteners	Species	Dose and exposure	Outcome	Refs
D-allulose	Male wistar rats	High fat diet with 5% allulose for 8 weeks	- Fermentation in the cecum by intestinal microorganisms leads to the production of SCFAs similar to those from dietary fiber - Decreased body-weight - Decreased enzyme activity related to lipid metabolism, such as fatty acid synthase and cholesterol acyltransferase	Higaki et al. (2022)
	Male C57BL/6 J mice	High fat diet with 5% allulose for 16 weeks	- Increased the abundance of Lactobacillus, Coprococcus and Coprobacillus - Decreased the abundance of Turicibacter, Clostridiaceae, Erysipelotrichaceae and Dorea	Han et al. (2020)
D-tagatose	Male landrace x yorkshire pigs	50 g/kg sucrose + 100 g/kg of D-tagatose for 18 days	- Fermented by gut microbiota in the cecum and colon, but not in the stomach or small intestine - Increased the abundance of D-tagatose degrading bacteria in the colon - Increased butyric and valeric acid production in the colon	Lærke et al. (2000)
	Human	30 g of raspberry jam with 7.5 g and 12.5 g of D-tagatose for 2 weeks	- Increased butyrate production after 2 weeks - Decreased iso-valeric acid productions - Decreased the pH for low tagatose treatment - Increased the abundance of lactobacillus only in men by high tagatose treatment	Venema et al. (2005)
Isomaltulose	Male Sprague–Dawley rats	Water with isomaltulose (10%, w/w) for 5 weeks	- Increased abundance of prebiotics bacteria such as Faecalibacterium and Phascolarctobacterium - Decreased abundance of pathogenic bacteria such as Shuttleworthia	Yang et al. (2021)
	C57BL/6 J mice	400 mg/kg b.w./day for 3 weeks	- Increased abundance of beneficial bacteria such as Akkermansiaceae, Marinifilaceae, and Anaerovoracaceae	Zhou et al. (2022)
Trehalose	The calves (crossbreed of Japanese Black and Holstein)	30 g/animal/day for 3 consecutive days	- No significance changes of pH after 22 and 55 days - Increased the total volatile fatty acids after 22 and 55 days - Increased relative abundance of Dialister and Eubacterium after 22 days - Increased relative abundance of Prevotella after 55 days - Decreased abundance of Clostridium spp.	Miura et al. (2021)
	in vitro remodeling	In vitro pathogenic model of Clostridioide difficile infection	- Increased abundance of beneficial bacteria, such as Finegoldia, Faecalibacterium and Oscillospira - Decreased abundance of pathogenic bacteria such as Klebsiella and Clostridium	Buckley et al. (2021)

Ace-K

Ace-K, which is approximately 200 times sweeter than sucrose, is a colorless and odorless crystalline powder with a decomposition point of 225 °C and is stable over a wide range of pH values (Irwin et al., 2005; Magnuson et al., 2016). Therefore, a small amount of Ace-K can impart sweetness similar to that of sucrose, and its properties such as high water solubility (270 g/L) and high-temperature stability make it a good ingredient for various food processing applications (Magnuson et al., 2016). Although Ace-K has several desirable processing characteristics such as intense sweetness, high water solubility, and stability, its use at high concentrations is limited by its persistent bitter aftertaste (Horne et al., 2002; Schiffman et al., 1979). Ace-K is widely used in a variety of food and beverage products such as soft drinks, dairy desserts, baked goods, jams, jellies, sugar-free gums and candies, tabletop sweeteners, and low-calorie sauces and dressings. Ace-K intake is typically so low that virtually no calories are added to the diet (Horne et al., 2002). Previous studies have shown that Ace-K consumption exerts minimal effects on fasting glucose, fasting insulin, and post-prandial glucose levels, homeostatic model assessment for insulin resistance, and insulin area under the curve (Kim et al., 2020). Ace-K does not bioaccumulate in tissues because of its rapid absorption upon oral ingestion and primary excretion in the urine, a characteristic shared by other artificial sweeteners (Le Wilson et al., 1999; Renwick, 1986). This, coupled with the negligible toxicity of its metabolic byproduct, acetoacetamide, which is produced in minuscule amounts, has led the United States Food and Drug Administration (USFDA) to deem additional testing unnecessary (Belton et al., 2020). Toxicity associated with Ace-K is rare and no significant adverse effects have been observed in humans (Karstadt, 2010). However, it has been found to be genotoxic and interferes with glucose fermentation by the intestinal flora (Bandyopadhyay et al., 2008). In addition, long-term studies in rats have shown that Ace-K is associated with impairment of peripheral nervous system and cognitive memory, inhibition of glycolysis, and depletion of functional ATP levels (Cong et al., 2013). Despite these findings, the overall impact on human health remains minimal because of the typically low levels of consumption.

