The role of nutritional factors in fat-free mass preservation and body composition changes after bariatric surgery: a systematic review of the available evidence

Abstract

Background

Bariatric surgery (BS) is an effective treatment for obesity, but it often leads to significant changes in body composition, including loss of fat-free mass (FFM). Nutritional intake plays a crucial role in influencing these changes. This systematic review aims to assess the impact of nutritional strategies on body composition, specifically FFM preservation, following BS.

Methods

A systematic literature search was conducted across PubMed, Scopus, EMBASE, Web of Science, and Google Scholar using combinations of terms including ("bariatric surgery" OR "RYGB" OR "sleeve gastrectomy") AND ("nutritional intake" OR "dietary factors") AND ("lean body mass" OR "fat-free mass" OR "body composition"). We included studies examining adults who underwent bariatric surgery and reported quantitative relationships between postoperative nutritional intake (assessed through dietary records, recalls, or biomarkers) and changes in LBM/FFM. Data were extracted and synthesized to evaluate the effects of specific nutritional factors including protein quantity/quality, macronutrient distribution, and micronutrient status on body composition outcomes.

Results

Inadequate protein intake (< 60 g/day or < 1.2 g/kg ideal body weight) was common post-BS and associated with significant FFM loss (5–15% reduction). Higher protein intake (≥ 60 g/day or ≥ 1.2 g/kg) preserved FFM, particularly during the first 6 months, though results varied due to differences in study design and assessment methods. Macronutrient analysis revealed carbohydrate intake > 130 g/day correlated with poorer weight loss outcomes, while fat intake > 60 g/day was associated with greater FFM loss (2–4% additional reduction). Conversely, fiber intake > 25 g/day demonstrated protective effects. One trial of the Dietary Approaches to Stop Hypertension (DASH) diet demonstrated mixed body composition impacts.

Conclusion

Adequate protein intake is crucial for minimizing FFM loss after BS. Personalized nutrition plans, early counseling, and regular follow-ups are vital for improving long-term outcomes. However, variability in study results highlights the need for more research to develop standardized nutritional guidelines for post-surgery care.

Introduction

According to estimates from the World Health Organization (WHO), global obesity prevalence has nearly tripled since 1975. Currently, over 1.9 billion adults live with overweight, with more than 650 million living with obesity. In 2016, 39% of adults aged 18 and older had overweight, while 13% had obesity [1]. Integrating hygienic-dietary measures (defined here as lifestyle modifications including balanced nutrition, regular physical activity, and behavioral adjustments to promote overall health) with pharmacological treatments can lead to a 10% reduction in body weight over the medium term (defined as a period of 6 months to 2 years); however, these approaches often fall short in sustaining weight loss over the long term [2]. Body weight, particularly fat mass, is a significant factor contributing to nutrition-related chronic diseases, including coronary heart disease, diabetes, and certain types of cancers. Bariatric surgery (BS) is regarded as the most effective treatment for severe obesity in eligible patients. Evidence indicates that BS can significantly improve or even resolve many obesity-related comorbidities, such as hypertension, due to substantial weight loss [3, 4].

BS is considered the most effective treatment for individuals with severe obesity [3]. Roux-en-Y gastric bypass (RYGB) and sleeve gastrectomy (SG) are the two most commonly conducted bariatric surgeries [5]. Despite successful weight loss outcomes, patients who undergo BS may still face the risk of nutrient deficiencies because of energy restriction, malabsorption, and food intolerances [6, 7]. Additionally, many patients experience substantial loss of fat-free mass (FFM; encompassing muscle, water, bone, and organs). In the first year following a laparoscopic RYGB, patients typically lose approximately 22% of their FFM [8]. Lean body mass is crucial for resting energy expenditure, functional capacity, muscle strength, and cardiovascular health [9]. In patients after BS, excessive loss of FFM can be detrimental, potentially slowing down weight loss or even leading to weight regain. [10, 11]. While some reduction in FFM is unavoidable during weight loss, excessive loss of FFM can compromise the effectiveness of BS and heighten health risks, such as a compromised immune system, reduced muscle strength, and increased risk of infections [12].

Factors such as age, sex, preoperative body mass index (BMI), presence of comorbidities, and psychological status are believed to influence both weight loss and FFM loss following BS [12]. The impact of postoperative variables, including nutritional factors, on variations in total weight loss (TWL) and FFM loss following BS is still a subject of ongoing debate. Specifically, the optimal levels of protein, calories, and micronutrients needed to preserve FFM post-BS remain unclear. Studies often vary in their methodologies, including differences in the timing of nutritional assessments, the types of dietary intake measurements used, and the statistical approaches to analyzing the data. These inconsistencies lead to conflicting recommendations, making it difficult for clinicians to develop standardized nutritional guidelines [13–16].

Regarding FFM loss after BS, nutritional studies have examined protein, calorie, and certain micronutrient intake, but the findings remain controversial [14–16]. The literature highlights the significance of micronutrient deficiencies in individuals who have undergone BS [17].

