Human muscle biopsies show mitochondrial function declining with age, and longitudinal fitness data shows that decline is not a straight line, it speeds up. Exercise training has changed the trajectory in a controlled trial. Nicotinamide riboside has changed the chemistry of aged muscle without changing the bioenergetics that people actually care about.
Longevity
Published by Astra, which offers some of the treatments discussed. Educational, not medical advice.
Mitochondria are the compartments inside cells that convert nutrients and oxygen into ATP, the molecule cells spend to do work. When people say mitochondrial function declines with age, they are usually pointing at one of two kinds of measurement: how much ATP a given amount of muscle tissue can produce, and how efficiently the muscle uses oxygen to do it.
One of the earliest careful human measurements of the second kind used phosphorus magnetic resonance spectroscopy, a noninvasive imaging method, to measure oxidative capacity directly in the calf muscle of adults ranging from young to elderly. Oxidative capacity was lower in the older group, and the study was notable for showing this without a muscle biopsy, in living tissue, at rest and during exercise.1
A separate study measured the other kind directly. Researchers took muscle biopsies from a range of adults and measured mitochondrial ATP production rate along with mitochondrial DNA and RNA abundance. Older adults showed lower mitochondrial ATP production, and the reduction tracked with lower mitochondrial gene expression rather than only a raw drop in mitochondrial count.2
Put together, two different methods, one imaging-based and non-invasive, one biopsy-based and direct, point at the same conclusion: aging human muscle makes ATP less efficiently per unit of tissue. Neither study is small in the sense of being a single case, but neither is a population-wide survey either. They are careful physiology studies in defined groups of volunteers, and that is the right way to read them: as solid measurements of a real phenomenon, not as an estimate of exactly how much function any one person has lost.
A natural assumption is that mitochondrial and aerobic function fall at a steady rate from early adulthood onward, so that losing capacity at 40 predicts the same rate of loss at 70. Longitudinal data on aerobic capacity in healthy older adults does not support that picture.
In a study that followed community-dwelling older men and women over repeated visits, tracking peak oxygen uptake as a marker of aerobic and mitochondrial capacity, the rate of decline accelerated with advancing age rather than remaining constant. People in their later decades lost aerobic capacity faster per year than they had earlier in the same study.3
This matters for how the topic should be framed to someone in their 40s or 50s. The honest version is not 'mitochondrial function falls at a fixed rate you can extrapolate,' it is 'mitochondrial and aerobic function fall, and the fall tends to speed up later, which is a reason to intervene earlier rather than to wait until the acceleration is already underway.'
The most encouraging finding in this literature is also the least exotic. A controlled exercise training study enrolled both young and older adults and assigned them to different training modes, then measured mitochondrial respiration and the rate of new mitochondrial protein synthesis before and after training.4
Training improved mitochondrial respiratory capacity in both age groups, and it did so partly through enhanced protein translation, meaning the cells were building mitochondrial machinery faster in response to the training stimulus. Older adults responded to training. That is the specific finding worth naming: aged muscle was not incapable of the same kind of adaptation seen in younger muscle, it responded to the same stimulus.
This is a controlled trial with a defined intervention, exercise training of specified modes, in a small number of young and older volunteers, not a population-scale outcomes study and not a study that measured whether the mitochondrial improvement translated into fewer falls or longer life. What it shows cleanly is that mitochondrial bioenergetics in aged human muscle is modifiable by training, in a study designed specifically to test that question.
NAD+, nicotinamide adenine dinucleotide, is a molecule mitochondria need to run the reactions that make ATP, and NAD+ levels are reported to fall with age in various tissues. That has made NAD+ precursors, including nicotinamide riboside, a heavily marketed category for exactly the promise this article is about: restoring youthful mitochondrial energy production.
A study gave nicotinamide riboside to older adults and directly measured its effect in skeletal muscle, the tissue where the mitochondrial decline described above was documented.5 The supplement did what it was expected to do at the chemistry level: it raised the NAD+ metabolome in muscle, meaning the pool of NAD+ and its related metabolites increased. It also shifted the transcriptome, the pattern of genes being expressed, toward an anti-inflammatory signature.