Aspartame

Aspartame, a white and odorless crystalline powder derived from the dipeptide methyl ester of L-aspartic acid and L-phenylalanine, is approximately 180–200 times sweeter than sucrose and has a maximum water solubility of 20 g/L at pH 2.2 and room temperature (Food, 2002). Aspartame is primarily used to sweeten carbonated beverages because it is unstable at temperatures below 30 °C and above 80 °C and stable at pH 4.3, making it unsuitable for cooking and baking (Choudhary and Pretorius, 2017; Prankerd et al., 1992). Since its approval, this characteristic has led to the widespread use of aspartame in over 6,000 different products, including but not limited to soft drinks, dessert mixes, frozen desserts, yogurt, chewable multivitamins, breakfast cereals, tabletop sweeteners, and pharmaceuticals (Rencüzoğulları et al., 2004). Aspartame is metabolized in the gastrointestinal tract to phenylalanine, aspartic acid, and methanol (Choudhary and Lee, 2018; Czarnecka et al., 2021). These metabolites, which are processed similar to other dietary substances, may be more toxic, with methanol potentially damaging the liver cells (Czarnecka et al., 2021). Phenylalanine, which is metabolized to tyrosine, poses a risk to individuals with phenylketonuria (PKU), whereas aspartic acid is metabolized to alanine and oxaloacetic acid (Choudhary and Lee, 2018). However, this risk is not significant in healthy individuals, and research on genotoxicity shows that aspartame is safe (Czarnecka et al., 2021; Otabe et al., 2019). Although allergic reactions to aspartame are rare, they may cause dermatitis such as systemic contact dermatitis in sensitive individuals or upon contact with allergens; however, this usually requires high doses (Geha et al., 1993).

Saccharin

Saccharin, a white crystalline solid chemically known as 1,1-dioxo-1,2-benzothiazol-3-one and a sulfonamide derivative of toluene, is approximately 300–500 times sweeter than sucrose but has an unpleasant bitter and metallic taste (Chattopadhyay et al., 2014; Horne et al., 2002). Saccharin has low water solubility and exists in several forms, including acidic, sodium, and calcium forms. Although normal saccharin has a solubility of 3.5 g/L in water at room temperature, sodium saccharin is significantly more soluble, with a solubility of 670 g/L (Dubois, 2012; Mahmood and Al-Juboori, 2020). Sodium saccharin is highly stable against hydrolysis, heat, and light (Mahmood and Al-Juboori, 2020), making it suitable for the manufacture of food and beverages. Although saccharin is not metabolized in the body and is mostly excreted in the urine and feces (Renwick, 1985), it may accumulate in organs such as the heart, liver, pancreas, adrenal glands, thymus, and testes (Mahmood and Al-Juboori, 2020). However, the amount of saccharin that accumulates in the bladder wall and plasma is low (about 1 μg/mL), and the residue is completely excreted in the urine within 96 h (Renwick, 1985). The harmful effects of saccharin have been debated in studies showing development of cancer in rats fed high levels of saccharin (Bryan et al., 1970); however, recent studies have shown that it is not harmful to humans (Jo et al., 2017; Mahmood and Al-Juboori, 2020). As with other alternative sweeteners, caution should be exercised when consuming large amounts of saccharin. However, the toxicity and carcinogenicity of saccharin and their relationship remain unclear (Kroger et al., 2006; Yilmaz and Uçar, 2015).

Sucralose

Sucralose, a disaccharide, is synthesized from sucrose via a selective process that replaces three hydroxyl groups with chlorine atoms, thereby significantly altering its structure (Molinary and Quinlan, 2012) and resulting in a compound that is approximately 600 times sweeter than sucrose (WIET and BEYTS, 1992). Sucralose is highly soluble in water over a wide range of temperatures and pH, thus enhancing its adaptability to various applications (Molinary and Quinlan, 2012). Its exceptional thermostability allows it to maintain its structure up to 119 °C without decomposition, even when exposed to temperatures ranging from 119 to 550 °C (de Almeida et al., 2009). In addition, sucralose is not affected by changes in pH and remains stable under acidic, neutral, and basic conditions (pH 3, 7, and 11), which ensures its stability in a wide variety of food and beverage formulations (Hutchinson et al., 1999). The combination of its intense sweetness and robust solubility, along with its stability under varying heat and pH conditions, make it an ideal sweetener for beverage, dairy, and confectionery applications.

Owing to the structural changes in sucrose during sucralose synthesis, the body does not recognize sucralose as a carbohydrate; therefore, it is not metabolized and is almost completely excreted unchanged (Knight, 1994). Only approximately 2% of the sucralose metabolites, which are considered toxicologically insignificant, are excreted in the urine (AlDeeb et al., 2013). Recent studies have identified two new biotransformation products of sucralose; however, their potential health effects are not yet completely understood (Bornemann et al., 2018).