Monitoring the nutritional status, body composition, and dietary intake of these patients both in the short term and long term is essential for planning effective interventions, maintaining health, and ensuring quality of life. Previous research has indicated a link between inadequate nutritional intake and reduced FFM. However, no systematic review has been conducted to evaluate the strength of the evidence supporting this association. Therefore, this systematic review aims to comprehensively assess the evidence regarding nutritional intake and its relationship with body composition changes in patients following BS. By synthesizing the available evidence, this review seeks to provide clarity on the impact of specific nutritional factors on FFM preservation and to identify gaps in the current literature, ultimately informing future research and clinical practice.

Material and methods

Search strategy

A systematic literature search was conducted across PubMed, Scopus, EMBASE, Web of Science, and Google Scholar from inception to September 2024, with no restrictions on language or publication date, combining terms for bariatric surgery ("bariatric surgery" OR "metabolic surgery" OR "gastric bypass" OR "Roux-en-Y" OR "RYGB" OR "sleeve gastrectomy" OR "SG" OR "adjustable gastric banding" OR "biliopancreatic diversion"), nutritional factors ("nutrition" OR "diet" OR "dietary pattern" OR "macronutrient" OR "protein intake" OR "amino acids" OR "micronutrient" OR "fiber" OR "carbohydrate" OR "fat intake" OR "supplementation"), and body composition outcomes ("body composition" OR "lean body mass" OR "fat-free mass" OR "muscle mass" OR "sarcopenia" OR "weight loss"), linked with Boolean operators (AND/OR) (e.g., "bariatric surgery AND protein intake AND fat-free mass") and restricted to human studies. We conducted a manual search by examining the references of important articles to uncover any additional studies that might have been missed in the electronic database searches. This review only includes articles that were published in English. Figure 1 depicts the process used for selecting studies in this review. Given the variability in comparisons among the included studies—such as differences in outcomes, and exposures/interventions—and the lack of sufficient data suitable for quantitative analysis and pooling, we opted for a qualitative systematic review. The systematic review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [18]. The pre-registration code for the systematic review and meta-analysis protocols in PROSPERO is 1041878.

Fig. 1.

PRISMA flow diagram for selection process of the studies

The study selection process is shown in the PRISMA flowchart (Fig. 1) presented in the Results section.

Eligibility criteria

This review included both randomized clinical trials and observational studies involving post-BS patients. Studies that assessed nutritional intake after surgery using methods such as 24-h food recalls, dietary food records, or food frequency questionnaires, as well as trials that examined the impact of nutritional interventions on body composition changes following BS, were included. There were no restrictions regarding age or sex. In contrast, we excluded reviews, editorials, studies using non-human models, and studies published in languages other than English. To be eligible for inclusion in this review, studies needed to examine the relationship or effects of dietary pattern/single food/calorie intake/nutrients or supplements with body composition changes in individuals with obesity who had undergone BS. We excluded studies with participants who had cachexia or severe mental and cognitive disorders that might impede adherence to a structured nutritional plan.

Table 1 presents the PICOS (population, intervention, comparator, outcome, and setting) criteria used in this systematic review. Due to the methodological approach used, ethical approval was not required for this study.

Table 1.

PICOS (population, intervention/exposure, comparator, outcome, and setting) criteria used to perform the systematic review

PICOS	Criteria
Population	Obese individuals who underwent bariatric surgery
Intervention/exposure	Any nutritional exposure/intervention
Comparator	Control group (if applicable)
Outcome	Body composition (e.g., fat mass, muscle mass, lean mass), anthropometric indices
Setting	Observational studies and clinical trials

Study selection

The selection process was carried out in two phases. Before Phase 1, all identified citations were screened for duplicates, and these were removed. In Phase 1, two reviewers (BA and ZS) independently evaluated the titles and abstracts of all citations from the electronic databases. Articles that did not meet the inclusion criteria were excluded. In Phase 2, the same reviewers independently applied the inclusion criteria to the full text of the articles. One reviewer thoroughly examined the reference lists of the selected studies. Any disagreements in either the first or second phase were resolved through discussion until the two authors reached a consensus. If a consensus was not reached, the third author (MV) was involved in making the final decision.

Data extraction and quality assessment of the studies

Data extraction was performed independently by two reviewers (BA and ZS), with any discrepancies resolved through consensus. We created a data extraction sheet to record details such as the first author, year of publication, country, study design, follow-up duration (if applicable), sample size, age and sex of participants, intervention/exposure, type of surgery, nutrient(s) or food or dietary pattern exposure (for observational studies), study groups and intervention duration (for clinical trials), body composition outcomes, and main findings. The quality of observational studies was assessed using the Newcastle–Ottawa Scale (NOS) [19], while the Jadad-Oxford Scale [20] was used to evaluate randomized clinical trials. The NOS assesses three key methodological aspects: the selection of study participants (0–4 points), adjustment for confounders (0–2 points), and determination of outcome indicators (0–3 points). A study that received a score of 7 to 9 points was considered high quality [19]. The Jadad-Oxford scale assigned 0 or 1 point for each of five criteria: (1) randomization presence, (2) randomization method, (3) double-blinding, (4) double-blinding method, and (5) description of withdrawals and dropouts. Scores of 3 or higher were deemed high quality, while scores of 2 or lower were classified as low quality [20].

