Men burn more because there is more of them to burn#
A 70 kg man and a 70 kg woman do not burn the same number of calories a day, and the reason is unglamorous. When whole-body MRI was run on 468 adults aged 18 to 88, men averaged 33.0 kg of skeletal muscle against women's 21.0 kg — 38.4 percent of body mass versus 30.6 percent1. Muscle and organs are the tissue that costs money to keep running. Fat is close to free. Two people can post the same number on a bathroom scale and be carrying twelve kilograms' difference in the machinery that generates the bill.
Take that difference out of the arithmetic and the "slower female metabolism" mostly evaporates — and in the largest free-living measurement anyone has made of the question, it does not merely evaporate, it reverses. That result is the reason this article exists, and it is not the one most people expect. TDEE explained covers what daily burn is assembled from; this is about the single input people most often mistake for a verdict on their body.
The rebuttal is older than most of the internet#
The idea that women are metabolically disadvantaged has been getting argued down in the nutrition literature since at least 1982, when a review in the American Journal of Clinical Nutrition ran under the title "Body composition and resting metabolic rate: the myth of feminine metabolism." Its argument was that the field had two incompatible habits: a clinical one that indexes resting rate to body surface area, which produces sex-specific normal values and makes women look low, and a physiological one that indexes it to metabolically active tissue, which does not2.
The physiological perspective is one of an asexual metabolism that differs in the mass of active cells.
That sentence is forty-four years old and still carries the whole load. It also makes a testable prediction: match two people on lean tissue and the sex term should stop earning its keep. Researchers have tested it repeatedly, and the results are close enough to unanimous on the big claim and genuinely split on the small one.
What survives the adjustment — and what doesn't#
Absolute burn is not in dispute. Among 190 collegiate athletes, resting metabolic rate averaged 2,595 ± 433 kcal/day in the 98 men and 1,709 ± 308 in the 92 women, a gap of nearly 900 calories. Normalize those same measurements to body mass or to fat-free mass and the difference stops reaching significance — p = 0.064 against body mass, p = 0.084 against fat-free mass — with body mass the strongest single predictor in both sexes4.
A respiratory-chamber study reached a different verdict. Across 235 healthy adults — 114 men, 121 women — 24-hour energy expenditure ran 124 ± 38 kcal/day higher in the men after adjustment for fat-free mass, fat mass and age, and the authors put sedentary 24-hour expenditure roughly 5 to 10 percent lower in women once body composition, age and activity were accounted for3.
| Study | Who | Measurement | Sex difference after adjusting for body composition |
|---|---|---|---|
| Ferraro 1992 | 235 adults, respiratory chamber | Sedentary 24-h energy expenditure | Men higher by 124 ± 38 kcal/day |
| Jagim 2023 | 190 collegiate athletes | Resting metabolic rate | Not statistically significant (p = 0.084) |
| Tooze 2007 | 450 adults 40–69, doubly labeled water | Free-living total energy expenditure | Women higher by 182 kcal/day |
Those three rows are not three answers to one question. They are answers to three questions that look alike, and the thing separating them is named in the middle column.
Put the same question to free-living people and the sign flips#
The third row is the one worth slowing down for. In 450 men and women aged 40 to 69, total energy expenditure was measured by doubly labeled water — the reference method for free-living burn, which tracks the disappearance of two isotopes from your body water rather than modelling anything. Unadjusted, women's daily expenditure came in 591 kcal/day below men's, which is what you would predict from body size alone. Adjusted for fat-free mass, women's expenditure was 182 kcal/day higher than men's. The authors traced it to activity: physical activity energy expenditure, adjusted for fat-free mass, ran 188 kcal/day greater in the women5.
Read Ferraro and Tooze side by side and the disagreement resolves into a design difference rather than a contradiction. Ferraro's measurement is explicitly sedentary — a sealed chamber where the activity term is suppressed by the apparatus. Tooze's is free-living, where activity is roughly a quarter of the total and belongs to whoever earns it. The residual sex gap in resting metabolism is small and contested; the sex gap in daily movement is not fixed by biology at all, and in this middle-aged American cohort it ran in women's favor by enough to swamp the resting difference. Which is a useful corrective to a common reading — the largest genuinely variable term in your day is the one nobody assigns to sex, and it is covered in NEAT: the calorie burn you're ignoring.
One caution on generalizing the flip. These were 450 middle-aged adults in one American study, and physical activity patterns are cultural and occupational before they are anything else. The finding does not license "women burn more than men." It licenses something narrower and more durable: the sign of the adjusted difference is set by activity, not by chromosome, so any claim about the gap that ignores what the two groups were doing all day is measuring the wrong thing.
Your calculator prices your sex at 166 calories flat#
This is where the abstraction turns into a number on a screen. The Mifflin-St Jeor equation, the standard behind most calculators, predicts resting energy expenditure as 10 × weight + 6.25 × height − 5 × age, then adds 5 for men and subtracts 161 for women6. Feed it identical weight, height and age and it will hand a woman a number exactly 166 calories lower — every time, for everybody.
That constant is doing the job of a body-composition measurement it never took, and because it is a constant, it cannot scale. A heavily trained woman carrying unusual lean mass gets the same 166-calorie deduction as a sedentary woman of the same weight, and a soft, untrained man gets the same 5-calorie credit as a rower. The deduction is right on average and wrong in a specific, predictable direction for anyone whose body composition is unusual for their sex — which is exactly the population most likely to be looking up their number in the first place.
