'At least I don't have to deal with it.' It's a thought many women have when their period disappears under high training load or low energy availability. For others, it doesn't even register as a problem — it's assumed to be a normal side effect of training hard. It isn't. The reality is more complicated. A missing period is not a neutral event. It is a signal from your body that something has gone wrong with its energy accounting, and the downstream effects extend well beyond reproduction.
Why periods go missing
The most common cause in active women is hypothalamic amenorrhea (HA), a functional suppression of the hypothalamic-pituitary-ovarian axis. In plain terms: your brain detects insufficient energy to support both athletic output and reproductive function, and shuts down the latter.
Skipping periods is often linked to eating too little or disordered eating. But it is not exclusively so. Women who are eating sufficiently by standard metrics can still experience hypothalamic amenorrhea (HA) if their energy expenditure significantly outpaces intake — a pattern the International Olympic Committee (IOC) has formalised under the term Relative Energy Deficiency in Sport (RED-S). You don't have to be undereating for this to happen. You simply have to be underfuelling relative to your training demand, which is easy to do without realising it, particularly in endurance sports.
What actually happens when oestrogen drops
Amenorrhea means oestrogen levels fall, sometimes significantly. Oestrogen is not just a reproductive hormone. It has systemic effects throughout the body.
Bone density: Oestrogen is central to bone remodelling. Low oestrogen accelerates bone resorption and slows new bone formation. Even short periods of amenorrhea can cause measurable bone density loss, and in young women, this matters because peak bone mass is established in the twenties. Some bone density loss from HA may not be fully recoverable. (Mountjoy et al., IOC Consensus Statement, 2018)
Cardiovascular health: Oestrogen has protective effects on the endothelium (the lining of blood vessels) and on blood fat and cholesterol markers. Chronic low oestrogen negatively affects cardiovascular risk markers.
Injury risk: Lower oestrogen affects tendon and ligament function, increasing soft tissue injury risk.
Cognitive function and mood: Oestrogen plays a role in serotonin regulation and neuroprotection. Low oestrogen is associated with increased depression and anxiety.
The athletic performance paradox
Here's what makes Relative Energy Deficiency in Sport (RED-S) particularly insidious: in the short term, training under energy restriction can feel productive. Weight may drop. Times may improve. It is only later, often when stress fractures, persistent fatigue, or mood deterioration accumulate, that the full cost becomes apparent.
Performance eventually declines. Research consistently shows that athletes with RED-S have impaired muscle protein synthesis, reduced training adaptation, slower recovery, and higher injury rates than those with adequate energy availability. The short-term performance gains are borrowed against a long-term debt.
What recovery looks like
Recovery from hypothalamic amenorrhea requires restoring energy balance, which almost always means eating more, and often means reducing training load. The hypothalamus needs to receive a consistent signal that energy is sufficient before it reinstates reproductive function.
This process takes time. For some women, periods return within a few months of dietary changes. For others, particularly those who have been amenorrhoeic for years, the process is longer. Working with a sports dietitian who specialises in RED-S is the most effective route.
If you have lost your period and are unsure why, or have been told it's 'normal for athletes,' it is worth pushing for further investigation. It is not normal. It is a symptom.
The bottom line
A missing period is not a performance optimisation. It is a warning sign. If yours has gone, take it seriously. The downstream effects, on bone density, cardiovascular health, mood, and long-term performance, are real and cumulative. Fuel your training. Protect your cycle. The two are not in conflict.
There is also a less-discussed consequence: the menstrual cycle acts as a regular clearance mechanism. Without it, certain metabolic byproducts and inflammatory markers that would ordinarily be flushed during menstruation can accumulate. Your period is not just reproductive — it is part of your body's housekeeping system.