The advice was written for someone else: fuelling and cramping for women endurance athletes
Ask a group of women runners where their fuelling plan came from and a familiar answer comes back. A training partner, a coach, a race guide or a podcast, almost all of it built on research conducted largely on men.
That is not a rhetorical point. It is a measurable feature of the evidence base, and it shows up exactly where women’s physiology diverges most from the assumptions baked into standard advice.
The evidence gap is real
Women remain under-represented across sport and exercise medicine. The editorial “Under-representation of women is alive and well in sport and exercise medicine”, published in BMJ Open Sport and Exercise Medicine on 9 May 2023 (doi 10.1136/bmjsem-2023-001606), sets out the shape of it: systematic reviews demonstrate that female athletes are under-represented in sports and exercise research, female first and last authorship on scientific publications is less than 25 per cent, and women hold less than 25 per cent of editorial-board leadership roles in sports sciences.
The consequence is not that existing advice is wrong. It is that the advice was calibrated on a population whose hormonal environment does not fluctuate across a monthly cycle, and then handed to a population whose does.
What actually changes across the cycle
Three things shift in ways that matter for endurance work, and none of them appears in a standard fuelling guide. Read what follows as the direction the literature reports rather than as a number to plan against. Reported effects are small; they differ between studies, and they remain unsettled because of the under-representation described above.
Core temperature is reported as slightly higher in the luteal phase. That sounds trivial until you layer it on a mid-summer long run, where heat strain can arrive a little earlier for the same effort.
Plasma volume and fluid regulation also move, which can change sweat rate and how you feel at a given intake. Some athletes notice this as bloating, others as an unusual thirst response, and many notice nothing at all. That spread between individuals is the practical point, and it is why generic guidance struggles here.
Substrate use is reported to shift as well, with a tendency toward greater fat oxidation in some phases. For a long steady effort, this rarely shows up. For a hard session at the top of your range, it may be the difference between finishing the set and fading through it.
The biggest lever is not a supplement
Before any of that is worth optimising, there is a larger one. Low energy availability, where intake does not cover both training demand and basic physiological function, is the condition underneath Relative Energy Deficiency in Sport (RED-S), and it is regularly mistaken for something else.
It can show up as persistent fatigue, stalled progress, poor recovery, disrupted or absent periods, frequent minor illness and a general sense that training is getting harder rather than easier. Muscle symptoms including cramping can sit downstream of it.
This is the one area where the answer is not a product, and we would rather say so than sell around it. Thin is not faster. If you are training hard and consistently under-fuelling, no electrolyte strategy compensates, and chasing one delays the actual fix. The Australian Institute of Sport publishes resources on energy availability written for athletes and coaches, and if several of those signs are familiar, that is a conversation for a sports dietitian or doctor rather than a change to your race-day plan.
Iron status belongs in the same category. Deficiency produces fatigue that looks exactly like poor fitness, and it is diagnosed with a blood test rather than inferred from how a session felt. If fatigue persists through a block where the training and the fuelling both look right, that is the test to ask for.
Cramping: what the evidence actually supports
Exercise-associated muscle cramping is where the popular explanation has moved on, and the advice has not caught up.
The dehydration and electrolyte-depletion model is intuitive and has proven hard to demonstrate. Cramping occurs in athletes who are well hydrated; it typically affects only the muscles working hardest rather than the whole body, and it does not track neatly with sweat or sodium losses. A better explanation is altered neuromuscular control, where fatigue disturbs the balance between the reflexes that excite and inhibit a muscle and leaves it prone to involuntary contraction. Our explainer on what actually causes cramps goes through the mechanism.
Two things follow. Cramping is more closely related to training load and pacing than to how much you drank, so conditioning and race-day effort are the first places to look. And it is why the old pickle-juice trick works at all: not the sodium, but the strong flavour hitting sensory nerves in the mouth and throat and producing a neuro-response that settles the muscle. CrampFix is built on that same mechanism, and CrampFix’s own testing describes relief within 30 seconds to 2 minutes, without asking you to carry a jar of brine.
Hydration and electrolytes still matter a great deal for performance, thermoregulation and how you feel. They are simply not the whole cramping story, and treating them as though they were leads athletes to increase intake season after season without the problem shifting.
Build your own data
Given a research base that cannot yet give you a personalised answer, the useful move is to collect your own.
Log your longer and harder sessions for three months with four fields: date, session type, how the effort felt against expectation, and where you were in your cycle. Add sweat rate on a handful of sessions by weighing yourself before and after. A log covering several cycles is the only dataset that is actually about you, which is more than any published guidance can offer while the evidence base looks the way it does.
Then change one variable at a time. If heat tolerance drops in a predictable window, move the session or the start time before you add more fluid. If gut comfort changes, shift the timing of pre-session food rather than the food itself. Our guide to replenishing electrolytes gives you a baseline to adjust from, and the running hub covers the race-day end.
One rule survives all of this. Nothing new on race day.
The short version
Standard endurance advice is a reasonable starting point and a poor finishing point for women. Cover energy availability first, check iron if fatigue persists, treat cramping as a neuromuscular and training-load question rather than only a hydration one, and personalise the rest from your own logged data.
Treat the generic plan as a hypothesis, not an answer. Then get the order right: fuel the training, log the sessions, and carry CrampFix for the cramp that arrives anyway on the day that matters. The running hub is where to start on race-day execution and nutrition supplement planning.
This article summarises the published position as at 14 August 2026. It is general information for athletes and coaches, not individual medical or dietetic advice.