The body is built from cells
The body is a community of tens of trillions of cells, and the biology of ageing is largely the biology of how well those cells keep themselves running. The hallmarks are, from this angle, the maintenance systems that falter.
At cell level, the core upkeep jobs are energy, DNA repair, protein quality control, and the signals cells send one another — and daily life reaches each.
Energy: the mitochondria
Cells run on energy produced mostly by mitochondria. Their capacity tends to decline with age, and because energy underlies every other cellular job, that decline ripples outward. Mitochondrial stress also acts as a signal in its own right, not merely an energy shortfall.
Here they sit as one maintenance system among several — energy first among equals.
Keeping DNA intact
DNA is continually damaged and continually repaired. Healthy cellular function depends on that repair keeping pace; ageing reflects, in part, the slow accumulation of what the repair systems miss — the genomic-instability hallmark, seen from inside the cell.
Protein quality control and autophagy
Proteins must be folded correctly and cleared when damaged, or they aggregate and jam the cell. Autophagy is the cell’s recycling system for worn-out components, and its decline is a recurring theme in ageing — enhancing it extends lifespan across model organisms.
How cells talk to each other
Cellular health is not only what happens inside a cell but how cells signal to one another — hormonally, immunologically and through inflammation. The dominant age-related shift is toward chronic, low-grade inflammation, which links single-cell maintenance to whole-body decline.
How the way you live reaches the cell
The cellular systems above are not sealed off from daily life — they are where nutrition, exercise, sleep and stress ultimately act. Exercise drives mitochondrial adaptation in muscle; sleep supports clearance processes in the brain; nutrient-sensing pathways read the food supply.
Nutrition, training and sleep act on those same pathways. Lifestyle and cellular biology are one story at two scales.