Table 2 presents information about the studies included in the systematic review.

Table 2.

Characteristics and outcomes of the included studies (presented in chronological order, starting with the most recent)

First author (reference no.)	Year, country	Study design, follow-up	Sample size (M/F)	Age (year)	Intervention/exposure	Surgery type	Outcomes analyzed	Main findings	Study quality*
Schiavo [22]	2024, Italy	Non-randomized interventional trial, 1 month	57 (NM/NM)	18–65	Group 1: supplementation with P + BCAAs + Vit.D (n = 31) Group 2: protein (n = 26)	SG	TBW, FM, FFM, and MS	P + BCAA + Vit.D supplementation is more effective than protein alone in determining FM loss and is associated with a lower decrease in FFM and MS, without interfering with clinical status in patients 1 month after SG	4
Zhan [23]	2024, China	Retrospective cohort, 1 month	138 (42/96)	18–60	Group 1: consumed SPB Group 2: consumed CPB	BS other than RYGB	BMI, weight, FFM, FFMI, % FFM, SMI	SPB supplements may be more effective in preventing early FFM loss after BS, compared to CPB supplements, particularly among male patients. Therefore, SPB supplements may be recommended to patients undergoing BS	6
López-Gómez [24]	2024, Spain	Ambispective observational cohort, 1 month	44 (11/33)	18–65	Group 1: low-protein, low-calorie diet formula Group 2: high-protein, low-calorie diet formula	SG	ASMI, FFMI, FI, TBW, BMI	In patients undergoing SG surgery, the addition of a normocaloric, hyperproteic formula managed to slow down the loss of muscle mass and increase the loss of fat mass with no differences on total weight loss	5
Rashidbeygi [25]	2024, Iran	Randomized controlled trial, 16 weeks intervention	98 (27/71)	> 18	Group 1: low-calorie DASH diet Group 2: low-calorie diet	SG	Weight, BMI, FM, FFM	Following the calorie-restricted DASH diet for 16 weeks in patients who had BS with weight regain 2 years after surgery improved weight, BMI, and FM reduction compared to calorie-restricted control diet	6
Moslehi [12]	2024, Iran	Cross-sectional	146 (33/113)	> 18	Macronutrient quality index, carbohydrate quality index, fat quality index, and healthy plate protein quality index	SG	Weight, BMI, FM, FFM	Adherence to a high-protein, high-fiber diet after SG may enhance surgical success by improving total weight loss and preventing excessive fat-free mass loss	5

Alshamari [26]	2022, Qatar	Randomized controlled trial, 1, 3, 6 months post-LSG	48 (18/30)	18–60	Intervention group: received daily protein supplements containing 20 g of protein Placebo group: received zero protein supplements	SG	Weight, BMI, EWL%, TWL%, muscle mass, FM, fat percentage, fat mass loss, muscle mass loss	Protein supplement post-LSG has no significant impact on weight, muscle mass, or fat mass loss. However, the protein supplement helped in maintaining muscle mass and preventing muscle mass loss	6
Lamarca [27]	2021, Brazil	Randomized controlled trial, 2–7 years post-RYGB	63 (7/56)	40.3 ± 8.3	Group 1: placebo (control [CON]; n = 17); group 2: whey protein supplementation (PRO; n = 18); group 3: RT combined with placebo (RTP; n = 13); group 4: RT combined with whey protein supplementation (RTP + PRO; n = 15)	RYGB	Weight, BMI, FFM, SMM, FM, body fat	Combined RT and adequate protein intake via supplementation can increase FFM and SMM in the late postoperative period without changing REE. These associated strategies were effective in improving muscle-related parameters and potentially in improving the patients’ physical function	7
Hirsch [28]	2020, USA	Randomized controlled trial, 12 weeks	49 (6/43)	43.7 ± 10.7	Group 1: consumed ready-to-drink protein shakes Group 2: followed standard-of-care recommendations	NM	Weight, BMI, FFM, FM, FM%	Patients were able to achieve the recommended 60 g of protein per day post-surgery, but the provision of ready-to-drink protein shakes may help bariatric patients achieve higher post-surgery protein intakes. Both groups experienced significant decreases in weight, BMI, and body composition; results varied depending on the method of body composition used. Changes in FFM measured by BIS may reflect greater changes in TBW as opposed to actual muscle mass	5

