Short answer: mitochondria convert the food you eat and the oxygen you breathe into ATP, the molecule every cell spends to do work. That conversion depends on NAD+ as an electron carrier. NAD+ declines with age, mitochondria accumulate damage, and the cell's ability to clear the damaged ones slows — which is the mechanistic story behind what people experience as age-related loss of energy.
What mitochondria actually do
Mitochondria are organelles inside almost every cell, present in the hundreds or thousands in high-demand tissue like heart and muscle. Their job is oxidative phosphorylation: taking electrons stripped from nutrients and passing them down a chain of protein complexes, using the energy released to pump protons across a membrane, then letting those protons flow back through an enzyme that manufactures ATP.
ATP is the universal currency. Muscle contraction, nerve firing, protein synthesis, active transport across membranes — all of it is paid for in ATP. A typical adult turns over roughly their own body weight in ATP each day, which is possible only because it is recycled continuously rather than stored.
Where NAD+ comes in
NAD+ is the shuttle. It accepts electrons from nutrient breakdown, becomes NADH, and delivers them to the electron transport chain, where it is regenerated as NAD+ to go again. Without adequate NAD+, the chain does not receive its input, and ATP production slows regardless of how much food is available.
NAD+ is also consumed — not just recycled — by two other groups of enzymes: PARPs during DNA repair, and sirtuins during their regulatory work. That creates competition for a shared pool, which is why NAD+ sits at the intersection of energy metabolism, DNA repair and cellular regulation rather than in energy alone. See How NAD+ Precursors Actually Work.
Why output declines with age
Several things happen together, and they reinforce each other.
NAD+ availability falls. Zhu and colleagues (2015) measured NAD+ directly in healthy human brain using magnetic resonance spectroscopy and confirmed an age-related decline in NAD+ concentration, alongside a shift in the NAD+/NADH redox state.
Damage accumulates. The electron transport chain leaks a small fraction of electrons, producing reactive oxygen species. That is normal and even useful as a signal at low levels, but when production exceeds the antioxidant systems available to neutralise it, the result is oxidative stress — and mitochondria sit closest to the source.
Clearance slows. Cells dispose of damaged mitochondria through mitophagy, a targeted form of autophagy. Fang and colleagues (2016) showed this process is itself partly NAD+ dependent, so falling NAD+ impairs the very mechanism that would remove the damaged mitochondria. Dysfunctional units accumulate because the cleanup crew is underfunded.
What genuinely improves mitochondrial function
Exercise, by a wide margin. Endurance training stimulates mitochondrial biogenesis — the creation of new mitochondria — via the PGC-1α pathway. Nothing in a bottle matches the effect size of regular aerobic training. This is the least commercial and most important point in this article.
Sleep. Metabolic clearance and repair processes run predominantly during sleep. Chronic short sleep works directly against mitochondrial maintenance. See How to Increase Deep Sleep.
NAD+ precursors. Supplying NMN or nicotinamide riboside raises blood NAD+ in human trials, and Yoshino and colleagues (2021) reported improved muscle insulin sensitivity in prediabetic women after ten weeks of NMN. The honest framing is that the mechanism is well characterised and the human outcome data is early — small trials, short durations. See NMN, NAD+ Full Stack and NMN Dosage and Side Effects.
Cordyceps is studied for oxygen utilisation and ATP production specifically, which is a mitochondrial rather than stimulant mechanism — covered in Caffeine-Free Energy.
The antioxidant caveat worth knowing
It is intuitive to think that if reactive oxygen species damage mitochondria, flooding the system with antioxidants must help. The research has not borne that out simply. Low levels of reactive oxygen species act as signals that trigger adaptive responses — including the mitochondrial adaptations to exercise — and Ristow and colleagues (2009) found high-dose antioxidant supplementation around training blunted some of the health-promoting adaptations to exercise rather than improving them.
The picture is one of balance rather than elimination, which is why this article does not tell you to take more antioxidants.
Frequently Asked Questions
Quick answers to the questions readers most often ask.
What is mitochondrial function?
It refers to how efficiently mitochondria convert nutrients and oxygen into ATP, the energy currency every cell spends. The process is oxidative phosphorylation: electrons stripped from nutrients pass down a protein chain, pumping protons across a membrane whose return flow drives ATP synthesis.
Why does mitochondrial function decline with age?
Three things compound. NAD+ availability falls, so the electron carrier feeding the chain becomes scarcer. Oxidative damage accumulates in mitochondria, which sit closest to where reactive oxygen species are produced. And mitophagy, the process that clears damaged mitochondria, is partly NAD+ dependent, so it slows exactly when it is needed most.
How can I improve mitochondrial function?
Regular aerobic exercise by a wide margin, because endurance training stimulates mitochondrial biogenesis through the PGC-1α pathway. Adequate sleep supports the repair processes that run overnight. NAD+ precursors have a well characterised mechanism with early human outcome data.
What is the relationship between NAD+ and mitochondria?
NAD+ is the electron carrier that delivers electrons from nutrient breakdown to the electron transport chain. Without adequate NAD+, ATP production slows regardless of nutrient availability. NAD+ is also consumed by PARP enzymes during DNA repair and by sirtuins, so those processes compete for a shared pool.
Do antioxidants improve mitochondrial function?
Not straightforwardly. Low levels of reactive oxygen species act as signals that trigger adaptive responses, including the mitochondrial adaptations to exercise. High-dose antioxidant supplementation around training has been found to blunt some of those adaptations rather than improve them. The goal is balance, not elimination.
References
- Zhu XH et al. In vivo NAD assay reveals the intracellular NAD contents and redox state in healthy human brain and their age dependences. PNAS. 2015.
- Yoshino M et al. Nicotinamide mononucleotide increases muscle insulin sensitivity in prediabetic women. Science. 2021.
- Fang EF et al. NAD+ replenishment improves lifespan and healthspan in ataxia telangiectasia models via mitophagy and DNA repair. Cell Metab. 2016.
- Ristow M et al. Antioxidants prevent health-promoting effects of physical exercise in humans. PNAS. 2009.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.
