Cellular Bioenergetics (ATP Production Capacity)

Mitochondrial biogenesis asks how many mitochondria a cell has. Cellular bioenergetics asks how well each one works. The two come apart with age, and the functional side declines first.

The reference dataset is a Mayo Clinic study of 146 healthy sedentary people aged 19 to 89, 86 women and 60 men, who had vastus lateralis biopsies and treadmill testing [1]. Mitochondrial ATP production rate fell about 8% per decade per gram of muscle, and still fell about 5% per decade after normalising per milligram of mitochondrial protein, so the loss is not only fewer mitochondria [1]. VO2max fell about 8% per decade, 5% after adjusting for leg lean mass (P < 0.001), and leg lean mass plus ATP production rate together explained 86% of the variance in VO2max [1]. Cytochrome oxidase subunit COX3 abundance fell 10% per decade and COX4 8% per decade, both P < 0.001 [1]. Oxidative DNA damage tracked along: 8-oxo-dG in people aged 65 to 80 sat roughly 25% above those aged 20 to 35, P < 0.01 [1].

Courtney Peterson, Darcy Johannsen and Eric Ravussin collected the muscle-level numbers: in adults aged 70 to 84 versus 20 to 34, citrate synthase activity was 19% lower and mitochondrial capacity 17% lower while OXPHOS protein content was unchanged [2]. Phosphorus MRS in elderly adults showed a 21% lower P/O ratio, meaning less ATP per oxygen consumed [2]. One study found a 50% reduction in oxidative capacity per muscle volume and a 30% reduction per mitochondrial volume [2]. Muscle mass itself declines just under 1% per year, and strength falls about three times faster than mass [2].

Marta Gonzalez-Freire and colleagues confirmed the coupling in the Baltimore Longitudinal Study of Aging: 38 participants aged 24 to 91, mean age 68.6, with ex vivo high-resolution respirometry, post-exercise 31P-MRS, treadmill aerobic capacity and strength testing all moving together [3].

What moves it: urolithin A, a gut metabolite of ellagitannins that triggers mitophagy. Sophia Liu and David Marcinek randomised 66 adults aged 65 to 90, mean age 71.7, to 1,000 mg urolithin A or placebo for 4 months; muscle endurance in the first dorsal interosseus rose by 95.3 contractions to fatigue against 11.6 on placebo at 2 months [4]. Anurag Singh's 88-person ATLAS trial in overweight sedentary adults aged 40 to 65 tested 500 mg and 1,000 mg for 120 days with no exercise programme [5]. Neither trial measured lifespan.