Saiki [29]	2020, Japan	Retrospective cohort, 3 years	89 (42/47)	41.9 ± 9.0	Nutritional status was collected from all patients	LSG	Weight, BMI, visceral fat area (VFA), subcutaneous fat area (SFA), SMM	In the insufficient weight loss group, snacking and eating out habits were often observed. In the excessive weight loss group, the frequencies of binge eating were high, and a decrease in SMM due to low protein intake was observed. Furthermore, weight regain was shown 12 months after LSG in both groups. In the average weight loss group, there were fewer problems in weight loss outcomes, nutrition intake and body composition	6
Gu¨nes [30]	2019, Turkey	Randomized controlled trial, one month	60 (9/51)	18–65	Group 1: received 1.2 g/kg/day protein support Group 2: received the standard diet	LSG	Weight, BMI, FM, FFM	Protein support in the first month after LSG prevents FFM loss	5
Oppert [31]	2018, France	Randomized controlled trial, 18 weeks	76 (NM/NM)	42.4 ± 9.9	Group 1: usual care (controls [CON]); Group 2: usual care and additional (whey) protein intake (PRO); Group 3: or usual care, additional protein intake, and supervised strength training (PRO + EX)	RYGB	Weight, BMI, FM, LBM	No difference was found regarding weight and lean body mass loss through the combination of resistance training and additional protein intake for 6 months after RYGB	4
dos Santos [32]	2018, Brazil	Cross-sectional and controlled study	90 (90 females) RYGB group (n = 49); Control group (n = 41)	45.0 ± 9.0	Serum concentrations of 25(OH)D	RYGB	Weight, BMI, total muscle mass, total skeletal muscle mass, SMMI, FM	There was no association between variables related to body composition and 25(OH)D concentrations. On the other hand, vitamin concentrations correlated negatively to BMI variation after undergoing surgery	5

Schiavo [35]	2017, Italy	Prospective, 12 months	60 [60 males]	18–65	Male patients received either (1) a normal protein diet (n = 30) with protein intake 1.0 g/kg of IBW, or (2) a protein-enriched diet (n = 30) with protein intake 2.0 g/ kg of ideal body weight	LSG	Weight, FM, FFM	Protein-enriched diet is more effective than normal protein diet in determining FM loss and is associated with a lower decrease in FFM and RMR, without interfering with renal function in male patients after LSG	5
Sherf Dagan [33]	2016, Israel	Prospective cohort, 6 and 12 months	77 (32/45)	42.7 ± 9.4	Daily protein intake	LSG	Weight, BMI, WC, FM, FFM, excess wight loss	A large proportion of the patients during the first year after LSG, do not meet the currently recommended daily protein intake of at least 60 g/d. The results emphasize the importance of an adequate protein intake after LSG and supports the currently recommended protein intake goal of ≥ 60 g/d as an efficient strategy to better preserve FFM post-LSG	6
Lopes Gomes [34]	2016, Brazil	Randomized controlled trial, 16 weeks	30 [30 females]	45 ± 11	Participants were treated with hypocaloric diet and randomized to receive or not supplementation with whey protein, 0.5 g/kg of the IBW	RYGB	Weight, FFM, FM	Whey protein supplementation promoted body weight and FM loss in women with long-term weight regain following RYGB	4
Schollenberger [36]	2016, Germany	Randomized controlled trial, 6 months	20 (3/17)	18–65	Group 1: the protein (PRO) group, or to 2) the control (CON) group, which received an isocaloric placebo	LSG (n = 15) or RYGB (n = 5)	Weight, BMI, excess wight loss, absolute weight loss, FM, LBM	Protein supplementation after BS improves body composition by enhancing loss of body FM and reducing loss of LBM within the 6-month-follow-up	5
Moizé [37]	2013, Spain	Prospective, 12 months	50 (9/41)	44.0 ± 1.7	Dietary advice and daily protein intake were assessed prior to, and at 2 and 6weeks, 4, 8, and 12 months after surgery	SG (n = 25) or GB (n = 25)	Weight, BMI, WC, FM, LTM	The data provide supportive evidence for the protein intake goals of > 60 g/d or 1.1 g/kg IBW/d as a being associated with better LBM preservation in the BS patient	6

Dodsworth [38]	2012, Australia	Randomized controlled trial, 12 months	47 (9/38)	18–65	Group 1: protein-enriched diet (90–100 g of protein per day) Group 2: usual care	laparoscopic adjustable gastric banding	Weight, BMI, WC, FM, FFM	No differences in protein intake, weight or body composition were observed between the intervention and usual care groups. Relative protein intake for the study sample as a whole was not a significant predictor of weight or body composition at any time point	6
Raftopoulos [39]	2011, USA	Prospective, 12 months	427 (287/140)	42.7 ± 11	Mean total protein daily, daily protein intake, and compliance with a daily protein intake > 1 g/kg/d	RYGB	BMI, %EWL, %BF, %LM	Excellent compliance with a daily protein intake of ≥ 1 g/kg/d at 12 months after RYGB is feasible and might result in the benefits of increased weight loss, a decreased percentage of body fat, and improved percentage of lean mass	7
Clements [40]	2011, USA	Unblinded, randomized control trial, 8 weeks	30 (1/29)	> 18	Group 1: received 24 g of HMB/Glu/Arg dissolved in water twice daily Group 2: received no supplement	LGB	BMI, FM, LBM	There is a significant decrease in weight, BMI, LBM, and RMR in all subjects after LGB, and these changes were not affected by the use of HMB/Glu/Arg. However, its consumption (78 cal per serving) did not adversely affect weight loss in the experimental group	4
Andreu [41]	2010, Spain	Prospective, 12 months	101 (25/76)	43.2 ± 1.0	Daily protein intake	laparoscopic Roux-in-Y gastric bypass (LGBP) or LSG	Weight, BMI, FM, FFM	The study underscores the value of protein supplementation for the achievement of the recommended daily protein intake in the bariatric patient. However, the data do not help to define a protein intake goal as critical in determining the FFM and protein status changes following LGBP or LSG	6