Neotame

Neotame, a derivative of aspartame also known as n-[n-(3,3-dimethylbutyl)-l-aspartyl]-l-phenylalanine-1-methyl ester, is produced by reducing the alkylation of aspartame and 3,3-dimethylbutyraldehyde (O’donnell, 2012). This white and odorless crystalline powder is slightly soluble in water, with a solubility of 12.6 g/L at room temperature, and is significantly sweeter than other sweeteners; it is approximately 8,000 times sweeter than sucrose and 30–80 times sweeter than aspartame (Nofre and Tinti, 2000). Neotame exhibits a pH stability similar to that of aspartame (O’donnell, 2012); however, unlike aspartame, neotame is more stable over a wide range of acidic pH (Nofre and Tinti, 2000). Neotame is rapidly metabolized and almost completely eliminated and does not accumulate in the body. When food containing neotame is ingested, half of the sweetener is excreted in feces and the other half is excreted in urine as de-esterified dimethylbutyl-aspartyl-phenylalanine (DMB-Asp-Phe) almost immediately (Chattopadhyay et al., 2014; Nofre and Tinti, 2000). Therefore, neotame metabolites do not affect the body (Mayhew et al., 2003; Ruiz-Ojeda et al., 2019). Although the metabolic pathway of neotame generates negligible amounts of methanol, it is a safe and stable sweetener without any reports of toxicity (Chattopadhyay et al., 2014; Nofre and Tinti, 2000; Whitehouse et al., 2008).

Physicochemical and biological properties of rare sugars

Recently, the use of rare sugars has generated considerable interest in the sugar industry as their caloric content is lower than that of sucrose (Ahmed et al., 2021; Kroger et al., 2006). Defined by the International Society of Rare Sugars as “monosaccharides and their derivatives” that are rarely found in nature, initial attempts to chemically synthesize rare sugars faced challenges because of the formation of waste and by-products, as well as the need for cascade steps and deprotection of functional groups (Giffhorn et al., 2000; Zhang et al., 2017). Prof. Izumori proposed an innovative strategy in 2002, suggesting that natural monosaccharides such as D-glucose, D-galactose, and D-fructose can be converted via enzymatic methods (epimerization, isomerization, and oxidation–reduction), a process known as the Izumori strategy (Granström et al., 2004). With the advancement of research technology, non-Izumori strategies, such as phosphorylation-dephosphorylation cascade reactions, epimerization, and enzymatic condensation, have been introduced, stimulating active research on the rare sugars of disaccharides (Zhang et al., 2017). This review discusses the enzymatic methods for the synthesis of mono- and disaccharide rare sugars.

D-allulose

D-allulose, also known as D-psicose, is a monosaccharide that is an epimer of D-fructose, with the molecular formula of C₆H₁₂O₆. It has a near-zero calorie content and regulates glucose and lipid metabolism (Kimura et al., 2017). D-allulose has approximately 70% of the sweetness of sucrose; it contains a ketone group as a reducing agent and has been shown to undergo Maillard reaction at 90 ℃. (Namli et al., 2021). Unlike D-glucose and D-fructose, heating amino acids, polypeptides, and proteins with D-allulose results in the formation of disulfide and non-disulfide cross-links at low melting temperatures (Ogawa et al., 2017). These Maillard reaction products not only increase the water-holding capacity, but also improve antioxidant activity (Bolger et al., 2021; Sun et al., 2007). Therefore, D-allulose may be used as a sugar substitute in the food industry (Zhang et al., 2023). Although D-allulose can be extracted from natural sources such as plants and animals, these methods of D-allulose production are associated with low yields and high processing costs (Oshima et al., 2006). In contrast, the method of enzymatically producing D-allulose using D-allulose 3-epimerase has many advantages such as its environmental friendliness, high specificity, simplicity, and efficiency (Armetta et al., 2019). D-allulose is generally recognized as safe (GRAS) by the USFDA. Almost 70% D-allulose is primarily absorbed in the small intestine and 30% is excreted in the feces (Tsukamoto et al., 2014). Previous studies have reported the absence of liver damage or adverse symptoms in healthy subjects who consumed 5 g D-allulose per meal for 12 weeks (Hayashi et al., 2010). In addition, toxicity studies on allulose produced via bioconversion have not reported significant toxicity in rats (An et al., 2019).

