Effect of yogurt and pH equivalent lemon juice on salivary flow rate in healthy volunteers – An experimental crossover study

Abstract

Background

Xerostomia is a common clinical problem, and different medications have been tried in its management. In the present study, routine dietary products are used to assess their effect on salivary flow.

Aim

To assess the efficacy of yogurt and lemon juice on increase in salivation and its comparison with that of unstimulated saliva.

Materials and Methods

A total of 40 volunteers (aged 19–48) were selected. The pH of yogurt was calculated, and equivalent pH lemon juice was prepared. First, normal resting saliva was collected as baseline followed by every 1 min for 5 min. Patients were given lemon juice or yogurt and then crossed over to the other group to assess the impact of the stimulants on salivary flow from 1 to 5 min.

Results

The results were analyzed statistically. Comparisons between baseline saliva secretion and that by yogurt and lemon juice (using the ANOVA test) showed that there was a significant increase after treatment at the end of the experiment for both yogurt and lemon juice. However, yogurt showed a significant increase in saliva secretion compared to baseline than lemon juice.

Conclusions

Our findings suggest that yogurt is a potential candidate for the treatment of dry mouth.

Introduction

Saliva plays a critical role in the maintenance of oropharyngeal health [1]. Dry mouth must not be considered a minor problem in the population, since it constitutes a phenomenon with many aspects relative to oral function as well as quality of life [2]. The salivary fluid is crucial for defense against viral, bacterial, and fungal infections, for remineralization of dental enamel and dentine, and for taste sensation [1–4]. Complaints of a dry mouth and diminished salivary output are common in older populations [1, 5–7], and clinical studies have shown that 25% of geriatric individuals in the general population suffer from xerostomia and related problems in the absence of any disease. Nevertheless, an increase of cases of xerostomia has been recently reported among young adults [8, 9].

Upon meeting dry mouth patients, dentists face a challenging problem difficult to treat. Treatment with lubricants or salivary substitutes and salivary stimulation by gustatory or masticatory methods may produce an improvement, but xerostomia recurs once the active treatment is interrupted [10]. Secretagogues like pilocarpine [11, 12], and cevimeline [13, 14] have been studied extensively as pharmacological agents to counter hyposalivation; nevertheless, more than one third of patients show adverse effects analogous to those produced by other cholinergic drugs, including gastric upset; perspiration; tachycardia; bradycardia; arrhythmia; increases of pulmonary secretions, muscular tone, and urinary frequency; and blurred vision [15, 16]. In a recent study, individuals with xerostomia expressed their desire of a nonpharmacological method for their treatment; however, none of the presently available standard treatments fulfill these expectations [17].

Ancient therapies have advocated various dietary products like citrus fruits and herbs in the treatment of dry mouth; these are often stimulators of appetite and, in turn, of saliva secretion [18]. Yogurt is one of the most widely distributed dairy products. Yogurt in different forms with diverse local names is made throughout the world [19]. The use of yogurt dates back many centuries, although there is no accurate record of the date when it was first made. According to legend, yogurt was first made by the ancient Turkish people in Asia [20]. Yogurt is derived from the Turkish word jugurt, describing any fermented food with acidic taste. Its manufacture involves the use of specific symbiotic/mixed culture of Lactobacillus bulgaricus and Streptococcus thermophiles. The uniqueness of yogurt is attributable to the symbiotic fermentation involved in its manufacturing [21].

Lemon, known for its citric acid content and sour taste, is a renowned home remedy for dry mouth. Studies have shown that the effect of pilocarpine on physiologic saliva secretion is not as strong as the effect of lemon juice [22] and yogurt [23]. Various studies have speculated the use of natural yogurt and probiotic yogurt against S. mutans in the prevention of dental caries [24–26] and in hyposalivation [23]. In this study, the efficacy of natural yogurt on saliva secretion was investigated and compared with the effect of lemon juice.

Materials and Methods

This clinical crossover study was conducted at College of Dental Sciences, Davangere. A total of 40 healthy volunteers comprising of 13 males and 27 females aged 20–49 years were selected. Exclusion criteria were as follows: specific systemic diseases; smokers, history of drug administration that included anticholinergics, tricyclic antidepressants (TCAs), or anticancer drugs; and a history of radiotherapy of the head and neck and lactose intolerance. The study was approved by local research Ethics Committee of College of Dental Sciences, Davangere.