Swenson [42]

2007, USA

Randomized controlled trial, 12 months

32 (3/29)

Group 1: 39.7 ± 7.6 Group 2: 41.7 ± 9.8

Group 1: received a low-fat control diet based on American Heart Association recommendations Group 2: received la ow-carbohydrate, high-protein diet based on the South Beach Diet

laparoscopic gastric bypass

Weight, BMI, FM, FFM

For both diets similar significant within group changes were observed for body composition between the baseline and the 6-month follow-up visit

5

BS bariatric surgery, TBW total body weight, FM fat mass, FFM fat-free mass, FFMI fat free mass index, FI fat index, MS muscle strength, P protein, BCAA branched-chain amino acid, Vit.D vitamin D, SPB short peptide-based, CPB complex protein-based, SMI skeletal muscle mass index, BMI body mass index, SG sleeve gastrectomy, ASMI appendicular skeletal mass index, DASH dietary approach to stop hypertension, LSG laparoscopic sleeve gastrectomy, EWL excess weight loss, TWL total weight loss, RT resistance training, SMM skeletal muscle mass, BIS bioelectric impedance, LBM lean body mass, SMMI skeletal muscle mass index, WC waist circumference, RMR resting metabolic rate, GB gastric bypass, LTM lean tissue mass, BF body fat, LM lean mass, HMB beta-hydroxy-beta-methylbutyrate, Glu glutamine, Arg arginine, IBW ideal body weight.

^*The quality of observational studies was assessed using the Newcastle-Ottawa Scale (NOS), and the Jadad-Oxford Scale was used to evaluate clinical trials.

Definition of terms

Lean body mass (LBM) refers to the weight of everything in the body except fat, including muscle, bones, water, and organs, while fat-free mass (FFM) is often used synonymously with LBM but can sometimes exclude certain components like water. In this systematic review, we focus on FFM, which encompasses all body components except fat tissue. We have chosen to use FFM consistently throughout the manuscript to align with current literature and avoid confusion, as it is more commonly referenced in recent studies. While LBM is often used interchangeably with FFM, our consistent use of FFM aims to provide clarity and precision in discussing the impact of nutritional strategies on body composition changes following bariatric surgery [21].

It is important to note that common techniques include Bioelectrical Impedance Analysis (BIA), which estimates body composition through electrical resistance; dual-energy X-ray absorptiometry (DXA), known for its precision in measuring fat and lean mass; and skinfold measurements, which provide practical assessments of body fat percentage through caliper measurements at specific sites. These methods play a crucial role in evaluating the impact of nutritional strategies on body composition changes following bariatric surgery.

Results

Selected studies

After evaluating 1843 abstracts from databases (excluding duplicates), screening titles and abstracts, and identifying additional records from other sources, a thorough assessment of 104 full-text articles was performed. Among these, 41 studies were found to be irrelevant to the current systematic review and were thus excluded from further assessment. These studies were excluded because they did not meet the predefined inclusion criteria. Common reasons for exclusion included not examining the relationship between nutritional intake and body composition changes, not involving post-bariatric surgery patients, or utilizing study designs that were not eligible for inclusion (e.g., reviews, case reports). In total, 22 studies (comprising 10 observational studies and 12 trials) [12, 22–42] with 1,832 participants, published between 2007 and 2024, met the criteria for inclusion in the systematic review.

Characteristics of the observational studies

Table 2 provides a chronological overview of the descriptive details of the included studies, organized from the most recent to the earliest. A total of 1,222 participants were included across the 10 observational studies [12, 23, 24, 29, 32, 33, 35, 37, 39, 41] evaluated in this systematic review. Three studies were conducted in Spain [24, 37, 41], Italy [35], other studies were conducted in China [23], Iran [12], Japan [29], Brazil [32], Israel [33], and another one in USA [39]. Two studies were cross-sectional [12, 32], while eight followed a cohort design [23, 24, 29, 33, 35, 37, 39, 41]. Of the cohort studies, five employed a prospective cohort design [33, 35, 37, 39, 41], while the others used either an ambispective observational cohort design [24] or a retrospective design [23, 29]. Two studies focused exclusively on either females [32] or males [35], while the remaining eight studies [12, 23, 24, 29, 33, 37, 39, 41] included both female and male participants. In one study [22], the participants' sex was not specified. The participant numbers varied from 44 to 427 across the studies. Body composition was assessed using bioelectrical impedance analysis (BIA) in eight studies [12, 23, 24, 29, 33, 35, 39, 41], whereas two studies utilized dual-energy X-ray absorptiometry (DXA) [32, 37].