D-tagatose

D-tagatose has attracted interest in the food industry because of its low calorie content and glycemic index (Noronha et al., 2018). It is a stereoisomer (epimer) of D-fructose that differs in the configuration of the hydroxyl group on the fourth carbon (Que and Gray, 1974). D-tagatose, which is approximately 90% as sweet as sucrose, has the advantage of being lower in calories than sucrose by only 1.5 kcal/g (Levin, 2002). In addition, it is highly soluble in water (dissolves up to 160 g per 100 mL at 20 °C), and is available in both white crystal and powder forms (Grant and Bell, 2012). The stability of D-tagatose is reported to be robust, with a melting temperature of 134 °C and pH stability of 2 to 7 (Oh, 2007). Although D-tagatose occurs naturally in heated cow milk, its commercial production relies on the enzymatic isomerization of D-galactose using L-arabinose isomerase (Roh et al., 2000). D-tagatose received GRAS status from the FDA in 2001 (Levin, 2002). Although the World Health Organization (WHO) has not established an acceptable daily intake (ADI) for D-tagatose, high consumption is not expected to cause toxic effects (Sokołowska et al., 2022). Preliminary animal studies on D-tagatose suggest that it may reduce lipoprotein and blood glucose levels by lowering the level of total cholesterol (Donner et al., 2010). D-tagatose is primarily absorbed in the small intestine, with a significant amount being metabolized and a small amount excreted in the feces (Normén et al., 2001). Clinical studies have not shown any significant adverse effects on liver function or health in individuals consuming moderate amounts of D-tagatose (Boesch et al., 2001). This compound has been evaluated for its safety and is associated with a low risk of toxicity in humans at normal consumption levels (Lu et al., 2008).

Isomaltulose

Isomaltulose, also known as palatinose, is a reducing functional disaccharide composed of D-glucose and D-fructose linked by an α-1,6 glycosidic linkage (Shyam et al., 2018). It contains calories (4 kcal/g) similar to those of sucrose; however, its glycemic index (GI) is significantly lower (32) than that of sucrose (72) (Maresch et al., 2017). Isomaltulose is more resistant to acid hydrolysis, although its thermal stability is lower than that of sucrose (Sawale et al., 2017). In addition, because of the presence of the α-1,6-glycosidic linkage in isomaltulose, its digestion rate in the small intestine is four to five times slower than that of sucrose (Oosthuyse et al., 2015). Sucrose isomerase (SIase), the primary enzyme used for isomaltulose production, catalyzes the conversion of sucrose to isomaltulose (Zhang et al., 2021). A recent study reported that in Corynebacterium glutamicum, a GRAS microorganism, SIase was successfully expressed with high purity and yield compared to that in Escherichia coli (Liu et al., 2021). Isomaltulose has been classified as GRAS in the USA and approved for safety assessment as a novel food ingredient under the Novel Food Regulations in the European Union, Australia, and New Zealand (Sawale et al., 2017). Despite the undefined ADI of isomaltulose by the WHO, it has been approved because of its lower intestinal glucose absorption and slower metabolism than sucrose (Holub et al., 2010; Tonouchi et al., 2011). Isomaltulose, a low-GI sweetener, shows gastrointestinal tolerance comparable to that of sucrose and is well-tolerated at dietary intakes of up to 10% for 13 weeks in rat models (Jonker et al., 2002). Adverse effects were not observed in immunotoxicity and neurotoxicity screening (Lina et al., 2002).

D-trehalose

D-trehalose, a disaccharide composed of two glucose molecules linked by an α, α-1,1-glycosidic bond, has unique properties, including low caloric value and high stability under heating and acidic conditions (Kaushik and Bhat, 2003). Although D-trehalose has the same caloric value as sucrose (4 kcal/g), it is used in the food industry as a sugar substitute because of its physicochemical properties such as sweetness intensity (40% sucrose), glass transition temperature, and water-holding capacity (Chen et al., 2022; Russ et al., 2014; Simperler et al., 2006). Enzymatic production is preferred over chemical methods for the commercial production of D-trehalose because of its low cost and simplicity, which utilizes three main biosynthetic pathways involving: (i) trehalose-6-phosphate synthase and trehalose-6-phosphate phosphatase, (ii) maltooligosyl-trehalose synthase and maltooligosyl-trehalose trehalohydrolase, and (iii) trehalose synthase (Cai et al., 2018; Vandercammen et al., 1989). Although the ADI for D-trehalose has not been reported by the WHO, it is well tolerated even at a high dietary intake of 10% (Richards et al., 2002). Clinical studies have not reported any significant adverse effects of moderate D-trehalose consumption on liver function or other health symptoms (Stachowicz et al., 2019). Safety evaluations in animal models revealed high median lethal dose, suggesting low risk of toxicity in humans at conventional dietary levels (Madonna et al., 1989).

Relationship between alternative sweeteners and intestinal microbiota

Although artificial sweeteners are not absorbed by the body and are largely excreted in the urine or feces, some residual artificial sweeteners may be metabolized in the body or remain in the colon (Bornemann et al., 2018). Therefore, the metabolites of both artificial and residual sweeteners can affect the gut microbiota, leading to changes in the community (Ruiz-Ojeda et al., 2019). The effect of artificial sweeteners on the human gut microbiota may be a double-edged sword, either potentially beneficial or detrimental. Although studies have revealed changes in the gut microbiome owing to the use of artificial sweeteners, the amount of artificial sweeteners used in these studies was higher than that consumed in reality. Additionally, the effects of artificial sweeteners on the human gut microbiome remain unexplored (Conz et al., 2023). Despite these complexities, our understanding of the relationship between artificial sweeteners and gut microbiota should be improved for human health and well-being.

