Increasing nicotinamide adenine dinucleotide (NAD+) levels with precursors like nicotinamide riboside (NR) has gained popularity as a strategy to combat against age-related diseases associated with dysfunction of the cell’s powerhouse, the mitochondria. Clinical trials evaluating the effects of supplementing with NR to boost NAD+ levels have sought to determine whether taking NR has these effects in humans.

Although previous studies in rodents have indicated that supplementation with NAD+ precursors like NR stimulates metabolic health by improving mitochondrial generation and turnover in skeletal muscle, the oral delivery of NR has only been shown to raise human NAD+ levels in blood. Scientists from Denmark published a study in the Journal of Physiology where they tested the effects of  long-term (12 week) NR supplementation on NAD+ content and mitochondrial function in human skeletal muscle. They found that NR supplementation does not increase the NAD+ content or mitochondrial function in the skeletal muscle of obese and insulin-resistant men, suggesting  that the effects of NR supplementation on skeletal muscle in rodents will not translate to humans.

Metabolic health depends on mitochondrial function, which progressively deteriorates as a result of decreased NAD+ levels and metabolism in aging mice and human tissue. Researchers have linked this decline in function with age-associated diseases like obesity, insulin resistance, and type 2 diabetes, which result in dysfunctional regulation of metabolism in skeletal muscle. Previous research suggested the naturally-occurring vitamin B3, NR, increases NAD+ levels in mice, which decline with age. The coenzyme NAD+ functions in cellular reactions that generate energy and also serves to stimulate the function of enzymes involved in maintenance of cellular health.

For these reasons, this team of scientists therefore looked at skeletal muscle in human tissue to find out whether NR supplementation increased NAD+ levels and improved mitochondrial function in humans. To do so, they performed a clinical trial with 40 participants that received 1000 mg NR or placebo twice daily for 12 weeks. This study did not provide evidence that NR improves NAD+ levels in the skeletal muscle of men with obesity and insulin resistance.

(Dollerup et al., 2020 | Journal of Physiology) NR supplementation does not increase NAD+ levels in human skeletal muscle. The figure indicates no differences in NAD+ levels before and after placebo and before and after 12 weeks of NR supplementation.

The investigators also found that NR supplementation did not affect the abundance of mitochondria in skeletal muscle. Research has demonstrated that NR treatment of rodents induces the cellular production of mitochondria in skeletal muscle thereby increasing the abundance of mitochondria. To determine whether NR supplementation had a similar effect in humans, the team of researchers measured the levels of proteins from mitochondria after 12 weeks of NR supplementation and found no noticeable differences. They also measured the levels of citrate synthase, an enzyme with cellular activity that correlates to mitochondrial abundance, and found no changes after NR supplementation.

(Dollerup et al., 2020 | Journal of Physiology) NR supplementation did not increase mitochondrial abundance in human skeletal muscle. Activity levels of citrate synthase, indicative of  mitochondrial abundance, did not increase in human skeletal muscle with NR supplementation.

The research team examined whether NR supplementation increased reactions that use oxygen to convert nutrients into cellular energy (i.e., respiration) and found that NR did not improve respiration in skeletal muscle. They found no differences when they used a respirometer, a device that measures the exchange of oxygen, to compare respiration before and after NR supplementation. “Following 12 weeks of supplementation, NR treatment did not improve the overall mitochondrial oxidative capacity in human skeletal muscle,” stated the scientists in their study.

“Our data do not support the hypothesis that dietary NR supplementation has significant impact on skeletal muscle mitochondria in obese and insulin-resistant men. Future studies on the effects of NR on human skeletal muscle may include both sexes and potentially provide comparisons between young and older people,” stated the team. Future studies could also examine the effects of NR supplementation on cellular NAD+ content and mitochondrial function in age-related conditions other than obesity and insulin resistance.