The thing underneath it has an ugly name: low energy availability. What you eat minus what training burns funds everything else your body does. Below about 30 calories per kilo of lean tissue a day the body switches off the optional parts ¹. Reproduction goes first: the period thins and stops. Bone formation drops within days ². Thyroid falls, which is the cold hands and the low resting metabolism ³. The driver is the energy gap, not the miles.

A lost period is a symptom, not a badge. Swimmers with suppressed cycles stalled their 400m time for a year while teammates on the same training improved about 8% ⁴. Athletes in low energy availability took roughly four and a half times more bone injuries in a season, and risk rises with each triad component ⁵⁶. It's common: 22-58% of elite female endurance athletes are affected ⁷. And the bone you lose in your twenties is largely the bone you keep at sixty-five ⁸.

This reaches past the Olympic runner. The recreational exerciser who runs fifty kilometres a week, eats clean, and hasn't had a period in years is the modal case. Men get a parallel version, testosterone falling where oestradiol does ⁹. Adolescents carry the highest stakes: peak bone mass is set before twenty ⁸.

The whole fix is more food, held long enough to work. Refeeding 300 to 500 calories a day restored cycles in most athletes over six months ¹⁰.

Recovery runs fast to slow.

• Weeks: warmth returns first, thyroid climbs, sleep thickens ³.
• One to three months: the fog lifts, workouts feel like running, libido returns.
• Three to twelve months: the period returns, often after years, and the plateau breaks ¹⁰.
• A year on: bone density stops falling, and caught young it rebuilds.