Clinical trials

A recognized driver for cardiovascular complications of type 2 diabetes mellitus (T2DM) is impaired plasma glucose homeostasis as consequence of skeletal muscle insulin resistance. Insulin-mediated plasma glucose disposal in skeletal muscle comprises oxidative glucose disposal (cellular glucose uptake for oxidation) and non-oxidative glucose disposal (NOGD; cellular glucose uptake for storage as glycogen), both processes being impaired in T2DM patients. Excessive intrahepatic fat accumulation (particularly monounsaturated (MUFA) and saturated (SFA)) is commonly observed in T2DM patients and tightly associates with plasma glucose dysregulation. It has been hypothesized that skeletal muscle insulin resistance redistributes circulating glucose away from muscle which together with hyperinsulinemia promotes intrahepatic lipid accretion via de novo lipogenesis (DNL). As saturated lipids is the final product of DNL, improving skeletal muscle insulin sensitivity, next to enhance plasma glucose homeostasis, might lower intrahepatic lipid content particularly intrahepatic saturated lipids. Regular exercise is a cornerstone in the treatment of T2DM and to improve skeletal muscle insulin sensitivity. Interestingly, a conventional exercise program (aerobic-type combined with strength-type exercise) restores insulin-stimulated oxidative glucose disposal in T2DM patients to levels observed in age-matched normoglycemic subjects. Non-oxidative glucose disposal (NOGD), however, does not improve upon such conventional exercise programs. In this regard, for full restoration of compromised glucose disposal, it is pivotal to come up with effective training methods to target NOGD. High intensity interval training (HIIT) has the potential to expands the glycogen synthesis capacity in athletes by repetitive cycles of glycogen depletion/repletion, hence holds promise to improve NOGD in T2DM patients. Of note, HIIT also lowers the intrahepatic fat content in pre-diabetes individuals. Nevertheless, whether HIIT reduces the intrahepatic fat content and modifies its composition in T2DM patients is unknown. In this regard, it is hypothesized that HIIT expands the NOGD capacity in skeletal muscle of overweight/obese type 2 diabetes patients. By doing so, it is postulated that HIIT improves skeletal muscle insulin sensitivity and therefore benefits the 24 hours glycaemic profile in T2DM patients. In line, it is hypothesized that the HIIT-mediated improvements on NOGD and skeletal muscle insulin sensitivity coexist with the reduction of intrahepatic lipid content -particularly reduced saturated lipids- via lowering DNL.

The prevalence of age-related chronic diseases (like obesity, type 2 diabetes and cardiovascular diseases) is mounting worldwide, reaching pandemic proportions. These age-related chronic diseases are associated with diminished skeletal muscle mitochondrial function in humans. Nicotinamide adenosine dinucleotide (NAD) is a coenzyme that regulates mitochondrial function, therefore, plays an important role in energy metabolism. Importantly, it has been shown that high cellular NAD+ levels as well as a high NAD+/NADH ratio promote metabolic and mitochondrial health. In contrast, NAD+ bioavailability declines upon aging in humans as well as in animal models of metabolic disorders and type 2 diabetes. These findings fuel the notion of boosting the NAD+ bioavailability in order to improve metabolic disturbances and mitochondrial dysfunction in aged individuals. Supplementation with nicotinamide riboside (NR), a naturally occurring form of vitamin B3, boosts cellular NAD+ levels. However, in contrast to animal studies, NR supplementation in humans has so far been unsuccessful in improving skeletal muscle mitochondrial function, exercise capacity or insulin sensitivity. Interestingly, Recently, it has been suggested that metabolic conditions where NAD+ levels become limited, is needed for NR supplementation to exert beneficial health effects. This metabolic condition could be achieved by exercise. However, studies combining NR and exercise are lacking, and that is why we will perform the present study.