Observational studies examining the association between nutritional factors and body composition changes following bariatric surgery

Protein supplementation and fat-free mass preservation

Several studies highlight the role of protein intake in mitigating fat-free mass (FFM) loss post-surgery. A 2024 retrospective cohort study [23] compared short peptide-based (SPB) and complex protein-based (CPB) supplements in 138 bariatric surgery (BS) patients, excluding Roux-en-Y gastric bypass. SPB supplements significantly reduced FFM loss in both females (β = − 1.14, P = 0.047) and males (β = − 2.36, P = 0.024), with males also showing reduced percentage FFM loss (P = 0.049). Similarly, López-Gómez et al. [24] found that a normocaloric, hyperproteic supplement preserved appendicular skeletal muscle mass and FFM better than a low-protein diet in sleeve gastrectomy patients. Schiavo et al. [35] observed that a protein-enriched diet (2.0 g/kg ideal body weight) led to greater fat mass reduction and less FFM loss than a normal protein diet (1.0 g/kg) in male patients. Dos Santos et al. [32] and Andreu et al. [41] noted mixed results, with protein supplementation aiding intake goals but not always correlating with FFM preservation.

Macronutrient quantity and quality

Macronutrient composition influences weight loss and body composition. A cross-sectional study [12] of 146 adults 2–4 years post-sleeve gastrectomy found that higher carbohydrate intake reduced total weight loss, while increased protein intake lowered odds of poor weight loss. Greater fat intake correlated with FFM loss, but high fiber intake protected against excessive FFM loss. Notably, "macronutrient quality indices" (e.g., source/processing of carbs/fats/proteins) did not significantly affect outcomes. Saiki et al. [29] highlighted extremes in weight loss (%TWL ≤ 19.9% or ≥ 35.0%) in Japanese patients, linking insufficient loss to snacking/eating out and excessive loss to binge eating and low protein intake, both leading to weight regain.

Protein intake thresholds and surgical outcomes

A 12-month cohort study [33] of 77 sleeve gastrectomy patients identified ≥ 60 g/day protein as protective against FFM loss, particularly in women (8.9% vs. 12.4% loss). Another study [37] of 50 gastric bypass/sleeve gastrectomy patients reinforced this, showing ≥ 60 g/day or 1.1 g/kg ideal body weight reduced lean tissue loss at 4 and 12 months, with protein intake being the primary predictor. Raftopoulos et al. [39] further supported these findings, linking higher protein intake to greater excess weight loss and lean mass retention after Roux-en-Y gastric bypass.

Confounding factors and limitations

While protein intake is consistently associated with better outcomes, variability exists. Andreu et al. [41] found no direct link between protein intake and FFM changes despite supplementation, suggesting other factors (e.g., exercise, adherence) may play a role. Vitamin D deficiency [32] showed an inverse correlation with BMI reduction but no direct association with body composition, underscoring the complexity of post-surgical metabolic adaptations.

Characteristics of the interventional studies

Table 2 provides a chronological overview of the descriptive details for the included interventional studies. In total, 610 participants were involved across the 12 interventional studies reviewed [22, 25–28, 30, 31, 34, 36, 38, 40, 42]. Three studies were conducted in USA [28, 40, 42], two in Brazil [27, 34], other ones in Iran [25], Qatar [26], Turkey [30], France [31], Germany [36], Italy [22], and Australia [38]. These studies were published from 2007 to 2024. Nine trials included both female and male participants [25–28, 30, 36, 38, 40, 42], one focused exclusively on females [34], and two studies did not specify the participants' sex [22, 31]. The subjects in the included studies were all over 18 years old, and sample sizes ranged from 20 to 98 participants. The nutritional interventions included the intake of supplements like protein and amino acids, as well as dietary management strategies such as calorie restriction, high-protein diets, and low-calorie dietary approach to stop hypertension (DASH) diets. Two of the trials combined physical exercise with nutritional interventions [27, 31]. The studies did not provide any information on adverse effects. Body composition was assessed using BIA in eight articles [22, 25–27, 30, 34, 36, 38]. Two studies utilized DXA [31, 40], while one study employed bioelectrical impedance spectroscopy (BIS) [28] for evaluation. In one study [42], body composition was assessed using air displacement plethysmography.

Interventional studies exploring the impacts of nutritional factors on body composition changes after bariatric surgery

Protein supplementation and body composition

Multiple studies highlight the benefits of protein supplementation in preserving FFM and improving body composition post-surgery. Schiavo et al. [22] found that whey protein combined with BCAAs and vitamin D (P + BCAAs + Vit.D) significantly reduced fat mass loss (18.5% vs. 13.2%, P = 0.023) and mitigated FFM decline (4.1% vs. 11.4%, P < 0.001) compared to protein alone in sleeve gastrectomy patients. Similarly, Güneş et al. [30] reported that protein intake (1.2 g/kg/day) reduced sarcopenia risk, while Gomes et al. [34] observed greater fat loss without FFM reduction in weight-regain patients after RYGB with whey protein supplementation. However, Alshamari et al. [26] found no significant muscle mass preservation with 20 g/day protein post-SG, and Dodsworth et al. [38] noted poor compliance with protein-enriched diets after LAGB, limiting efficacy.

Combined interventions: protein and exercise

The synergistic effects of protein and exercise were demonstrated in two studies. In post-RYGB patients, whey protein combined with resistance training (RTP + PRO) significantly increased FFM and skeletal muscle mass compared to placebo [27]. Oppert et al. [31] also observed that protein supplementation plus supervised strength training (PRO + EX) improved lower-limb muscle strength in women, though lean mass changes were comparable across groups, suggesting exercise may require longer durations to impact body composition structurally.

