Your period is a vital sign: RED-S, hormones and metabolic health

You train hard, you eat "pretty well", but something feels off. You're tired all the time, your period has gone quiet or keeps changing, you pick up niggles, and progress has stalled. Lots of active women live with this for years and assume it's just part of training. Often it isn't. It's your body telling you it doesn't have enough fuel.

The one idea to remember

Your brain only runs your cycle when it senses enough fuel. A small area of the brain (the hypothalamus) sends regular hormone pulses that tell your ovaries to work. When energy stays too low for too long, those pulses slow down and can stop (Loucks & Thuma, 2003). Your body is simply saving energy for the things it needs to survive.

That one link explains a lot: missed periods, weaker bones, low energy, poor recovery and stalled performance.

What is RED-S, in plain words?

RED-S stands for Relative Energy Deficiency in Sport. It means you're not eating enough to cover your training and everything else your body does each day. The International Olympic Committee describes it as a problem that can affect hormones, bones, immunity, gut, heart, mood and performance, in both women and men (Mountjoy et al., 2023).

It's a scale, not a switch. A short, planned dip in energy (say, a heavy training week) usually bounces back quickly. The problem is when it goes on for weeks or months.

Your period is a vital sign

Think of your period like a monthly health report. It's the cheapest early warning sign you have.

• Only 13% of cycles are exactly 28 days. In data from over 600,000 cycles, most were longer or shorter (Bull et al., 2019). Variation is normal.
• A normal range is roughly 21 to 35 days between periods.
• A regular period is reassuring, but not proof. In one study of exercising women, about half of cycles had hidden hormone changes even when periods looked normal (De Souza et al., 2010).
• A missing period is a clear signal. It is never just "normal for an athlete".

On the pill? Most hormonal contraception replaces your natural cycle, so a monthly bleed doesn't tell you whether your own hormones are working. You'll need to lean on the other signs below.

Signs your body may be short on fuel

• Periods that are missing, very irregular or getting further apart
• Feeling tired, flat or cold most of the time
• Poor sleep, low mood, irritability or anxiety around food
• Getting ill often or taking ages to recover
• Stress fractures or bone injuries that keep coming back
• Training harder but getting slower or weaker
• Low sex drive, hair thinning or brittle nails

Hormone health and metabolic health are linked

When fuel is low, your body turns things down to save energy: thyroid activity, oestrogen and the hormones that build bone and muscle. That's why under-fuelling can show up as low energy, feeling cold, gut problems and slower metabolism rather than just a missed period. Low oestrogen also matters for bone. Most bone is built by your late teens, and long-term low oestrogen can mean bone loss that's hard to win back (Gordon et al., 2017).

What about cycle syncing?

You may have seen advice to change your training or diet for each phase of your cycle. The research says cycle phase has, at most, a small effect on performance for most women (McNulty et al., 2020). It's not disproven, but there's no strong evidence for strict phase plans yet. Getting enough fuel matters far more than timing your workouts to your cycle.

What you can do yourself, starting today

• Track your cycle: note the first day of each period and how long each cycle is. An app or a notes page is fine.
• Note how you feel: energy, sleep, mood, hunger and soreness, a quick 1 to 5 score each day. Patterns show up within a few months.
• Eat around training: something before, and carbohydrate plus protein soon after.
• Don't cut carbs to save calories: carbohydrate is the main fuel for training, and low carb intake is a common cause of low energy.
• Eat more on bigger days: heavy sessions, long runs and double days need more food, not the same plate.
• Don't diet during heavy training or racing blocks.

When to see a doctor

See your GP or a sports doctor if your periods have stopped for three months, a cycle runs longer than 45 days, or you have a stress fracture (De Souza et al., 2014). A missing period can have other causes, such as PCOS, thyroid problems or pregnancy, so a doctor needs to rule those out first (Gordon et al., 2017). Take your cycle and symptom notes with you.

How we help

We don't diagnose anything. That's your doctor's job, and we'll encourage you to be seen first if your periods have stopped or you've had a stress fracture. What we handle is the fuelling and training side, working alongside your doctor rather than instead of them.

• Fuelling plans. We work out what your training, work and sleep actually cost, then build your eating around your week: what to have before, during and after sessions, and how much more you need on the big days. Not a generic diet.
• Blood testing. Available in Australia and the UK only. Ask us about UK arrangements. For Australian clients, we order practitioner-ordered tests through accredited labs: a finger-prick kit at home, or a collection centre near you. We look at iron and ferritin, thyroid, oestradiol and the metabolic markers next to your training and your symptoms, then read the results with you and adjust the plan. Any diagnosis still comes from your doctor.
• Coaching. Fuelling, training load, sleep and recovery tracked together, with check-ins and a direct line to the coaching team between them, so a slide gets caught before it turns into a missed period or a stress fracture. Online worldwide, in person in Sydney.

Be patient with it. Energy and mood often improve within weeks, but periods can take many months to return. Recovery is mainly about eating more, especially carbohydrate, and sometimes training a little less (De Souza et al., 2014). If you'd rather talk it through than read another article, book a free 30-minute call and we'll tell you honestly whether this looks like a fuelling problem or something for your doctor first.