What it did not do was produce the bioenergetic improvement that would validate the marketing claim, meaning a measurable gain in mitochondrial respiratory or ATP-producing capacity in that aged muscle. The molecule moved. The chemistry underneath it changed in a plausible direction. The functional outcome that supplement buyers actually want, more usable cellular energy from their existing mitochondria, was not shown.
This is worth sitting with rather than glossing over, because it is a case where an intervention did something real and measurable, just not the thing being sold. Raising a metabolite pool and shifting gene expression are legitimate findings. They are not the same claim as 'restores mitochondrial function,' and the study that actually tested for that improvement did not find it.5
| Intervention | What it changed | What it did not change |
|---|---|---|
| Exercise training4 | Mitochondrial respiration and protein translation rate, in both young and older adults | Not designed to test long-term outcomes like disability or mortality |
| Nicotinamide riboside5 | NAD+ metabolome and anti-inflammatory transcriptomic signature in aged muscle | Bioenergetic, mitochondrial function measures in that same aged muscle |
Mitochondrial dysfunction is included as one of the hallmarks of aging in the most recent update to that consensus framework, alongside processes such as genomic instability, cellular senescence, and altered intercellular communication.6 Being named a hallmark means researchers consider it a process that contributes causally to aging phenotypes across tissues, not merely a downstream marker that happens to correlate with getting older.
That framing is useful for understanding why this field attracts so much investment and, correspondingly, so much premature commercial claim-making. If mitochondrial dysfunction is genuinely a driver of aging rather than a side effect, then an intervention that reliably improved it in people would be a meaningful finding, worth the attention it gets. The gap is between that scientific premise, which is reasonably well supported, and the leap to 'this specific product improves it in you,' which for most products on the market has not been tested at the level the exercise trial above was tested.
The honest current state: the biology of decline is measured and real, named as a hallmark, and it accelerates. The best-supported way to intervene is training, tested directly and successfully in older adults. The most heavily marketed supplement route, NAD+ precursors, has shown metabolic and transcriptomic effects in aged human muscle without showing the functional mitochondrial improvement the marketing implies.
Mitochondrial function in human muscle really does decline with age, the decline really does speed up later in life, and exercise has better trial evidence for changing that trajectory than any supplement sold for the purpose, including the one Astra offers.
Astra Editorial, reviewing the mitochondrial aging literature
In human muscle, yes, by two independent measurement approaches: noninvasive oxidative capacity imaging and biopsy-based ATP production rate, both lower in older adults than younger adults.12
No. Longitudinal tracking of aerobic capacity in healthy older adults found the rate of decline accelerates in later decades rather than staying constant.3
Exercise training. A controlled trial in young and older adults found training improved mitochondrial respiration and protein translation in both age groups.4
In the trial that tested this directly in aged human skeletal muscle, nicotinamide riboside raised the NAD+ metabolome and shifted gene expression toward an anti-inflammatory pattern, but did not deliver the bioenergetic, mitochondrial-function improvement the supplement is marketed around.5
The current consensus framework lists mitochondrial dysfunction as one of the hallmarks of aging, meaning researchers treat it as a contributing process, not only a downstream marker.6
The trial evidence for exercise improving mitochondrial function in older adults is stronger and more direct than the trial evidence for any supplement, including NAD+ precursors, tested for the same outcome.45
It is a different delivery route than oral nicotinamide riboside and has not been tested in the same aged-muscle bioenergetics study design referenced here. It is a reasonable next read if you want to understand NAD+ biology specifically, with the same honest caveat that exercise has the stronger mitochondrial-function evidence to date.
If mitochondrial and metabolic energy is the concern, be honest about the hierarchy: exercise training has the strongest human trial evidence for improving mitochondrial function in older adults, of anything discussed here. NAD+ injection therapy is a route to raising NAD+ availability, the same molecule pool that nicotinamide riboside raised in aged muscle without a proven bioenergetic payoff in that trial. It is not a substitute for training, and Astra is not going to claim otherwise.
This guide is educational and is not medical advice. Compounded medications are not FDA-approved. Speak with a licensed physician about your own care.