NAD+ plays a pivotal role in maintaining cellular homeostasis, a function that declines with age. There is evidence that decreased NAD+ levels are associated with age-related metabolic disorders, including obesity and diabetes. The precursor of NAD+, nicotinamide mononucleotide (NMN), is key in the process of mammalian NAD+ biosynthesis. In model organisms, NMN supplementation could increase NAD+ levels in multiple tissues and reverse age-related abnormalities (e.g., diabetes mellitus, insulin resistance, and aging). Human trials have not been conducted in this area until recently.
Human clinical trials of NMN
Dr. Samuel Klein’s group at Washington University published the first NMN trial in humans in Science on April 22, 2021. 25 overweight or obese postmenopausal women with prediabetes were randomized to receive NMN supplementation for 10 weeks. The study is a randomized, double-blind, placebo-controlled study – the most rigorously controlled type of study possible to unravel the causal connection between NMN supplementation and physiological function. As it turns out, NMN enhances muscle insulin sensitivity, insulin signaling, and remodeling in humans. For the first time, this study provides evidence that MNM benefits human metabolism.
A study of 25 postmenopausal women with prediabetes and overweight or obesity was conducted. In this study, 12 were assigned to placebo and 13 to NMN (250 mg/day). A number of tests were performed to monitor the outcome before and after treatment, including body composition measurements, insulin sensitivity tests, and cellular effects analysis of muscle biopsies. Due to the small size of the study and the relatively low doses, the short trial period, as well as the small number of participants, the statistical power is quite low. As a result, only significant effects of NMN supplementation can be detected statistically based on this study design. Future studies are needed to gain a complete picture of the effects of NMN.
NAD+ levels in the blood increased following NMN treatment
NMN-produced metabolites, N-methyl-2-pyridone-5-carboxamide (2PY) and N-methyl-4-pyridone-5-carboxamide (4PY), significantly increased in blood plasma ten weeks after treatment.
Furthermore, the levels of NAD+ inside blood cells (PBMC) were significantly elevated after NMN treatment. These results supported the notion that oral NMN supplementation could indeed elevate NAD+ levels in the blood.
Muscle NAD+ turnover was increased with NMN treatment
NMN treatment did not increase the NAD+ level in muscle biopsies. However, NAD+ turnover in muscle was significantly increased by NMN supplementation, as suggested by increases in 2PY and 4PY metabolites. Thus, NMN may be able to promote both NAD+ synthesis and utilization in muscle. Although it may seem as if NAD+ levels were unchanged, there was likely more NAD+ produced and consumed compared to the placebo group.
NMN increases muscle insulin sensitivity
Metabolic disorders associated with aging are characterized by insulin resistance. A person’s pancreatic islets B cells release insulin when their blood glucose levels rise, promoting glucose absorption in major metabolic tissues, including skeleton muscle. Consequently, the tissues will absorb glucose and the blood glucose level will drop. People with metabolic disorders such as obesity and type II diabetes, however, do not respond well to insulin, and their blood glucose level remains high even with high blood insulin levels. Known as insulin resistance, this condition will exacerbate metabolic disorders.
Based on this study, muscle insulin sensitivity was assessed by calculating the rate at which glucose is released from the muscle after being stimulated by insulin. The researchers measured how quickly and efficiently glucose was taken up from blood in response to insulin. Metabolically, people with higher insulin sensitivity have higher insulin-stimulated glucose disposal rates.
As a result of 10 weeks of NMN supplementation, compared with placebo, insulin-induced glucose disposition rate increased by 25 * 7% subjects. Improvements in insulin sensitivity in the muscle are clinically relevant and comparable with those observed with *10% weight loss (equivalent to roughly 20 pounds weight loss). Based on this data, NMN administration improved insulin sensitivity and possibly metabolic health by increasing NAD+ turnover rather than affecting muscle NAD+ concentration.
An interesting finding was that NMN only affected insulin sensitivity in muscle. NMN didn’t affect other important variables associated with insulin resistance, including indices of liver and adipose tissue insulin sensitivity, intra-abdominal adipose tissue volume, intrahepatic triglyceride content, and fasting plasma glucose, insulin, and adiponectin concentrations. Results indicate that NMN has selective beneficial effects on insulin-mediated glucose metabolism in skeletal muscle.
NMN treatment upregulated genes required for muscle remodeling/regeneration
In addition, they used RNA-sequencing to examine gene expression in muscle samples following NMN treatment. The number of genes differentially expressed during insulin infusion was 60 times greater than with placebo. As a result of NMN treatment, the PDGF binding pathway was enriched the most, and muscle PDGFR* as well as downstream targets of PDGF signaling, were significantly induced.
In addition to managing cell growth and division, the PDGF also plays a key role in muscle growth and regeneration. Additionally, NMN treatment increased insulin-stimulated mTOR phosphorylation (another master regulator of cell growth) and gene expression of myogenic markers, which indicate increased skeletal muscle remodeling and regeneration.
Trials of NR failed to improve insulin sensitivity
Moreover, contrary to our findings, randomized controlled trials conducted in middle-aged and older-adult men found that treatment with nicotinamide riboside (NR) did not affect whole-body or muscle insulin sensitivity. In comparison with the administration of 250 mg NMN in this trial, these NR trials used higher doses of NR for a similar length of time: 2000 mg for 12 weeks in Dollerup et al. (2018); 1000 mg for 3 and 6 weeks in Elhassan et al. (2019) and Remie et al. (2020).
As the rate-limiting factor in NAD+ synthesis, NMN may offer more metabolic benefits than NR. Unfortunately, further study is required due to the heterogeneity of the population and the limited sample size.