Do you have a slow metabolism? What the measurements say

Everyone knows someone who eats whatever they like and stays thin, and someone who "gains weight looking at bread". The usual explanation is metabolism: you are born with a fast one or a slow one, end of story. Scientists have measured this for decades, in sealed rooms and with specially labeled water that tracks every calorie burned. The measurements tell a different story.

What "metabolism" means

Most of the calories you burn in a day go to keeping you alive: heartbeat, breathing, brain, warmth, repair. That resting burn is about two thirds of the total for a typical office worker. The rest goes to moving and to digesting food.

Resting burn is not a personality trait. In a study of 150 Scottish adults, 63 percent of the differences between people were explained by lean mass (muscle, organs, bone), another 6 percent by fat mass, 2 percent by age. About a quarter was left unexplained, which works out to a couple hundred calories a day between two people of the same build, not a thousand. There is no "slow metabolism" type hiding in there.

It did not slow down at 30

The biggest study of daily energy burn pooled measurements from 6,421 people, from babies of 8 days to adults of 95, in 29 countries. Once body size was accounted for, metabolism was flat from age 20 to 60. It only begins to fall after 60, by under one percent a year. The one group burning calories unusually fast for their size was one-year-olds.

So the weight that arrives in your thirties is not a slower furnace. It is more sitting, shorter sleep, bigger portions and more stress, often all at once.

Where the real differences hide

Fidgeting. In a Mayo Clinic experiment, 16 lean volunteers were overfed by 1,000 calories a day for 8 weeks. Fat gain varied 10-fold between them. The ones who gained least had, without meaning to, started moving more: standing, pacing, fidgeting. That unconscious movement swung by several hundred calories a day and predicted who stayed lean.

Appetite. The best-known "obesity gene", FTO, was confirmed in a study of 38,759 people. The 16 percent of Europeans who carry two copies of the risk version weigh about 3 kg more on average than people with none. When researchers looked at why, the carriers burned the same calories at rest; they ate more, and chose more energy-dense food. The gene works through hunger.

A ceiling. Among 332 adults in five countries, from Ghana to the United States, the most active people did not burn proportionally more. Past a moderate level of activity, total daily burn flattened out. The body adjusts.

The one way to actually get a slower metabolism

Crash dieting. Researchers followed 14 contestants of The Biggest Loser, who had lost an average of 58 kg in a few months on the show. Their resting burn dropped by about 600 calories a day during the show. Six years later, with much of the weight back, it was still about 500 calories a day below what their size predicted. The body remembers a famine and defends against the next one.

Slow loss, enough protein, and strength training to keep muscle soften that response. A plan a person can live with beats any plan that ends. Why the weight comes back after a diet, and what helps keep it off, is in its own article.

What your DNA adds

Not a furnace setting. What the file does hold are leanings in appetite and fullness: FTO, MC4R and a few dozen smaller markers that nudge how quickly you feel full and how much you reach for. Knowing you lean toward a big appetite does not change the physics, but it does tell you which lever to pull: protein and fiber at meals, sleep, and food that is not sitting on the counter. We covered FTO in detail in its own article.

Sources

1. Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine. American Journal of Clinical Nutrition, 2005. PubMed 16280423
2. Pontzer H, Yamada Y, Sagayama H, et al. Daily energy expenditure through the human life course. Science, 2021. PubMed 34385400
3. Levine JA, Eberhardt NL, Jensen MD. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science, 1999. PubMed 9880251
4. Frayling TM, Timpson NJ, Weedon MN, et al. A common variant in the FTO gene is associated with body mass index and predisposes to childhood and adult obesity. Science, 2007. PubMed 17434869
5. Speakman JR, Rance KA, Johnstone AM. Polymorphisms of the FTO gene are associated with variation in energy intake, but not energy expenditure. Obesity, 2008. PubMed 18551109
6. Cecil JE, Tavendale R, Watt P, Hetherington MM, Palmer CN. An obesity-associated FTO gene variant and increased energy intake in children. New England Journal of Medicine, 2008. PubMed 19073975
7. Pontzer H, Durazo-Arvizu R, Dugas LR, et al. Constrained total energy expenditure and metabolic adaptation to physical activity in adult humans. Current Biology, 2016. PubMed 26832439
8. Fothergill E, Guo J, Howard L, et al. Persistent metabolic adaptation 6 years after "The Biggest Loser" competition. Obesity, 2016. PubMed 27136388