Preparation

Since some additives are added to industrial yogurt, we used homemade natural yogurt. The samples were analyzed for their pH using a pH meter, and equivalent pH lemon juice was prepared by dissolving water to natural lemon concentrate. They were kept in a cool place (4–8 °C in a refrigerator) until the test day.

Procedure

In order to collect saliva samples from each subject, 18 bottles were needed. Sampling of each volunteer was performed with six measurements including salivation without stimulation and salivation after being given yogurt and lemon juice. All samplings were done over a time period of 5 min with 1 min intervals. Saliva samples were collected in well-sealed, capped containers and were labeled. In order to prevent the evaporation of saliva, the containers’ caps were screwed tightly. The exact weight of each container was measured using a sensitive digital scale (Sartorius CP124S, Germany) before and after saliva collection. Differences between the weights before and after saliva collection were recorded for further analyses. Volunteers were asked to refrain from eating and drinking for one and half hour before starting the saliva collection. For the unstimulated saliva secretion, subjects were given a brief description of the study; they were asked to rinse their mouth with tap water (placebo) and to rest for 1 min. After swallowing their saliva, they were asked to wait 40 s without swallowing and then spit out into preweighed containers. This measure was repeated five times per subject. After 1 min, they were asked to collect the contents of their mouths in each minute for 5 min, without swallowing the saliva in between the measurements. After the collection of unstimulated saliva, the volunteers were given a 15-min break.

Stimulated saliva secretion was assessed by collecting saliva within 15 min using the protocol of Dawes [18]. A half-hour wash-out period was implemented between each course of treatment. Twenty patients were given yogurt, and the other 20 patients were given lemon juice, after which they were crossed over. To detect the impact of pH on saliva secretion, yogurt and lemon juice were tested immediately after preparation. For the stimulated saliva flow rate evaluation, subjects were asked to rinse their mouth with tap water and then to rest for 1 min. After swallowing their saliva, they put 5 mL of yogurt or lemon juice in their mouth. After 40 s, they spat out the entire contents of the mouth (5 mL stimulant + stimulated saliva) into a preweighed cup. The test was repeated five times after a rinse and a 1-min rest period. In both cases, cups were immediately weighed to prevent evaporation. After three sessions, 15 measurements for each of the unstimulated and stimulated whole saliva flow rates were averaged for each subject.

Statistical analysis

All data in the table are shown as means ± SD. A one-way ANOVA SPSS software was used for intergroup comparison and post-hoc Tukey for pairwise comparison. A repeated measurement test was applied to analyze the trend for the changes in saliva secretion within each group and also between different groups. The differences were highly significant (p < 0.001).

Results

In this study, 40 healthy volunteers (aged between 20 and 49 years) were enrolled. First, each participant collected his/her unstimulated saliva as a baseline after which the subjects were given yogurt and lemon juice. Comparisons between baseline saliva secretion and the saliva secretions induced by yogurt and lemon juice (using the ANOVA test) showed that there was a significant increase after treatment at the end of the experiment for both yogurt and lemon juice (Fig. 1). However, yogurt showed a significant increase in saliva secretion compared to baseline than lemon juice. The amount of saliva secreted peaked at the first minute and gradually decreased. Though the quantity of saliva secreted decreased at the end of 5th minute, it was still significantly higher compared to baseline.

Fig. 1.

Graphical representation of comparison of weight differences (g) over time between the three groups

Unstimulated saliva secretion (Table I)

Table I.

Mean saliva weight (g) of unstimulated and stimulated (lemon juice and milk curd) groups at the end of the each minute

Time (min)	1	2	3	4	5
Unstimulated	0.3267	0.321	0.3	0.2979	0.2801
Lemon juice	1.6798	1.1881	0.9833	0.7455	0.6115
Yogurt	1.7743	0.8899	0.6641	0.5307	0.4057

The average unstimulated salivary flow of 0.3008 mL/min was obtained in 5 min duration, and it ranged between 0.28 and 0.33 mL/min. Unstimulated saliva was highest at the end of second minute and gradually decreased in quantity at the end of 5 min. This was considered as a baseline to which the effects of lemon juice and yogurt were compared.

