The present study revealed the following main findings: 1) parameters of muscle damage were elevated on the day following acute resistance exercise; 2) the decrease of carbohydrate oxidation along with RQ was observed with exercise; and 3) the muscle soreness and metabolic changes were mitigated by the consumption of Lactobacillus helveticus–fermented milk in pre- and post-exercise. Previously, it had been unclear whether dietary intervention can improve metabolic impairment after muscle-damaging exercise. Our observations primarily demonstrate that dietary fermented milk improves the impairment of glucose metabolism associated with exercise-induced muscle damage in humans. Previously, consumption of milk (unfermented) partially attenuates the muscle damage ; therefore, the placebo trial, which used unfermented milk, may have also suppressed muscle damage to some extent. However, our results showed that fermented milk is more effective than milk.
Generally, it is well-known that a single bout of exercise elevates glucose uptake for a period of time post-exercise [25–27]. However, we have shown that insulin-mediated glucose uptake in muscle is decreased by muscle-damaging exercises, but not by non-muscle-damaging exercises . Therefore, the decreases of carbohydrate oxidation and respiratory quotient in the present study are suspected to be caused by insulin-dependent glucose uptake in damaged muscle. This decrease of glucose uptake is presumably due to a reduction of glucose transporter protein 4 (GLUT4) translocation via the insulin signaling pathway, which is the rate-limiting step in glucose metabolism. It has been reported that some inflammatory cytokines and chemokines, such as TNF-α and interleukin-1, attenuate the activity of insulin-mediated signaling in muscle cells [28, 29]. In addition, oxidative stress can also decrease glucose uptake by reducing the activity of insulin-mediated signaling [9, 30, 31]. In the damaged muscle on the day following the exercise, these cytokines and oxidative components are elevated [32–34], which could lead to the impairment of insulin-dependent glucose uptake. However, because we did not find any significant changes of inflammatory markers and oxidative products in serum and urine, the response is considered to be limited to muscle tissue, rather than the whole body, as suggested in a previous study .
Previously, we demonstrated that Lactobacillus helveticus–fermented milk attenuates delayed-onset muscle damage after acute exercise in rat . In this study, phagocyte infiltration and inflammatory cytokines expression, markers of inflammation in damaged muscle, were markedly reduced by the consumption of fermented milk. In addition, lipid peroxide levels were elevated after exercise, although the fermented milk uptake significantly reduced this elevation. Therefore, these observations suggest that fermented milk consumption mitigates inflammation and oxidative stress as well as muscle damage, which results in improvements in glucose metabolism by maintaining the insulin signaling pathway. In the present study, we found that ORAC, a marker of antioxidant capacity, was reduced in the placebo trial, but not in the fermented milk trial. Thus, the inhibitory effect of glucose metabolic impairment and muscle damage may be associated with elevated antioxidant levels caused by the consumption of fermented milk. Previously, we demonstrated that, in the skeletal muscle of rats, fermented milk upregulates expression of antioxidant enzymes, such as superoxide dismutase-2, catalase, and glutathione S-transferase α-1 . In addition, heat shock protein 70, a chaperone protein that can function as an antioxidant and an anti-inflammatory agent, was also elevated by the consumption of fermented milk . These observations suggest that fermented milk improves glucose metabolism and muscle damage at least, in part, by controlling endogenous antioxidant and anti-inflammation factors, a hypothesis that is supported by the ORAC results in the present study.
Although the detailed mechanisms of the effects of fermented milk on mitigating muscle damage remain unclear, small peptides present in fermented milk may be the causable agent, because fermented milk is more effective than unfermented milk. Fermented milk is manufactured by fermenting skim milk with a starter culture containing Lactobacillus helveticus. During this process, the proteins in skim milk are digested by Lactobacillus and converted into small peptides, which are more easily absorbed by the intestines compared to amino acids or large oligopeptides. Such peptides may also have additional physiological benefits aside from their use as a source of protein. Several studies have reported that peptides from fermented milk have various salutary effects, including an antihypertension effect, improvement of arterial stiffness, and immune regulation [12, 15, 35]. The present study suggests that small digested peptides in fermented milk may contribute to increasing the level of antioxidants in muscle. In future studies, we will attempt to detect the specific small peptides present after the consumption of fermented milk.