Understanding Metabolism: What You Can and Can't Change

Metabolism gets blamed for a great deal. A slow one explains stubborn weight; a fast one explains a friend who eats anything. Products promise to boost it, and diets are accused of wrecking it.

Most of this is wrong, and the corrections are more useful than the myths. Metabolism is simply the sum of all chemical processes keeping you alive — and in the context of weight, it means total daily energy expenditure. It varies less between people than commonly assumed, and it is far harder to boost than the marketing suggests.

Total daily energy expenditure breaks into four parts of very different sizes, and knowing the proportions immediately clarifies which levers matter.

Between two people of the same age, sex, height and body composition, basal metabolic rate typically varies by around 10% in either direction. That is real, and it is not the two-fold difference folklore implies.

In absolute terms, a 10% difference on a 1,600 kcal BMR is about 160 kcal a day — meaningful over a year, and considerably smaller than the differences people attribute to it.

Where large differences do appear, the usual explanation is body composition rather than metabolic luck. Lean tissue is more metabolically active than fat, so someone carrying more muscle at the same weight genuinely burns more. That is measurable with the Lean Body Mass Calculator, and it is not mysterious.

The component that differs most between individuals is the one nobody markets: non-exercise activity thermogenesis. Fidgeting, standing, pacing, gesturing, general restlessness.

In overfeeding studies, NEAT has been shown to vary by several hundred calories a day between individuals given identical surplus calories — some people spontaneously moved more and gained far less. That variation dwarfs any supplement effect.

It also falls during a calorie deficit, largely unconsciously, which is a substantial part of why weight loss plateaus. You are not imagining being less inclined to move; that is a genuine adaptive response.

A long list of interventions have effects that are unmeasurable, trivial, or purely marketing.

Green tea, cayenne, apple cider vinegar and most thermogenic supplements produce effects too small to detect against normal daily variation. Eating six small meals rather than three does nothing — the thermic effect scales with how much you eat, not how many sittings.

Even building muscle is oversold here. A kilogram of muscle burns roughly 13 kcal a day at rest, not the 50 or 100 frequently claimed. Gaining five kilograms of muscle — years of work — adds perhaps 65 kcal a day. Muscle is enormously worth having; it is just not a metabolic transformation.

Losing weight does reduce energy expenditure, and by slightly more than body size alone predicts. This is adaptive thermogenesis, and it is genuine.

The size is usually in the range of 10–15% below what would be expected for the new body weight, driven by hormonal changes — reduced leptin and thyroid hormone — plus reduced NEAT. It is the mechanical reason a deficit stops working and a plateau arrives.

But it is not permanent metabolic damage, and it does not make weight loss impossible. It means the target needs recalculating as you get lighter, which the Calorie Calculator will do at your current weight rather than your starting one.

The claim that eating too little causes your body to hold onto fat is a distortion of something true. Severe restriction does reduce expenditure — but never by enough to stop weight loss while a genuine deficit persists.

The Minnesota Starvation Experiment, the classic reference here, found substantial metabolic adaptation in men on roughly half their maintenance intake. They still lost about 25% of their body weight. Adaptation slowed the process; it did not reverse it.

When someone eats very little and does not lose weight, the overwhelmingly common explanation is under-reporting of intake — a well-documented phenomenon that is not dishonesty but a genuine difficulty in estimating portions. The response is more careful measurement, not more restriction.

Everything above describes healthy adults, and there are real medical conditions that alter metabolic rate — which is why 'it is probably not your metabolism' should never become 'it is never your metabolism'.

Hypothyroidism reduces metabolic rate and is common, particularly in women, and it is diagnosed with a simple blood test. Cushing's syndrome, polycystic ovary syndrome and some other endocrine conditions affect body composition and energy balance. Several widely prescribed medications — certain antidepressants, antipsychotics, corticosteroids and beta blockers — are associated with weight gain.

The distinguishing features of a genuine problem are usually accompanying symptoms rather than weight alone: persistent fatigue, cold intolerance, hair or skin changes, unusual menstrual patterns. If those are present alongside unexplained weight change, that is a reason to see a doctor rather than to adjust your calorie target.

The honest list is short and unglamorous, but the items on it work.

Move more throughout the day. Walking, standing, taking stairs and generally being less sedentary adds more to total expenditure over a week than most training programmes, at almost no recovery cost.

Build and preserve muscle — not for the resting burn, which is modest, but because it maintains function, protects metabolic rate during weight loss, and makes everything else easier. Adequate protein and resistance training do this.

And sleep properly. Restriction raises appetite, impairs glucose tolerance, and shifts weight loss away from fat toward lean mass. It is the lever people most often ignore while searching for a supplement.