Effect of Ace-K on the gut microbiota

Ace-K at the dose of 37.5 mg/kg body weight/day for 4 weeks in CD-1 mice affected the gut microbiota and body weight gain, and altered bacterial community composition. In particular, body weight gain, and changes in the gut bacterial composition and fecal metabolome were significantly sex-dependent. Ace-K increased body weight gain in male rats, but not in female rats (Bian et al., 2017a). In male mice, Ace-K treatment significantly increased the abundance of Bacteroides, Anaerostipes, and Sutterella. Conversely, in female mice, Ace-K treatment led to a significant reduction in the abundance of several genera, including Lactobacillus, Clostridium, an unassigned Ruminococcaceae, and an unassigned Oxalobacteraceae, while increasing the abundance of Mucispirillum (Bian et al., 2017a). Ace-K was not toxic at the doses tested, and we detected changes in the gut microbiota composition and metabolome in a 28-day oral toxicity study in Wistar rats (Murali et al., 2022). While some changes in the relative abundance of bacteria were detected after Ace-K treatment, these changes were not clearly observed in males, whereas a distinct variability in bacterial abundance was observed in females, mainly in the family Verrucomicrobiaceae. The median number of gut bacteria in human feces from individuals who consumed 1.7–33.2 mg/day Ace-K in their diet for 4 days did not differ significantly between Ace-K consumers and non-consumers, although a difference in overall diversity was observed between the two groups. Analysis of the functional composition of the gut microbiota of Ace-K consumers and non-consumers using PICRUSt did not reveal any significant differences between the two groups, suggesting that Ace-K intake was not associated with the functional capacity of the gut microbiota (Frankenfeld et al., 2015). However, this study was limited by the short duration of sweetener use and small number of Ace-K consumption groups.

Effect of aspartame on the gut microbiota

Eight weeks of low-dose aspartame (5 − 7 mg/kg/d in drinking water) consumption by high-fat diet (HFD)-fed mice reduced energy intake and body weight gain compared to that of the controls, but exacerbated glycemia and insulin resistance. Not only did aspartame consumption increase the total abundance of Enterobacteriaceae and Clostridium leptum, but the interaction of HFD with aspartame increased the abundance of bacteria of the genus Roseburia compared to that in the non-aspartame group. Aspartame also inhibits the increase in the Firmicutes: Bacteroidetes ratio, which is typically increased by HFD (Palmnäs et al., 2014). Another study showed that maternal consumption of low-dose aspartame, in conjunction with a high-fat/sucrose diet, may disrupt weight regulation, glucose control, and gut microbiota in both mothers and offspring, especially early in life, even without the direct consumption of low-calorie sweeteners by the offspring (Jodi et al., 2020). A higher relative abundance of Clostridium leptum was detected in obese female Sprague Dawley (SD) rats and their offspring that consumed aspartame. In addition, aspartame consumption by pregnant SD rats affected the microbiota composition in the cecum of their offspring. Mothers who consumed aspartame showed decreased abundance of Enterococcaceae, Enterococcus, and Parasutterella, but increased abundance of a group of bacteria known as Clostridium cluster IV compared to mothers who received water. Regarding the offspring, both males and females from aspartame-consuming mothers showed an overabundance of Porphyromonadaceae compared to those from mothers who consumed water (Jodi et al., 2020). However, two previous studies in healthy adults reported that aspartame consumption had little or no effect on the gut microbiota composition, bacterial abundance, or predicted gene function (Ahmad et al., 2020; Frankenfeld et al., 2015). The feces of healthy adults were incubated with aspartame-based artificial sweeteners containing M. denticulata for 24 h to determine changes in the gut microbiota (Gerasimidis et al., 2020). The results showed that aspartame-based sweeteners significantly decreased the concentrations of Bacteroides/Prevotella and significantly increased the growth of Bifidobacterium and B. coccoides. However, the effects of aspartame-based sweeteners on the gut microbiome are mostly due to maltodextrin, suggesting that the contribution of aspartame is not significant (Gerasimidis et al., 2020). Suez et al. found that 240 mg aspartame/kg body weight/day for 2 weeks did not significantly affect the composition of the gut microbiota but did induce functional changes in the gut microbiota. In particular, microbial functions related to polyamine metabolism are significantly altered in the aspartame group (Suez et al., 2022a).

