Background Nicotinamide riboside (NR) is an NAD+ precursor that boosts cellular NAD+ concentrations. Preclinical studies have shown profound metabolic health effects after NR supplementation. Objectives We aimed to investigate the effects of 6 wk NR supplementation on insulin sensitivity, mitochondrial function, and other metabolic health parameters in overweight and obese volunteers. Methods A randomized, double-blinded, placebo-controlled, crossover intervention study was conducted in 13 healthy overweight or obese men and women. Participants received 6 wk NR (1000 mg/d) and placebo supplementation, followed by broad metabolic phenotyping, including hyperinsulinemic-euglycemic clamps, magnetic resonance spectroscopy, muscle biopsies, and assessment of ex vivo mitochondrial function and in vivo energy metabolism. Results Markers of increased NAD+ synthesis—nicotinic acid adenine dinucleotide and methyl nicotinamide—were elevated in skeletal muscle after NR compared with placebo. NR increased body fat-free mass (62.65% ± 2.49% compared with 61.32% ± 2.58% in NR and placebo, respectively; change: 1.34% ± 0.50%, P = 0.02) and increased sleeping metabolic rate. Interestingly, acetylcarnitine concentrations in skeletal muscle were increased upon NR (4558 ± 749 compared with 3025 ± 316 pmol/mg dry weight in NR and placebo, respectively; change: 1533 ± 683 pmol/mg dry weight, P = 0.04) and the capacity to form acetylcarnitine upon exercise was higher in NR than in placebo (2.99 ± 0.30 compared with 2.40 ± 0.33 mmol/kg wet weight; change: 0.53 ± 0.21 mmol/kg wet weight, P = 0.01). However, no effects of NR were found on insulin sensitivity, mitochondrial function, hepatic and intramyocellular lipid accumulation, cardiac energy status, cardiac ejection fraction, ambulatory blood pressure, plasma markers of inflammation, or energy metabolism. Conclusions NR supplementation of 1000 mg/d for 6 wk in healthy overweight or obese men and women increased skeletal muscle NAD+ metabolites, affected skeletal muscle acetylcarnitine metabolism, and induced minor changes in body composition and sleeping metabolic rate. However, no other metabolic health effects were observed. This trial was registered at clinicaltrials.gov as NCT02835664

Slowing down mRNA translation in either the cytoplasm or the mitochondria are conserved longevity mechanisms. Here, we found a non-interventional natural correlation of mitochondrial and cytoplasmic ribosomal proteins (RPs) in mouse population genetics, suggesting a translational balance between these two compartments. Inhibiting mitochondrial translation in C. elegans through mrps-5 RNAi repressed overall cytoplasmic translation. Transcriptomics integrated with proteomics revealed that this inhibition specifically reduced the translational efficiency (TE) of mRNAs required in growth pathways while increasing the TE of stress response mRNAs. The coordinated repression of cytoplasmic translation is dependent on atf-5/Atf4 and is conserved in mammalian cells upon inhibiting mitochondrial translation pharmacologically with the antibiotic doxycycline. Lastly, extending this in vivo, doxycycline repressed cytoplasmic translation and RP expression in the livers of germ-free mice. These data demonstrate that inhibiting mitochondrial translation initiates an atf-5/Atf4-dependent cascade leading to coordinated repression of cytoplasmic translation, which could be targeted to promote longevity. Keywords longevity / ribosomes / mitochondrial translation / cytoplasmic translation / translational balance Highlights • Mitochondrial and cytoplasmic RP levels balance in a natural stoichiometric ratio • Blocking mitochondrial ribosomes in worms and mice reduces cytoplasmic translation • This translational balance is ATF4/atf-5 dependent and conserved in human cells • Translational efficiency of RP transcripts changes in response to ratio requirement

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