I train hard, lost weight fast, and my periods stopped. Why?

Yes, fast weight loss with periods that have stopped in someone training hard is worth a prompt doctor visit. Rapid unintentional weight loss, missed periods and tiredness together are a pattern doctors take seriously at any age, and the combination with heavy training and a restrictive diet points at a specific, common cause: the body running on too little fuel for its workload. Doctors call it low energy availability or RED-S, and stopped periods are its clearest alarm bell.

The first step is a GP or family doctor, not a specialist: a proper history, an exam, and investigations, with further management or referral guided by the results. The panel our doctors would expect: a pregnancy test first, a complete blood count, iron profile, thyroid profile, vitamin B12 and folate, urea and electrolytes, and blood sugar. A vegetarian diet without dairy makes the iron, B12 and folate checks more important, not less.

A dietitian is often exactly the right second step, after the medical assessment rather than instead of it. Once the doctor has ruled out or found the medical drivers, a dietitian rebuilds intake to match the training load. If food rules, guilt, or fear of eating are part of the picture, saying that to the doctor matters most of all: that door leads to the right help fastest, and it is a door doctors open kindly.

Missed periods from underfueling are not a cosmetic issue; over time the same deficit thins bone. The earlier the assessment, the simpler the fix usually is.

Source: International Olympic Committee, consensus statement on relative energy deficiency in sport