NAD+ metabolism and cardiometabolic health: the human evidence

Commentary on ‘Nicotinamide mononucleotide increases muscle insulin sensitivity in prediabetic women’ by M. Yoshino et al., Science, 2021. The metabolic cofactor nicotinamide adenine dinucleotide (NAD+) has long been recognized as an essential molecule for cellular energy production and homeostasis due to its key role in cellular reduction–oxidation reactions. More than a century after this activity was first described, our understanding of NAD+ has evolved well beyond redox and energy metabolism. In fact, NAD+ serves as a co-substrate for enzymes contributing to vital cellular functions, including epigenetic and transcriptional regulation, post-translational protein modifications and cell signalling.1 Accordingly, recent years have witnessed soaring interest in different aspects of NAD+ metabolism, with the ultimate goal of harnessing its potential to treat chronic age-related diseases, which are typically multifactorial and lack efficient pharmacotherapies. Amongst these, cardiometabolic disorders represent an urgent and unmet medical need with an ever-growing prevalence due to the current demographic shift towards older populations, modern-day sedentary lifestyles and hypercaloric diets. Interestingly, intracellular NAD+ levels progressively decline with ageing and obesity, while the salutary effects of exercise and caloric restriction(definition) coincide with increases in NAD+.1 Accordingly, NAD+ has been posited as a potential actionable target to recapitulate the benefits of these healthy lifestyle modifications.2 In fact, a growing body of preclinical evidence suggests that oral supplementation of NAD+ precursors—otherwise known as vitamin B3 derivatives—improves metabolic health in rodent models of ageing, obesity, and diabetes.1,2 However, the clinical evidence backing such health benefits in humans is still lagging behind. In this regard, Yoshino et al.3 recently reported in Science the results of a randomized, placebo-controlled, double-blind trial testing the short-term effects of the NAD+ precursor nicotinamide mononucleotide (NMN) on body composition and insulin sensitivity in prediabetic women. In this small trial of 25 obese or overweight participants, NMN (250 mg/kg) did not affect body weight or composition as compared to the placebo group, yet improved measures of insulin sensitivity. Neither placebo nor NMN changed body mass index, fat-to-lean mass ratio, intra-abdominal fat levels, or hepatic triglycerides following the 10-week supplementation period (with the caveat that the NMN group began with lower hepatic triglycerides). Similarly, NMN did not affect plasma insulin, glucose, haemoglobin A1c, or cholesterol levels. However, hyperinsulinaemic-euglycaemic clamp testing revealed an improvement in insulin sensitivity over the course of NMN treatment, as determined by the rate of insulin-induced glucose clearance (normalized to fat-free body mass). While the placebo group showed no detectable change in insulin sensitivity, NMN-treated patients exhibited an average 25% increase in glucose disposal from baseline. At the same time, skeletal muscle insulin signalling was improved after NMNs supplementation, as denoted by the higher expression and phosphorylation levels of AKT—a major downstream effector of insulin action—in skeletal muscle biopsies of NMN-treated patients after insulin infusion. It is important to note that the AKT immunoblot analysis focused on within-groups (before vs. after treatment) changes and so, in absence of a direct comparison between the two groups on the same gel, no conclusion can be made on between-groups (NMN vs. placebo) differences. Regardless, the positive NMN effect on insulin sensitivity appeared to be skeletal muscle-specific, as both hepatic and adipose tissue insulin sensitivity indices were almost identical before and after NMN administration. Intriguingly, NMN did not increase NAD+ in skeletal muscle, despite doing so in peripheral blood mononuclear cells. Instead, the skeletal muscle content of nicotinamide metabolites, such as N-methyl-nicotinamide, N-methyl-2-pyridone-5-carboxamide, and N-methyl-4-pyridone-5-carboxamide, showed a multi-fold increase, consistent with prior studies using the alternative precursor nicotinamide riboside.4,5 Finally, NMN did not enhance skeletal muscle physical capacity, as evaluated by handgrip strength, nor did it improve mitochondrial respiratory function, as evaluated by high-resolution respirometery of quadriceps muscle biopsies. Although larger trials are warranted, this pilot study represents a significant step towards human translation of the health-promoting effects of NAD+ precursors. Like any other important study, this work raises several research questions worthy of future investigation. For instance, the authors performed an unbiased RNA-sequencing analysis of muscle biopsies obtained before and after NMN treatment, which revealed transcriptional up-regulation of platelet-derived growth factor (PDGF) signalling as well as collagen and extracellular matrix processing, at least upon insulin administration. These findings might be of relevance for future therapeutic testing of NAD+ precursors particularly in cardiovascular medicine. For example, two recent studies have reported that NAD+-replacement therapy by nicotinamide or nicotinamide riboside improves hypertrophy and myocardial stiffness in animal models of ageing, obesity, and heart failure with preserved ejection fraction; however, it remains to be determined whether altered extracellular matrix or PDGF signalling contribute to these effects.6,7 Another important question is why the results of this study differ from prior work using higher doses of nicotinamide riboside, which was found to modestly increase fat-free mass with no effect on insulin sensitivity.5,8 Further studies will be required to distinguish whether this reflects differences in the precursor molecule selected, the dose, the study population, or other more subtle aspects of t