Human Performance & Health › The Biological Foundation

This page is the foundation the rest of Human Performance & Health rests on: what cells are, how they make energy, how they repair themselves, and what the research says about damage, defense and the signals cells respond to. Each section names its source. Later pages on energy and fatigue, nutrition, sleep and training build on it.

How many cells

A 2023 analysis in PNAS puts a reference adult male at roughly 36 trillion cells, a reference adult female at about 28 trillion, and a child at around 17 trillion (Hatton et al., 2023).

The core systems

Energy: mitochondria. In skeletal muscle, “a single bout of exercise initiates signaling to provoke increases in mitochondrial biogenesis,” “balanced by the onset of organelle turnover carried out by the mitophagy pathway”; aging and disuse are linked to a dysregulated mitochondrial network, and the review’s authors describe exercise as a possible non-drug way to improve it (Hood et al., 2019).

Enzymes. The National Human Genome Research Institute defines an enzyme as “a biological catalyst” that “is almost always a protein” and “speeds up the rate of a specific chemical reaction in the cell”; its narrated definition adds that an enzyme is “usually a protein but could be RNA” (NHGRI Talking Glossary, Enzyme).

DNA repair. Jackson and Bartek write that “Cellular DNA is a sitting target for many toxic agents,” “from ionizing radiation to any number of chemicals in the environment,” to which “can be added errors that arise from physiological processes,” and that “The human body has evolved several systems to detect DNA damage and mediate its repair” (Jackson & Bartek, 2009).

Damage and defense. Exercise “increases mitochondrial formation of presumably harmful reactive oxygen species (ROS),” yet in a controlled trial “Exercise increased parameters of insulin sensitivity” “only in the absence of antioxidants”: vitamin C (1,000 mg a day) and vitamin E (400 IU a day) taken during training blocked the effect, and the authors concluded that “Supplementation with antioxidants may preclude these health-promoting effects of exercise in humans” (Ristow et al., PNAS 2009). See supplements. Chronic inflammation is one of the twelve hallmarks of aging in the 2023 Cell review listed below.

Nutrient sensing. The mechanistic target of rapamycin (mTOR) “coordinates eukaryotic cell growth and metabolism with environmental inputs,” “including nutrients and growth factors,” and regulates “many fundamental cell processes, from protein synthesis to autophagy” (Saxton & Sabatini, 2017). “Disabled macroautophagy” and “deregulated nutrient-sensing” are both on the 2023 list of hallmarks of aging.

Genes and epigenetics. The CDC: “Epigenetics refers to how your behaviors and environment can cause changes that affect the way your genes work,” and “Unlike genetic changes (mutations), epigenetic changes are reversible and do not change the sequence of DNA bases, but they can change how your body reads a DNA sequence.” It adds that “Your epigenetics change as you age, both as part of normal development and aging” and “because of exposure to environmental factors that happen over the course of your life” (CDC, epigenetics page, reviewed January 31, 2025).

Hormesis

Calabrese and Mattson define hormesis as “adaptive responses of biological systems to moderate environmental or self-imposed challenges,” and in their review the body’s dose responses are biphasic: a modest stimulation at low doses, typically 30 to 60 percent above controls, and harm at high ones, with exercise and fasting among their examples (Calabrese & Mattson, 2017). The Hood review describes the signaling exercise triggers, and the Ristow trial shows what happened when the signal was blocked. The sleep and recovery pages carry the sourced evidence on what too much load without recovery does.

How the other pages connect

Nutrition covers the materials food supplies; fitness covers training and the adaptations it produces; sleep covers sleep stages, including the mouse study in which sleep sped the brain’s clearance of waste (Xie et al., 2013, on Sleep Physiology & Architecture).

Going deeper

Each system on this page opens into its own guide over time: cellular energy, repair and cleaning, damage and defense, nutrient sensing, and genetics. Newer research areas such as NAD⁺ metabolism, senolytics, epigenetic clocks and therapeutic peptides are not covered on this site yet; when they are, each will be presented with its own sources and the state of its human evidence.

GoldBlock Workshop is educational. We explain how the body works. We do not diagnose, treat, or give personal medical advice. For decisions about your own health, talk to a qualified professional.

Sources and further reading

We build on the work of the researchers below, and we credit them by pointing to their published research rather than reproducing it.

• López-Otín & Kroemer, “Hallmarks of Health,” Cell (2021). Abstract read 2026-10-19 (QxMD listing): “Health is usually defined as the absence of pathology.” PubMed
• López-Otín, Blasco, Partridge, Serrano & Kroemer, “Hallmarks of Aging: An Expanding Universe,” Cell 186(2):243-278 (2023). Twelve hallmarks, including chronic inflammation, disabled macroautophagy and deregulated nutrient-sensing; abstract read 2026-10-19 (UCL Discovery copy). PubMed
• Hood, Memme, Oliveira & Triolo, “Maintenance of Skeletal Muscle Mitochondria in Health, Exercise, and Aging,” Annual Review of Physiology (2019). PubMed
• Calabrese & Mattson, “How does hormesis impact biology, toxicology, and medicine?” npj Aging and Mechanisms of Disease (2017). PubMed
• Jackson & Bartek, “The DNA-damage response in human biology and disease,” Nature 461:1071-1078 (2009). Abstract read 2026-10-19 (RePEc listing). PubMed
• Ristow et al., “Antioxidants prevent health-promoting effects of physical exercise in humans,” PNAS (2009). Abstract read 2026-10-19 (Kahn laboratory publication page). PubMed
• Saxton & Sabatini, “mTOR Signaling in Growth, Metabolism, and Disease,” Cell 168(6):960-976 (2017). Abstract read 2026-10-19 (MIT DSpace author manuscript). MIT DSpace
• National Human Genome Research Institute, Talking Glossary, “Enzyme” (read 2026-10-19). genome.gov
• Centers for Disease Control and Prevention, Genomics and Health, epigenetics page, reviewed January 31, 2025 (read 2026-10-19). cdc.gov
• Hatton et al., “The human cell count and size distribution,” PNAS (2023). PNAS