Effect of sucralose on the gut microbiota

A previous study has shown that the administration of sucralose to male C57BL/6 J mice at a dose of 5 mg/kg body weight/day for 3 or 6 months significantly affected the gut microbiota. The enriched bacterial functional genes are prominently associated with pro-inflammatory mediators (Bian et al., 2017b). The changes in the gut microbiota exhibited distinct patterns between the treated and control mice, with the severity of changes varying with the duration of sucralose treatment. An increase in the population of Ruminococcus and a decrease in the abundance of Lachnospiraceae, Dehalobacteriaceae, Anaerostipes, Staphylococcus, Peptostreptococcaceae, and Bacillus were observed in the gut microbiota of mice after the first 3 months of treatment. The gut microbiota began to reflect an increase in the populations of Akkermansia, Turicibacter, Roseburia, Clostridiaceae, and Christensenellaceae, along with a decrease in the abundance of Streptococcus, Lachnospiraceae, Dehalobacteriaceae, and Erysipelotrichaceae when the treatment was extended to 6 months. The authors argued that sucralose treatment increased the abundance of functional bacterial genes associated with pro-inflammatory responses, which may influence health outcomes (Bian et al., 2017b). Another study showed that the gut microbiome of sucralose-treated mice exhibited significant changes in functional gene enrichment over a 6-month period. In particular, the expression of genes associated with bacterial pro-inflammatory mediators, such as those responsible for lipopolysaccharide synthesis, flagella, fimbriae, toxins, and multidrug resistance, showed a significant increase (Zheng et al., 2022). Mice were administered various concentrations of sucralose, ranging from 0.0003 to 0.3 mg/mL. While these treatments did not affect body weight, the 0.0003 and 0.3 mg/mL doses significantly altered the gut microbiota, leading to an increase in Allobaculum abundance. In addition, the use of 0.0003 mg/mL sucralose significantly increased the population of potential pathogens, such as Tenacibaculum and Ruegeria, in the jejunum. In the ileum, 0.003 mg/mL sucralose significantly increased the abundance of Staphylococcus and Corynebacterium. Furthermore, doses of 0.0003 mg/mL and 0.3 mg/mL significantly decreased the population of Lachnoclostridium in the cecum (Zheng et al., 2022). These results suggested that different doses of sucralose differentially affected the gut microbiota in mice.

In contrast to the findings in mice, the human gut microbiota does not appear to show noticeable differences based on the amount or duration of sucralose consumption (Ahmad et al., 2020; Thomson et al., 2019). Méndez-García et al. showed that a subtle shift in the gut microbiota was observed when humans consumed 48 mg sucralose per day for 10 weeks. Although these changes were small, a long-term clinical experiment revealed a significant increase in Blautia coccoides and decrease in Lactobacillus acidophilus abundance. This suggests that although the effect of sucralose consumption on the gut microbiota may be less pronounced in humans than in mice, it does exist and warrants further investigation (Méndez-García et al., 2022).

Effect of saccharin on the gut microbiota

A previous study has shown that administration of 0.3 mg/mL saccharin to male C57BL/6 J mice in drinking water for 3 or 6 months significantly affected the gut microbiota. The control group showed changes in the gut microbiota; however, these changes were not significant. In contrast, the mice treated with saccharin showed significant changes in the gut microbiota, with differences observed between 3 and 6 months. In total, the abundance of 11 genera changed after 3 and 6 months of treatment. When the treatment was extended to three months, the abundance of Sporosarcina, Jeotgalicoccus, Akkermansia, Oscillospira, and Corynebacterium increased, whereas that of Anaerostipes and Ruminococcus decreased. After treatment extension to 6 months, the proportion of Corynebacterium, Roseburia, and Turicibacter in the gut microbiota increased, while that of Ruminococcus, Adlercreutzia, and Dorea decreased (Bian et al., 2017c). The abundance of Corynebacterium increased while that of Ruminococcus decreased at both three and six months; in contrast, Dorea population decreased specifically during the 6-month period. Saccharin affects the gut microbiota in mice, particularly by affecting pathogens, such as Corynebacterium, Ruminococcus, and Dorea (Bian et al., 2017c). In contrast, the gut microbiota of C57BL/6 J and C57BL/6 J mice with genetic ablation of sweet taste receptors (T1R2-KO) treated with 250 mg/kg body weight saccharin per day for 10 weeks did not change significantly (Serrano et al., 2021). Randomized healthy subjects treated with 400 mg/kg body weight sodium saccharin per day for two weeks did not show significant changes in the gut microbiota (Serrano et al., 2021). In a recent study, the gut microbiota of randomized human participants who consumed 180 mg/day saccharin along 5,820 mg/day glucose for 2 weeks did not show any alterations. However, a decrease in the abundance of Fusobacterium in the oral microbiota was observed in humans who consumed only 5,000 mg/day of glucose for two weeks (Suez et al., 2022b).

