Why Intermittent Fasting May Affect Women and Men Differently

The same eating schedule may not land the same way in every body. A new review of the research argues that intermittent fasting deserves to be read through the lens of biological sex, because women and men appear to respond to going without food in ways that overlap in some places and diverge sharply in others.

Intermittent fasting has become one of the most talked-about ways to eat, or rather not eat. The idea is simple: restrict food to certain windows of time, whether that means an eight-hour daily eating slot, alternate days of very low intake, or occasional longer fasts. The appeal is broad, and so are the claims made for it, from better blood sugar control to fat loss to slower aging. Most of the enthusiasm treats the practice as if it works more or less the same for anyone who tries it. A review published in the journal Nutrients pushes back on that assumption.

The authors gathered together the existing human and animal research on intermittent fasting and read it with one question in mind: does biological sex change the picture? This is what researchers call a narrative review, meaning the team synthesized and interpreted a body of published work rather than running a new experiment or pooling numbers into a single statistical estimate. That distinction matters for how much weight the conclusions can carry, and the authors are candid about it.

What they describe is a landscape of shared benefits and diverging costs. On the shared side, both women and men seem to gain from fasting in familiar ways. Body composition tends to improve, meaning shifts toward less fat and better distribution, and several markers of heart and metabolic health move in a favorable direction. These are the outcomes that have made fasting popular, and the review does not dispute that they appear across the sexes.

The divergence shows up in how the body's hormonal systems handle the stress of going without food. Fasting is, at its core, a period of reduced energy coming in. The review reports that women appear more sensitive in their reproductive and neuroendocrine function to that energetic shortfall. The neuroendocrine system is the bridge between the brain and the body's hormone-producing glands, and it is finely tuned to whether energy is plentiful or scarce. When energy availability drops too low, especially when combined with heavy exercise or during vulnerable reproductive stages, the female system seems more likely to register that as a signal to dial back reproductive function.

Men, by the review's reading, show a different pattern. They too undergo measurable hormonal adaptations under fasting, including changes in testosterone dynamics, but their functional outcomes appear to hold up despite those shifts. In other words, the internal numbers move without the same visible consequences. The authors are careful not to frame this as one sex tolerating fasting better than the other. The point is that the mechanisms and the trade-offs differ, and that averaging across everyone hides those differences.

Nothing in the review reduces to a single rule, because the response depends on many things at once. The specific fasting protocol matters, as does whether the diet during eating windows is nutritionally adequate, a person's baseline metabolic health, their life stage, and the clinical situation they bring to it. A young athlete training hard is not in the same position as a middle-aged person managing a chronic condition, and the review treats those contexts as genuinely distinct rather than variations on one theme.

This fits into a longer conversation in medicine about sex as a biological variable. For decades, much research was conducted in male animals or predominantly male participants and the results were assumed to generalize. The correction has been slow, and nutrition science is part of it. What this review adds is a focused argument that intermittent fasting is a domain where the sexes may respond differently enough that one-size prescriptions are hard to justify.

The review's own limits are important and openly stated. Studies that directly compare women and men under the same fasting protocol are scarce. Much of what the authors conclude is inferred by placing separate studies of men beside separate studies of women, which is a weaker foundation than a head-to-head comparison. A narrative review also reflects the choices of its authors in what to include and how to interpret it, without the systematic search and statistical pooling that give meta-analyses more authority. And because the underlying evidence mixes human and preclinical work, some mechanistic claims rest partly on animal studies whose relevance to people remains uncertain. None of the findings here establish that fasting causes any particular outcome in either sex; they describe patterns across a literature.

What this means in practice is modest but real. The review supports reading fasting advice with a degree of skepticism toward blanket claims, and it flags that low energy availability, particularly alongside intense training or during reproductive vulnerability, is where caution is most warranted for women. It does not tell anyone whether to fast, how long, or how often, and it is not a substitute for individual guidance. Someone considering intermittent fasting, especially with a health condition, during pregnancy or breastfeeding, or while training seriously, has good reason to discuss it with a clinician who knows their situation.

The larger takeaway is a call to the research community as much as to the public. If fasting responses genuinely track with sex, then studies that ignore that variable will keep producing averages that describe no one in particular. Building sex into the design of future fasting trials, the authors argue, is the way to turn a popular practice into advice that fits the person following it.