Inflammation

Inflammation is the immune system's response to damage or threat: blood flow increases, immune cells arrive, and signalling proteins called cytokines coordinate the clean-up and repair. Acute inflammation is what makes an injury swell and what allows it to heal.

It matters because the chronic, low-grade version is a different animal entirely. Persistent low-level inflammation — driven by visceral fat, poor sleep, smoking, inactivity and a poor diet — is implicated in atherosclerosis, insulin resistance and several cancers, and is measured clinically with hs-CRP.

In practice, exercise has a dual relationship with it: a session causes acute inflammation, while regular training lowers chronic inflammatory markers. This is also why routinely taking anti-inflammatory drugs or icing after training is now questioned — it can suppress the acute signalling that repair depends on.