Effect of neotame on the gut microbiota

Mice treated with 0.75 mg neotame/kg body weight/day for 4 weeks showed significant changes in their gut microbiota composition at the genus level. In particular, treatment was associated with a significant increase in the abundance of Bacteroidetes and a concomitant decrease in the abundance of Firmicutes. In addition, a significant decrease was observed in the abundance of two microbial families, Lachnospiraceae and Ruminococcaceae. Within these families, the populations of three different genera under Ruminococcaceae and five under Lachnospiraceae were significantly depleted (Chi et al., 2018). However, the relationship between neotame and the human gut microbiota has not been studied in detail. As it is largely excreted, neotame is considered safe for human consumption (Cao et al., 2020; Ruiz-Ojeda et al., 2019).

Artificial sweeteners such as Ace-K, aspartame, saccharin, sucralose, and neotame have unique benefits and potential drawbacks. Ace-K excels in sweetness and stability; however, it can produce a bitter aftertaste at high doses and may possess genotoxic properties. Aspartame, while sweet, is temperature-sensitive, which limits its use, and can be problematic for individuals with PKU due to the presence of phenylalanine. Saccharin, known for its sweetness, has been reported to have a metallic aftertaste. Although it has been linked to cancer in rats, no such association has been reported in humans. Sucralose is considered for its extreme sweetness and stability; however, further research is required to understand the health effects of its byproducts. Neotame offers the highest level of sweetness, with excellent stability and metabolic properties. The effects of these artificial sweeteners on gut health are varied. Consumption of Ace-K and aspartame may alter the balance of gut bacteria and potentially exacerbate insulin resistance. Sucralose and saccharin may also affect the gut microbiome and glucose metabolism; however, these effects are less pronounced in humans. Further research on the use of neotame should address the potential changes in the composition of the gut bacteria.

Relationship between rare sugars and the intestinal microbiota

Although rare sugars, microorganisms, and short-chain fatty acids (SCFAs) may act as prebiotics and improve the gut environment (Hughes et al., 2021), current literature on these is limited, as rare sugars are highly digested and absorbed in the human body (Ahmed et al., 2021). This section summarizes the prebiotic effects of and gut microbiome modifications caused by D-allulose, D-tagatose, isomaltulose, and D-trehalose based on recent articles.

Effect of D-allulose on the gut microbiota

A study reported the effect of D-allulose probiotic on acid production in milk using yogurt starter cultures of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. Among the probiotic strains commonly used in fermented milk and cheese production, only Lactococcus lactis H61 decreased acid production in the presence of D-allulose but not other sugars such as fructose, xylose, and sorbitol (Kimoto-Nira et al., 2017).

Yusaku et al. found that D-allulose intake significantly lowered the glucose levels in hyperglycemic (> 200 mg/dL) HFD-fed mice but did not affect the same (approximately 100 mg/dL) in normal glycemic chow-fed mice, as the anorexigenic effect of D-allulose is mediated by glucagon-like peptide-1 receptor activation (Iwasaki et al., 2018). Changes in the gut microbiota of mice on HFD may be indirectly related to weight control via changes in the gut environment, such as SCFA production. A previous study investigated the effects of a dairy diet containing 0–30% D-allulose on body weight and SCFA production in Wistar rats. This study showed that similar to dietary fiber, D-allulose is fermented in the cecum by intestinal microorganisms to produce SCFAs. It is noteworthy that compared to mice fed a HFD or HFD supplemented with erythritol (5%, w/w), HFD-fed mice supplemented with D-allulose (5%, w/w) had lower body weight and reduced levels of enzymes related to lipid metabolism, such as fatty acid synthase and cholesterol acyltransferase. However, SCFA production did not differ significantly between the D-allulose and HFD groups, except for increased butyrate production in the D-allulose group (Higaki et al., 2022). The gut microbiota analysis of HFD-fed mice receiving D-allulose showed a significant increase in the abundance of Lactobacillus, Coprococcus, and Coprobacillus, whereas that of Turicibacter, Clostridiaceae, Erysipelotrichaceae, and Dorea decreased significantly compared to that in the HFD-fed control group (Han et al., 2020). The increase in butyrate levels may be related to specific gut microbiome taxa, as Coprococcus is known to be involved in butyrate production (Ríos-Covián et al., 2016).

Effect of D-tagatose on the gut microbiota

Previous studies investigating D-tagatose fermentation by microbes revealed that only few types of human intestinal and dairy lactic acid bacteria, including certain strains of Enterococcus and Lactobacillus, are capable of fermenting D-tagatose. Interestingly, Bifidobacterium cannot ferment D-tagatose (Hans Bertelsen, 2001). The study showed that D-tagatose is fermented in the cecum and colon, but not in the stomach or small intestine, by the gut microbiota, which produces various compounds and gases. Pigs fed D-tagatose showed increase in the abundance of D-tagatose-degrading bacteria and elevated butyric and valeric acid production, particularly in the colon. In addition, the energy derived from post-microbial fermentation of D-tagatose was calculated in terms of SCFAs. With continuous consumption of the experimental diets, the fecal dry matter content and total number of anaerobic and D-tagatose-degrading bacteria showed an upward trend. Notably, the number of these bacteria was higher in pigs on the tagatose diet after 15 days (Lærke et al., 2000). Venenma et al. investigated the effects of D-tagatose on the fecal microbiota composition and SCFA production. Participants (12 men and 18 women) consumed 30 g raspberry jam with 7.5 g and 12.5 g D-tagatose for 2 weeks. The results showed that high levels of D-tagatose increased Lactobacillus levels only in men. All groups exhibited increased butyrate production in vitro and in vivo (Venema et al., 2005).