Specialized diets and macronutrient manipulation

Tailored dietary approaches showed mixed results. Rashidbeygi et al. [25] reported that a low-calorie DASH diet led to greater improvements in weight, BMI, and body composition than a standard low-calorie diet in SG patients with weight regain. Conversely, a randomized trial [42] found no difference in outcomes between a low-fat (AHA) diet and a low-carb/high-protein (South Beach) diet post-RYGB, with both achieving similar reductions in fat mass and FFM.

Novel supplements and unsuccessful interventions

Some innovative interventions yielded limited benefits. A trial [40] testing an HMB/glutamine/arginine supplement post-gastric bypass found no significant preservation of lean body mass despite weight loss. Hirsch et al. [28] noted that while protein shakes increased intake, FFM loss persisted, with measurement discrepancies (BIS vs. DXA) complicating interpretation. These findings underscore the need for further research on supplement efficacy and standardized assessment methods.

Discussion

This systematic review evaluated the impact of nutrition intake on body composition changes, particularly FFM, after BS. Despite the diverse geographic distribution and nutritional interventions across studies, the findings emphasize the importance of adequate protein intake in preserving LBM and reducing fat mass. However, the variability in study designs and nutritional strategies suggests that the results should be interpreted with caution.

Individuals who undergo BS experience early and significant changes in anthropometry and body composition, with studies showing a greater loss of lean body mass (LBM) (9.7% [3.3%]) compared to fat mass (− 7.7% [4.3%]) postoperatively [43]. However, body composition assessment methods, particularly impedance-based techniques, can influence these measurements. The rapid loss of FFM after BS is clinically concerning, as it can reduce metabolic rate, aerobic capacity, and functional ability, negatively impacting overall health and quality of life [22, 44]. FFM is vital for regulating basal metabolic rate, maintaining muscle strength, supporting functional capacity, and preserving health status. Therefore, minimizing FFM loss while reducing fat mass—termed “high-quality weight loss”—is considered the optimal outcome after BS [22].

Age, sex, preoperative BMI, and surgery type are important predictors of fat-free mass (FFM) loss after BS [45]. Diet significantly influences weight loss and body composition changes post-BS, but the specific dietary factors critical for optimizing outcomes remain unclear [46]. A meta-analysis of 12 studies linked insufficient protein intake (< 60 g/day) to substantial lean body mass loss [47]. Postoperatively, both calorie and protein intake typically decline [24], and most guidelines recommend consuming at least 60 g of protein daily [48]. However, many laparoscopic sleeve gastrectomy (LSG) patients consume less than this amount even 12 months after surgery [37, 41, 49], likely due to reduced gastric volume, intolerance to high-protein foods, and vomiting [33]. Early postoperative very low-protein diets may exacerbate body composition decline [24]. Studies assessing the relationship between adequate protein intake (≥ 60 g/day or ≥ 1.2 g/kg ideal body weight) and FFM loss show mixed results: some report no significant difference [41], while others find reduced FFM loss with sufficient protein intake at 4 to 12 months post-surgery [33, 37]. Notably, adequate protein intake appears most critical during the first 6 months after surgery [33]. A recent meta-analysis suggested that protein intake above recommended levels may enhance weight and fat mass loss but does not consistently preserve FFM, except possibly in LSG patients [51].

Adequate protein intake after BS is essential to prevent FFM loss, hair loss, poor wound healing, and, though rare, protein-calorie malnutrition [33]. To achieve the recommended minimum of 60 g/day, patients should prioritize high-quality, leucine-rich protein sources such as lean meats, fish, dairy, eggs, soy, and legumes [33, 52]. If dietary intake is insufficient, protein supplementation may be necessary. The beneficial effect of sufficient protein intake on FFM preservation, particularly after sleeve gastrectomy, may be linked to a lower non-protein calorie-to-nitrogen (NPC:N) ratio observed with higher protein intake [33]. Although there is no established NPC:N ratio for bariatric patients, intakes of ≥ 60 g/day are associated with a significantly lower NPC:N ratio at 6 months post-surgery, which may support better FFM retention during catabolic states [33, 53].

Research on protein intake and FFM preservation after BS shows mixed results. One study [12] found no significant association between protein intake and FFM loss 2–4 years post-sleeve gastrectomy, but instead identified higher fat intake as a predictor of increased FFM loss. This may be explained by fat intake's negative impact on insulin sensitivity, as insulin resistance during weight loss can lead to greater lean mass loss [54]. Formula diets-high in protein, low in carbohydrates and fat, with adequate micronutrients-have demonstrated effectiveness for both weight loss [55] and skeletal muscle preservation [56]. Preoperative formula diets may help prevent protein deficiency and muscle loss after laparoscopic sleeve gastrectomy [29]. The inconsistent findings across studies likely stem from methodological variations, including small sample sizes, different follow-up periods, diverse assessment techniques, and varying statistical approaches. Importantly, many studies failed to account for physical activity levels, a potentially significant confounding factor in body composition outcomes.

