Your mitochondria peaked at 30.

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The mitochondria are the little power plants inside every one of your cells. They turn food and oxygen into the energy you spend on every heartbeat, every step, every rep. From roughly age 30 onward, they thin out.

The definitive human-tissue study on this is the 2005 PNAS paper by Kevin Short and colleagues at the Mayo Clinic. 146 healthy adults across a 71-year age range. Muscle biopsies. Mitochondrial DNA copy number, mitochondrial protein content, and ATP production rate all fell in lockstep with age. By 60, the average muscle cell held about half the mitochondrial DNA it had at 25.

There are three separate ways the mitochondrial layer degrades from midlife onward, and they compound. Fewer of them per cell. Less efficient at converting fuel to ATP. Slower to be replaced when they get damaged.

At the biochemistry layer, the coenzyme that runs the sirtuins responsible for repairing and recycling mitochondria is NAD+. It drops roughly in half between age 40 and 60 in human skin biopsies. Massudi and colleagues at UNSW, PLoS ONE 2012. Blood NAD+ tells a slightly different story, but the tissue-level decline is consistent across studies.

The useful news is that PGC-1α, the master regulator of mitochondrial biogenesis, is one of the most responsive proteins in your body. It answers to stress. Which is where the interventions start.

Four things build mitochondria in humans, in trials, with peer-reviewed evidence you can actually read.

The first is interval training. In a 12-week trial at the Mayo Clinic, high-intensity interval training reversed the age-related decline in mitochondrial protein synthesis in adults over 65. Their post-training muscle proteome looked younger on a mass spec than it had before the intervention. Resistance training built strength, which matters for its own reasons. Interval training built the engine. Matthew Robinson and colleagues at Mayo, Cell Metabolism 2017. Three sessions a week for 12 weeks was the dose that mattered.

The second is heat. Repeated sauna exposure raised mitochondrial respiratory function in human skeletal muscle by 28 percent over six days of daily exposure. Hafen and colleagues at Brigham Young, Journal of Applied Physiology 2018. Which is one of the reasons the Finnish sauna cohorts we covered in June show roughly half the all-cause mortality at 4 to 7 sessions a week versus one.

The third is time-restricted eating. In a supervised feeding trial at Pennington Biomedical, Elizabeth Sutton and colleagues showed that a 6-hour daily eating window for 5 weeks improved insulin sensitivity, blood pressure, and oxidative-stress markers in prediabetic men, without weight loss to explain the change. A follow-up from the same group showed LC3A, a core autophagy gene, rose 22 percent in whole blood after just 4 days of the same protocol. Jamshed et al, Nutrients 2019. What has not been shown yet in humans is a direct increase in muscle mitochondrial capacity from TRE. A 3-week trial in adults with type 2 diabetes improved glucose regulation without changing mitochondrial function on muscle biopsy. Andriessen et al, Diabetologia 2022. The intervention likely works upstream in the recycling process, cleaning up existing mitochondria rather than adding new ones.

The fourth is the NAD+ pathway itself, which is where the biochemistry gets more interesting and where the evidence differs sharply by route of administration. Two forms of research-grade evidence in humans. 1,000mg of oral nicotinamide riboside daily for 6 weeks raised blood NAD+ in healthy middle-aged and older adults with no side effects, Chris Martens and colleagues at CU Boulder, Nature Communications 2018. 250mg of oral NMN daily for 10 weeks improved skeletal muscle insulin sensitivity in prediabetic postmenopausal women, Mihoko Yoshino and colleagues at Washington University, Science 2021.

Oral NAD+ precursors work, and they work slowly. The bottleneck is bioavailability. Which is where the medical-grade injectable route sits, and where the sponsor of this issue makes its case.

The stack for someone paying attention to this in their 40s is not any one of these four. It is drive the biogenesis signal with hard interval training, hold it there with heat, give the coenzyme layer the substrate it needs. Whether that substrate comes from an oral capsule, a licensed injection, or a season of consistent fasting is a question of bioavailability, cost, and how much friction you want in the loop. It is not a question of biology.

The rate at which your mitochondria thin from here is one of the few things you get a real vote on.

TBC

Until Wednesday.

Longevity Daily / The Building Decades