The present study evaluated the potential beneficial effect of kefir (KF) against fatigue. Furthermore, the composition of the gut microbiota is related to health benefits in the host; therefore, the study also investigated the effect of KF on the gut microbiota composition. Male ICR mice from four groups (n = 8 per group) were orally administered KF once daily for four weeks at 0, 2.15, 4.31, and 10.76 g/kg/day and were designated as the vehicle, KF-1X, KF-2X, and KF-5X groups, respectively. The gut microbiota was analyzed using 16S rRNA gene sequencing. The results showed a significant clustering of cecum after treatment in the vehicle, KF-1X, KF-2X, and KF-5X groups. The KF-2X and KF-5X groups showed a decreased Firmicutes/Bacteroidetes ratio compared with the vehicle group. In addition, anti-fatigue activity and exercise performance were evaluated on the basis of exhaustive swimming time, forelimb grip strength, and levels of serum lactate, ammonia, glucose, blood urea nitrogen (BUN), and creatine kinase (CK) after a swimming exercise. The exhaustive swimming time for the KF-1X, KF-2X, and KF-5X groups was significantly longer than that for the vehicle group, and the forelimb grip strength of the KF-1X, KF-2X, and KF-5X groups was also significantly higher than that of the vehicle group. KF supplementation also decreased serum lactate, ammonia, BUN, and CK levels after the swimming test. However, tissue glycogen content, an important energy source for exercise, increased significantly with KF supplementation. Thus, KF supplementation can alter the gut microbiota composition, improve performance, and combat physical fatigue.

Microbiota is currently an important issue in disease and health and many studies have revealed it to play an important role in physiological homeostasis and health promotion. Lactobacillus plantarum (L. plantarum), isolated from Taiwan pickled vegetables, is a well-known probiotic microorganism. In a recent animal study, it was shown that supplementation of mice with L. plantarum TWK10 (TWK10) could increase muscle mass, improve exercise performance and exert anti-fatigue effects. In order to examine the ergogenic effect of TWK10 supplementation on endurance performance in humans, we conducted a human double-blind placebo-controlled clinical study. A total of sixteen adult subjects over 20 years of age were recruited and randomly allocated to the placebo or TWK10 group (n = 8 each). The TWK10 group received 6 weeks of supplementation. Physiological assessments were conducted by exhaustive treadmill exercise measurements and related biochemical indexes. After 6 weeks of supplementation, levels of lactic acid, blood ammonia, blood glucose, free fatty acid (FFA) and creatine kinase (CK) were evaluated during exhaustive exercise. We were able to show that the TWK10 group had significantly higher endurance performance and glucose content in a maximal treadmill running test compared to the placebo group (P < 0.05), suggesting that TWK10 supplementation may be beneficial to energy harvest. Taken together, our results suggest that TWK10 has the potential to be an aerobic exercise supplement for physiological adaptation or an ergogenic supplement with health benefits for amateur runners.

Kefir is an acidic, carbonated, and fermented dairy product produced by fermenting milk with kefir grains. The Lactobacillus species constitutes an important part of kefir grains. In a previous animal study, kefir effectively improved exercise performance and had anti-fatigue effects. The purpose of this research was to explore the benefits of applying kefir to improve exercise performance, reduce fatigue, and improve physiological adaptability in humans. The test used a double-blind crossover design and supplementation for 28 days. Sixteen 20–30 year-old subjects were divided into two groups in a balanced order according to each individual’s initial maximal oxygen uptake and were assigned to receive a placebo (equal flavor, equal calories, 20 g/day) or SYNKEFIRTM (20 g/day) every morning. After the intervention, there were 28 days of wash-out, during which time the subjects did not receive further interventions. After supplementation with SYNKEFIRTM, the exercise time to exhaustion was significantly greater than that before ingestion (p = 0.0001) and higher than that in the Placebo group by 1.29-fold (p = 0.0004). In addition, compared with the Placebo group, the SYNKEFIRTM administration group had significantly lower lactate levels in the exercise and recovery (p < 0.05). However, no significant difference was observed in the changes in the gut microbiota. Although no significant changes in body composition were found, SYNKEFIRTM did not cause adverse reactions or harm to the participants’ bodies. In summary, 28 days of supplementation with SYNKEFIRTM significantly improved exercise performance, reduced the production of lactic acid after exercise, and accelerated recovery while also not causing any adverse reactions.

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