Effect of lemon juice on saliva secretion (Table I)

The average salivary flow after lemon juice intake was 0.8529 mL/min over a period of 5 min, and it ranged between 0.41 and 1.78 mL/min. It was highest at the first minute and gradually decreased in quantity at the end of 5 min. However, the salivary flow after intake of lemon juice was higher than baseline even at the end of 5 min.

Effect of yogurt on saliva secretion (Table I)

The average salivary flow after yogurt intake was 1.0147 mL/min over a period of 5 min, and it ranged between 0.61 and 1.68 mL/min. It was highest at the first minute and gradually decreased in quantity at the end of 5 min. However, the salivary flow after intake of yogurt was higher than baseline even at the end of 5 min.

Discussion

The present study shows that yogurt can significantly increase saliva secretion compared to the baseline secretion levels. Yogurt is a coagulated milk product that results from the fermentation of lactic acid in milk by Lactobacillus bulgaricus and Streptococcus thermophiles [28, 29]. It has a smooth texture with a mildly sour and pleasant avor. Studies have shown that a sour taste in combination with an organic stimulus has a strong role in the secretion of saliva. It is obvious that organic acids, including acetic acid, lactic acid, citric acid, and malic acid, at equal pHs and equal concentrations, cause greater effects in the perception of sour taste [27]. This study was designed to examine and compare the effects of yogurt (lactic acid) and lemon juice (citric acid) on salivation.

In one study using 1% citric acid, it was determined that the highest flow of saliva was reached at the end of the first minute and that, after 6 min, saliva secretion returned to normal [30]. Our study was limited to a 5-min duration, and significant differences were seen in saliva secretion compared to baseline at the end of each minute. The saliva flow was higher than baseline values even at the end of 5 min with both yogurt and lemon juice. Lemon juice showed higher salivary flow during the first minute in comparison with yogurt. However, yogurt showed better values from 2 to 5 min. This shows that yogurt shows sustained release of saliva for a longer duration than lemon juice. This may be attributed to the fact that yogurt forms a thin layer on the mucosa helping in sustained salivary flow over longer duration. However, no scientific evidence exists.

Another study has shown that milk curd has significant effect on salivary flow at different concentrations. The study also showed that the salivary secretion rates with milk curd were higher than that obtained with topical pilocarpine sans the side effects of pilocarpine administration [23].

In the present study, yogurt was used in natural form. Since the pH of the yogurt solutions applied in this study ranged from 4.25 to 4.32, similar concentrations of lemon juice with equal pH values were prepared. This part of the study was done to test whether or not the acidic properties of yogurt are responsible for the salivary secretion. Based on the findings, there was a significant increase in the salivary secretion rate using yogurt compared to lemon juice with an equal pH from the second minute. The results show that the salivary secretion enhancement caused by yogurt is not just a result of its acidic nature, and other possible mechanisms may contribute to the salivary stimulation. This finding matches with the study by Babaee et al. [23].

The results of this study emphasize the positive role of yogurt on saliva secretion, independent of its acidic taste or pH. The pH is not the only factor that is responsible for the increase in saliva secretion. Yogurt contains ions including sodium, potassium, calcium, and phosphorus. The increase in saliva production effect may be related to the sodium and calcium ions in the yogurt, or perhaps the yogurt increased the secretion of saliva via pharmacological mechanisms [23]. Lemon juice, though a potential salivary stimulant, is known to cause erosion of teeth leading to hypersensitivity. Yogurt consumption is not documented with any menacing side effects.

Conclusions

Through this study, it can be concluded that yogurt is a potential candidate in the treatment of dry mouth. Many people suffer from dry mouth or xerostomia due to diseases such as Sjögren’s syndrome, radiation therapy for head and neck tumors, or old age. Dry mouth is a common clinical problem, and different products have been proposed to improve it. Making products such as mouthwash or lozenges using yogurt can help to reduce dry mouth.

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Authors’ contribution

JM performed the data collection and study design; RA, data collection and study design; SAR, article’s drafting; SA, article’s critical editing; MA, statistical analysis and article’s critical editing; and OK, study design and article’s editing.

Conflict of interest

Authors declare no conflict of interest.

Funding Statement

None.