Fuelling around your menstrual cycle: what the science actually shows

Should you eat differently across your menstrual cycle to train and perform better? It's one of the hottest topics in female athlete nutrition right now, and you'll see confident "phase-based fuelling" plans everywhere. The honest answer is that the underlying science is young, often based on small studies, and the practical effects look modest. There are a few reasonable principles worth knowing, and a lot of overconfident advice built on thin evidence.

I'd rather tell you the truth than sell you a neat protocol. So here's what's reasonably supported, what's plausible but unproven, and what genuinely matters more than any of it.

Start with the big caveat

Female athletes have been badly under-represented in sports science. A lot of foundational research was done on men and assumed to transfer. The work that does exist on the menstrual cycle is frequently small, hard to control (hormone levels vary enormously between women and between cycles), and produces mixed results. One study of elite athletes found no meaningful difference in actual nutrient intake across cycle phases. So when you read a glossy "fuel for your follicular phase" plan, treat the precise prescriptions with healthy scepticism. The biology is real; the specificity of the advice often outruns the data.

What the two phases actually do

Your cycle splits roughly into the follicular phase (from the start of your period to ovulation, oestrogen rising) and the luteal phase (after ovulation, oestrogen and progesterone higher).

The physiology that has some support:

• Substrate use shifts. Higher oestrogen in parts of the cycle tends to nudge the body towards using more fat and sparing glycogen. Carbohydrate oxidation appears somewhat higher in the follicular phase. The effect sizes here are small and not consistent across studies.

• Resting energy expenditure may rise in the luteal phase, with estimates ranging widely, roughly 2-11%. That's a big range, which tells you how uncertain it is.

• Symptoms are real even where metabolism is murky. Many women report more fatigue, bloating and cravings in the late luteal phase. Whether that's metabolic or symptomatic, it affects how training feels.

The practical principles worth following

Given the uncertainty, I'd anchor on robust, sensible habits rather than precise phase prescriptions:

• Don't under-fuel carbohydrate. Whatever phase you're in, endurance work needs carbs. If anything, the theory suggests being a little more deliberate about carbohydrate around harder sessions in the higher-hormone luteal phase. This is low-risk advice that holds up regardless of the fine detail.

• Protein matters and may matter slightly more in the luteal phase, where there's some suggestion of higher protein turnover. Aim for adequate, evenly-spread protein anyway; that's good practice for any athlete.

• Use the late luteal phase as a signal, not a sentence. If you feel flat and bloated premenstrually, that's a reasonable time to lean into easy fuelling, comfort, hydration and not beating yourself up over a tough session. That's listening to your body, not rigid science.

• Iron, iron, iron. This is the one with the firmest footing. Monthly blood loss makes female athletes the highest-risk group for iron deficiency, which quietly wrecks endurance performance. Prioritise iron-rich food, pair plant sources with vitamin C, and get tested if you're persistently tired. This matters far more than any phase-based carb tweak.

Reasonably supportedWorth doingCarbsSlightly higher use follicular; glycogen sparing lutealFuel carbs properly all cycleProteinPossible higher luteal turnoverAdequate, spread protein anywayIronClear higher deficiency riskIron-rich food, test if flatEnergyPossible small luteal riseDon't restrict; eat to demand

The thing that matters most: don't under-eat

Here is where I'll be firm, because the evidence is much stronger than for any phase-tweaking. The biggest nutrition risk for female athletes is not getting the follicular carb ratio wrong. It's low energy availability, eating too little to support both training and basic physiology, which leads to Relative Energy Deficiency in Sport, or RED-S.

RED-S can disrupt or stop your periods, weaken bones, tank performance, harm immunity and mess with hormones. A regular menstrual cycle is actually a useful health signal: losing it is a red flag, not a convenient side effect of training. If your cycle disappears, that's a reason to eat more and seek help, not something to ignore. No clever phase-based plan does you any good if you're under-fuelled overall.

FAQ

Is phase-based fuelling a waste of time? Not a waste, but oversold. The general nudges, fuel carbs well, mind iron, support protein, don't restrict, are sound. The hyper-specific day-by-day plans claim more certainty than the science supports.

What if I'm on hormonal contraception? The natural cycle hormone swings are blunted or absent, so phase-based reasoning largely doesn't apply. Focus on the fundamentals: eat enough, fuel your training, mind your iron.

Should I track my cycle as an athlete? Yes, but mainly as a health and pattern signal. Noticing how you feel and perform across your cycle is useful self-knowledge. A missing or irregular cycle is the most important thing it can tell you.

In MOVE we build fuelling around eating enough real food to meet the demands of your training, which is the foundation under all of this. The cycle science will keep maturing, and we'll follow it honestly as it does. For now, fuel properly, protect your iron, never under-eat, and treat the confident phase-by-phase plans with the scepticism a young field deserves.