Why We Still Don't Know What to Feed Half the Population — and What Honest Nutrition Does About It

There is a strange gap at the center of nutrition science. We can tell you, with reasonable confidence, how a cup of coffee affects the average man’s metabolism. Ask the same question for a woman, and a fair answer has to begin with: it depends where she is in her cycle — and, honestly, we haven’t studied that nearly enough.

This isn’t a failure of any one lab. It’s the shape of the field. And menstrual-cycle nutrition — the question of whether, and how, what a woman needs to eat shifts across the month — is one of the clearest places to see it.

The gap is real, and it has a date on it

For most of modern medicine, “the human body” in a study meant a man’s body. Women of reproductive age were routinely left out of clinical trials — partly out of caution after past drug harms, partly because a monthly hormonal cycle was treated as an inconvenient source of “variability” to be excluded rather than understood.

It took a law to change the default. The NIH Revitalization Act of 19931 mandated, for the first time, that U.S. federally funded research include women and analyze results by sex. Before it, a U.S. Government Accountability Office review found that trials for 60% of approved drugs hadn’t included enough women to say much about them — and before 1990, “women’s health” was treated as more or less synonymous with reproductive health.

The correction is recent, and it is incomplete. A 2021 analysis in the Journal of Women’s Health2 found that NIH research funding still isn’t aligned with disease burden: across conditions that fall mainly on one sex, funding skews toward men’s in roughly three-quarters of cases. The consequences show up as an evidence gap that researchers are only now charting3.

What that gap looks like up close

Here is the gap made concrete. When you go looking for the best available studies on how the menstrual cycle affects appetite, metabolism, and blood sugar, this is what you find:

• The gold-standard study on how the cycle changes the brain’s insulin action — a rigorous design using metabolic clamps — enrolled eleven women (Kullmann et al., 2023, Nature Metabolism4).
• A key study on cyclical insulin sensitivity: twelve women.
• The neuroimaging study everyone cites on food cravings and the cycle: twelve women (Frank et al., 2010, Brain Research5).

These aren’t bad studies. They’re careful, and they’re the foundation the field stands on. But they’re small — not because the questions are unimportant, but because almost no one funded the work. That small-n is the gap, in miniature.

What the evidence actually supports (and what it doesn’t)

So what can we honestly say? A modest, real, and still-uncertain amount.

Energy and appetite. A 1995 study in the American Journal of Clinical Nutrition6 found women ate more in the luteal phase (the roughly two weeks before a period) than the follicular phase — about 9.3 vs 8.0 MJ/day. A 2025 meta-analysis7 put the average difference near ~168 kcal/day — but with wide variation between studies, and at least one recent, well-controlled study found no consistent difference at all. Directionally real; modest; contested.

Cravings. The brain does seem to respond to food differently across the cycle — but not the way the headlines say. The neuroimaging evidence points to a shift in selectivity (in the luteal phase, reward centers respond specifically to high-calorie cues), not simply “more cravings.” A subtle, human finding — and one worth getting right rather than flattening.

Period pain. Here the ground is a bit firmer: a double-blind crossover trial8 and a 2024 meta-analysis9 both found omega-3 supplementation reduced menstrual pain and painkiller use — though the reviewers themselves flag that many of the underlying studies are small or lower-quality.

Blood sugar. For women with type-1 diabetes, the cyclical shift in insulin sensitivity is real enough to matter clinically (Milionis et al., 202310) — a reminder that this isn’t only wellness-app territory; for some people it’s daily management.

And what it does not support: the elaborate “cycle-syncing diet” — the color-coded plans telling you to eat specific nutrients in each of four phases. Even Switzerland’s national nutrition body is blunt about it: “there is still no official recommendation for a diet that is adapted to your cycle.” The physiology has real kernels; the prescriptive four-phase diet industry has run far ahead of the science.

The loop — and how to break it

Here’s the trap. Because the evidence is thin, the responsible-sounding move is to say nothing. But saying nothing means no demand, no attention, no funding, no studies — and the evidence stays thin. The caution meant to protect women ends up freezing the very research that would serve them. That’s the loop.

There’s a way out that doesn’t require pretending. You can build for women’s health and tell the truth about the state of the science — in fact, doing both at once is the stronger move. A product that says “your luteal phase may nudge your appetite up a little — here’s something heartier and genuinely satisfying, and here’s what we do and don’t yet know” respects the person, supports her actual experience, and raises awareness of the gap itself. It can’t be debunked, because it never overclaimed. And every honest, well-sourced product that takes women’s physiology seriously is one more piece of the demand signal that, eventually, funds the studies.

How we build it

That principle is the whole design of what we’re building. We don’t sell certainty we don’t have. Where the evidence is solid — iron after menstrual blood loss, omega-3 for cramps — we say so plainly. Where it’s real but modest — the luteal appetite shift — we offer, we don’t prescribe. Where it’s thin — most of the ovulation-specific claims — we say that, too, and simply suggest something good. And the outright unproven “cycle-syncing” prescriptions we don’t dress up as science at all.

It would be easier to print a confident four-phase chart. It would also be wrong, and — for a field that has been failed by overconfidence and neglect in equal measure — a little insulting. Half the population has waited long enough for answers that respect both their bodies and their intelligence. Building honestly, now, even while the science is still catching up, is how the answers finally get funded.