Effect of isomaltulose on the gut microbiota

Isomaltulose, a potential prebiotic, has been observed to significantly stimulate the growth of several probiotic strains, including Lactobacillus and Bifidobacterium. This suggests its potential to positively influence the gut microbiota. In addition, isomaltulose was found to affect the production of SCFAs, in particular, acetic and propionic acids, during fermentation by probiotic strains. This interaction suggested that isomaltulose not only promotes the proliferation of beneficial gut bacteria, but also positively influences their metabolic profile (Su et al., 2021). Yang et al. investigated the prebiotic potential of isomaltulose in rats, focusing on the effect on gut microbiota composition, as well as on SCFA production. Water containing 10% isomaltulose increased the abundance of prebiotic bacteria, such as Faecalibacterium and Adlercreutzia, while reducing the abundance of pathogenic bacteria, such as Shuttleworthia. Furthermore, propionate and butyrate levels were higher in isomaltulose-treated rats (Yang et al., 2021). Zhou et al. investigated the protective effects of isomaltulose on the gut microbiota in normal control (NC) and dextran sulfate sodium (DSS)-induced ulcerative colitis groups. Isomaltulose treatment led to an increase in the abundance of beneficial bacteria, including Akkermansiaceae, Marinifilaceae, and Anaerovoracaceae. In particular, the relative abundance of Verrucomicrobiota increased considerably compared to that in the NC and DSS models. These bacteria are known for promoting gut health by producing SCFAs (Zhou et al., 2022).

Effect of trehalose on the gut microbiota

Chen et al. demonstrated that compared to fructooligosaccharide (FOS), trehalose significantly enhanced the growth of lactic acid bacteria associated with bacteriocins, particularly Lactococcus lactis C101910 and Lactococcus sp. GM005. They analyzed bacteriocin production by Lactobacillus animalis, Enterococcus durans L28-1, and Lactococcus lactis C101910 and found that bacteriocin expression by Lactococcus sp. GM005 was higher in culture media containing trehalose than in media containing glucose, FOS, or raffinose (Chen et al., 2007). Hiroto et al. investigated the effects of supplementing milk replacement with trehalose on both gut microbiota and SCFA production in calves. Their results showed that trehalose supplementation resulted in a healthier gut environment characterized by a decrease in Clostridium and an increase in Dialister and Eubacterium populations. This microbial shift correlated with changes in SCFA production, particularly an increase in the level of butyrate, which is known to improve gut health by increasing mucin production and protecting against pathogenic bacteria, such as Clostridium (Miura et al., 2021). Buckley et al. reported that trehalose can attenuate pathogenic Clostridioide difficile and prevent simulated infections. The gut environment remodeled by Clostridioide difficile infection is characterized by an increase in the abundance of beneficial bacteria, such as Finegoldia, Faecalibacterium, and Oscillospira, and a decrease in the population of pathogenic bacteria, including Klebsiella and Clostridium, induced by trehalose (Buckley et al., 2021). This series of studies demonstrated the potential of trehalose to promote a healthier gut environment by improving both gut microbiota composition and SCFA production.

Rare sugars, such as D-allulose, D-tagatose, isomaltulose, and D-trehalose, have emerged as healthier and lower-calorie alternatives to conventional sugars. These rare sugars may positively affect gut health. D-allulose can be used as a prebiotic in starter cultures of Lactobacillus and Streptococcus. In addition, D-allulose can modulate blood glucose levels, body weight, lipid metabolism, and the composition of the gut microbiome. The selective fermentation of D-tagatose by specific lactic acid bacteria impacts gut health, particularly via SCFA production. Isomaltulose positively affects the growth of beneficial bacteria, such as Lactobacillus and Bifidobacterium, and the production of SCFAs. The protective effects of isomaltulose on gut microbiota have also been discussed. Trehalose may increase the abundance of beneficial lactic acid bacteria and modulate gut microbiota and SCFA production. Despite the limited number of studies on these rare sugars, this review presents a detailed investigation on the potential prebiotics and the improvements in the gut environment. However, further studies are required to completely understand the long-term effects of these sugars.

Declarations

Conflict of interest

The authors declare that they have no conflict of interest.