To date, vitamin D is the only micronutrient that has been specifically studied in relation to body composition after BS. One study found that only BMI, and not other body composition measures, was independently associated with the RYGB group [32]. Evidence shows that vitamin D deficiency is common before BS and often persists long after surgery [57]. This deficiency has been linked to factors such as excess adiposity and the type of bariatric procedure performed [32].

A study showed that people who underwent BS and followed a low-calorie DASH diet had greater reductions in weight, BMI, and fat mass than those on a standard low-calorie diet, regardless of hypertension status [25]. Similar benefits of the DASH diet on weight and waist circumference have been observed in non-bariatric populations, as highlighted by a meta-analysis of 54 trials [58]. These effects are largely attributed to the DASH diet’s high fiber content from plant-based foods, which supports weight loss by delaying gastric emptying, reducing macronutrient absorption, and enhancing satiety [25, 58]. Fiber also promotes cholesterol excretion and lowers blood lipid levels [59]. The DASH diet’s focus on portion control and reducing high-calorie, processed foods further aids weight management [60]. Few studies have examined the DASH diet’s impact on FFM, LBM, or muscle mass. However, available evidence suggests that the DASH diet, especially when combined with calorie restriction and exercise, can lead to reductions in weight, fat mass, and FFM [25, 61, 62]. Adherence to the DASH diet may also influence insulin sensitivity, which is important for muscle maintenance, as greater LBM is associated with lower insulin resistance. Insulin plays a key anabolic role in muscle protein synthesis and maintenance, and insulin resistance can accelerate muscle loss, particularly in older adults with diabetes [63].

A study by Clements et al. [40] found significant reductions in weight, BMI, and LBM in all participants following LGB, with no differences observed between those receiving beta-hydroxy-beta-methylbutyrate, glutamine, and arginine (HMB/Glu/Arg) supplementation and controls. While this oral supplement has proven effective in restoring muscle mass in patients with cachexia related to cancer, rheumatoid arthritis, AIDS, and critical trauma [64–68], the lack of effect in patients following BS may be due to different physiological contexts. Unlike cancer or trauma patients who typically either recover normal function or succumb to their conditions, these patients experience prolonged weight loss for 12–18 months after LGB, suggesting they may require longer follow-up periods to observe potential benefits from HMB/Glu/Arg supplementation [64–68].

While nutritional intake is a major factor in post-surgical changes in LBM and FFM, it is not the only influence. Individual characteristics such as age, sex, preoperative BMI, comorbidities, and psychological status also play significant roles in the variability of FFM and LBM loss after BS. For example, older adults may have different metabolic responses, and psychological factors can affect dietary adherence and lifestyle changes, impacting body composition outcomes. This variability highlights the importance of personalized nutrition plans that consider these diverse factors. Future research should focus on how nutritional strategies interact with these individual variables to develop comprehensive, patient-centered guidelines for optimizing outcomes and preserving FFM after BS.

This systematic review is the first to comprehensively assess the impact of nutritional factors on body composition changes following BS, providing novel insights into how dietary interventions affect LBM and fat mass across diverse populations and study designs. Its inclusion of various nutritional strategies and methodologies enhances the breadth of evidence. However, limitations such as heterogeneity in study designs, sample sizes, and assessment methods may affect the generalizability of findings. Combining clinical trials with observational studies introduces variability in bias and methodology, complicating direct comparisons. Additionally, inconsistent reporting of protein intake and lack of detailed physical activity data highlight the need for more standardized, long-term research to establish clear, actionable nutritional guidelines for post-BS care.

Strength of the evidence

This systematic review found moderate-strength evidence linking higher protein intake (≥ 60 g/day or ≥ 1.2 g/kg) to reduced FFM loss after bariatric surgery, particularly within the first year, as consistently demonstrated across observational and interventional studies, though conflicting results reflect methodological variability in assessment timing and dietary recording. While observational studies showed moderate risk of bias (per Newcastle–Ottawa Scale) due to inadequate confounder adjustment, randomized trials scored higher on the Jadad-Oxford Scale but were limited by small samples and short follow-ups. Critical gaps remain, including limited evidence on micronutrients, macronutrient distribution, long-term (> 2 years) outcomes, and personalized nutrition approaches, underscoring the need for standardized, high-quality research to refine clinical guidelines.

Conclusion

This review underscores the importance of protein intake in minimizing FFM loss and maximizing fat mass reduction after BS. Personalized nutrition plans, considering individual factors, and early, consistent dietary counseling are key to preventing deficiencies and reducing complications. While high-protein diets and balanced macronutrient intake show promise, further well-designed, long-term research is needed to establish optimal nutritional strategies. Future guidelines should incorporate comprehensive macro- and micronutrient recommendations to enhance overall health and quality of life for people who have undergone BS.

Author contributions

B.A. and M.V. conceived and designed the study. B.A. and Z.S. conducted the systematic search, screened articles, and read the full texts for eligibility. B.A., S.A, and Z.S extracted data from the original studies and evaluated the studies for risk of bias. B.A. and K.M. wrote the first draft of the manuscript. B.A., M.V. critically revised the manuscript. All authors have read and approved the final manuscript.

Data availability

No datasets were generated or analyzed during the current study.

Declarations

Competing interests

The authors declare no competing interests.