Worth noticing: the same 1990 paper reports that fat-free mass was the best single predictor of resting expenditure it found, and publishes the corresponding formula — 19.7 × fat-free mass + 413 — with no sex term in it at all. The sex term exists because fat-free mass is expensive to measure and sex is free. Which equation to actually use, and where each one fails, is worked through in Mifflin-St Jeor vs Harris-Benedict.
The menstrual cycle moves the number a little, and the evidence is thinner than the claim#
The day-to-day question women actually ask is about the cycle, and here the literature is small and pulling in two directions. A meta-analysis of 26 studies covering 318 women found a small but significant effect favoring higher resting metabolic rate in the luteal phase (effect size 0.33, 95% CI 0.17 to 0.49, p < 0.001) — but in studies published since 2000 the same effect shrank and stopped reaching significance (0.23, 95% CI −0.00 to 0.47, p = 0.055)7.
A more recent systematic review of seven studies published between 2020 and 2025, totaling 124 participants, found five reporting a luteal-phase elevation and four of those reaching significance, with the size put at roughly 30 to 120 kcal/day, or 3 to 5 percent. Its authors flag the problem plainly: several included studies confirmed cycle phase by self-report rather than hormonal measurement, most were small, and a 3 to 5 percent shift sits inside the normal day-to-day biological and methodological variability of a resting measurement8.
So the fair summary is: probably real, probably small, probably around the size of the noise floor of the instrument used to detect it. Thirty to 120 calories is less than the spread between two consecutive readings on the same person, and far less than the error already sitting in any calculated target. If your intake and hunger track your cycle, that is a real pattern worth eating around; it is more likely driven by appetite than by a measurable change in the furnace.
Using a number that was never really about sex#
The practical shape of all this is short.
The gap is tissue, and tissue is partly visible to you. Body composition, not the letter on the form, is the variable that moves resting burn. That is also why copying someone else's target fails so reliably, a problem that scales with body size and is laid out in why height and body size drive calorie burn.
Treat the sex constant as a prior, not a finding. A formula that deducts 166 calories from every woman is encoding an average body composition. If yours is not average — and trained, older, or post-menopausal bodies frequently are not — the constant is the part of the estimate most likely to be wrong for you. What actually happens to burn with age and menopause is a separate and better-measured question, covered in does your metabolism really slow with age.
Then measure yourself out of the argument. Whatever the equation says, three to four weeks of weight trend against a steady intake will locate your real maintenance more precisely than any sex-adjusted formula can — the method in how to find your true maintenance calories.
Cunningham's 1982 framing still holds up: there is one metabolism, and people differ in how much active tissue they are running it on. Sex predicts that quantity well enough to be worth a term in an equation, and badly enough that it should never be read as a diagnosis.
FAQ#
Do women really have slower metabolisms than men?#
Not in the way the phrase implies. Absolute resting burn is clearly lower — about 1,709 versus 2,595 kcal/day in one study of 190 collegiate athletes — but that gap tracks body composition, and men average 33.0 kg of skeletal muscle to women's 21.0 kg. Adjust for fat-free mass and the difference either disappears or shrinks to a contested 3 to 10 percent, depending on the population and whether activity is included in the measurement.
How many extra calories do you burn during the luteal phase?#
Roughly 30 to 120 kcal/day, or 3 to 5 percent, in the studies that found an effect at all — and a 2020 meta-analysis found the effect stopped reaching statistical significance in trials published after 2000. That size overlaps the normal day-to-day variability of a resting measurement, so it is real enough to acknowledge and too small to build a target around.
Why does the Mifflin-St Jeor equation subtract 161 calories for women?#
Because it adds 5 for men and subtracts 161 for women, giving a flat 166-calorie offset at identical weight, height and age. That constant is a cheap stand-in for the body-composition measurement the equation never takes. The same paper's fat-free-mass formula — 19.7 × fat-free mass + 413 — has no sex term at all, which is the clearest evidence that the sex term was a proxy the whole time.
Sources#
- Janssen I, Heymsfield SB, Wang ZM, Ross R. Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr. J Appl Physiol. 2000;89(1):81-88.
- Cunningham JJ. Body composition and resting metabolic rate: the myth of feminine metabolism. Am J Clin Nutr. 1982;36(4):721-726.
- Ferraro R, Lillioja S, Fontvieille AM, Rising R, Bogardus C, Ravussin E. Lower sedentary metabolic rate in women compared with men. J Clin Invest. 1992;90(3):780-784.
- Jagim AR, Jones MT, Askow AT, et al. Sex differences in resting metabolic rate among athletes and association with body composition parameters: a follow-up investigation. J Funct Morphol Kinesiol. 2023;8(3):109.
- Tooze JA, Schoeller DA, Subar AF, Kipnis V, Schatzkin A, Troiano RP. Total daily energy expenditure among middle-aged men and women: the OPEN Study. Am J Clin Nutr. 2007;86(2):382-387.
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247.
- Benton MJ, Hutchins AM, Dawes JJ. Effect of menstrual cycle on resting metabolism: a systematic review and meta-analysis. PLoS One. 2020;15(7):e0236025.
- Hurtová A, Gimunová M, Beníčková M. Resting metabolic rate fluctuations across the menstrual cycle: a systematic review. Front Physiol. 2026;17:1778735.
