7.13 - Exercise dose and health evidence

Too little activity and disease risk

Physical activity includes movement in daily life, while exercise is planned activity intended to develop or maintain fitness. Its dose depends on frequency, duration and intensity; minutes alone do not describe the mechanical load of a contact sport or the physiological stress of endurance exercise. “Too much” and “too little” describe a dose in relation to a person and an outcome, rather than one universal boundary.

Low activity is associated with increased risks of obesity, cardiovascular disease (CVD) and type 2 diabetes. These are probabilities across populations: an inactive individual does not inevitably develop all three, and an active individual is not guaranteed protection.

Intervention evidence supports some of these links. A randomised trial in 173 sedentary postmenopausal women found lower body weight and body fat after an exercise intervention than after a stretching control. Its population and intervention limit generalisation; it does not predict an individual's weight change.

For CVD, a prospective study of 90,211 UK Biobank participants measured activity using accelerometers and followed later diagnoses. Higher activity was associated with lower CVD incidence. The less active participants also differed in factors such as smoking, illustrating why an observational association needs careful interpretation.

Excessive loading and joint injury

Joint tissues experience forces during movement, and their response depends on the load and the opportunity for repair. Articular cartilage covers the ends of bones at a synovial joint, providing a smooth surface that reduces friction and distributes loads. Repetitive excessive loading, collisions or a sudden twisting injury can damage joint structures. Damage can cause pain, inflammation and reduced movement; injury can also increase later osteoarthritis risk.

The specification's phrase “wear and tear on joints” describes a possible disadvantage of excessive exercise, but cartilage is living tissue, not a layer that is inevitably used up by all movement. Osteoarthritis involves changes across the joint. Type of activity, previous injury, load, age and other factors matter. A population cohort study of knee injuries found greater subsequent diagnosed knee osteoarthritis risk in injured than in uninjured young adults. This supports the importance of injury, rather than demonstrating that all exercise causes arthritis.

Evidence also challenges a simple “more running always means more damage” claim. In an Osteoarthritis Initiative cohort, self-selected running in adults aged at least 50 with knee osteoarthritis was not associated with worse structural progression. The runners selected their own activity: people with more painful knees might avoid running. This evidence does not establish that every loading pattern is harmless, especially at doses not represented in the sample.

Exercise stress and immune evidence

Prolonged strenuous exercise can alter immune-cell numbers and function, making possible suppression of the immune system a hypothesis that must be tested against an appropriate outcome. Suppression means reduced capacity to defend against pathogens; it is not simply any measured change in an immune cell.

During exercise, stress hormones can move lymphocytes into the blood. Their concentration may fall during recovery as cells leave the circulation. A blood sample measures one compartment at one time, so fewer circulating lymphocytes do not necessarily mean fewer cells in the whole body or poorer defence. Cell-tracking experiments in mice found redistribution between organs after exercise. This supplies a possible explanation for changing blood counts, with the limitation that animal evidence cannot alone establish human infection risk.

Some early exhaustive-endurance research reported changes in natural killer cell counts and immune function that raised concern about a recovery period of impaired defence. More recent work shows why context matters: a 2024 crossover experiment in 12 female endurance athletes found altered immune measures under low energy availability. The energy available after exercise, sleep, psychological stress and pathogen exposure can differ alongside training load; they must be considered when interpreting illness patterns.

An increase in reported sore throats after an event could reflect infection, airway irritation, travel or crowd exposure. It does not alone prove exercise suppressed immunity. Stronger evidence would combine repeated functional immune measurements with confirmed infections and a suitable comparison group, accounting for those competing explanations. Equally, uncertain evidence does not establish that sustained excessive training under all conditions is risk free.

The most defensible conclusion states the measured change, the conditions and the uncertainty, rather than treating the traditional “open window” of immune suppression as a universal law.