Nicotinamide riboside does not alter mitochondrial respiration, content or morphology in skeletal muscle from obese and insulin resistant men

KEY POINTS: This is the first long-term human clinical trial to report on effects of NR on skeletal muscle mitochondrial function, content and morphology •NR supplementation decreases NAMPT protein abundance in skeletal muscle •NR supplementation do not affect NAD metabolite concentrations in skeletal muscle •Respiration, distribution, and quantity of muscle mitochondria are unaffected by NR •NAMPT in skeletal muscle correlates positively with OXPHOS Complex I, SIRT3, and SDH ABSTRACT: Preclinical evidence suggest that the nicotinamide adenine dinucleotide (NAD+ ) precursor nicotinamide riboside (NR) boosts NAD+ levels and improves diseases associated with mitochondrial dysfunction. We aimed to determine if dietary NR supplementation in middle-aged, obese, insulin-resistant men affects mitochondrial respiration, content, and morphology in skeletal muscle. In a randomized, placebo-controlled clinical trial, 40 participants received 1,000 mg NR or placebo twice daily for 12 weeks. Skeletal muscle biopsies were collected before and after the intervention. Mitochondrial respiratory capacity was determined by high-resolution respirometry on single muscle fibres. Protein abundance and mRNA expression were measured by Western blot and qPCR analyses, respectively, and in a subset of the participants (placebo n = 8; NR n = 8) we quantified mitochondrial fractional area and mitochondrial morphology by laser scanning confocal microscopy. Protein levels of nicotinamide phosphoribosyltransferase (NAMPT), an essential NAD+ biosynthetic enzyme in skeletal muscle, decreased 14% with NR. However, steady-state NAD+ levels as well as gene expression and protein abundance of other NAD+ biosynthetic enzymes remained unchanged. Neither respiratory capacity of skeletal muscle mitochondria nor abundance of mitochondrial associated proteins were affected by NR. Moreover, no changes in mitochondrial fractional area or network morphology were observed. 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 genders and potentially provide comparisons between young and older people. This article is protected by copyright. All rights reserved.