Do you really have a slow metabolism?

Almost every week someone tells me they have a slow metabolism. They eat little and do not lose weight, so they conclude their body is not working.

In 1992, researchers in New York studied people who swore they could not lose weight on 1200-calorie diets. They measured what those people ate with a laboratory method, without relying on their word. They said they ate a little over 1000 calories a day. They were eating more than 2000. The exercise they reported was 50 per cent more than what they actually did (Lichtman and colleagues, 1992). And their metabolism, once measured, was normal.

This happens without bad intent. The olive oil in the pan, the sauce, the biscuits with your coffee, the weekend, none of it tends to make it into the numbers.

Before you conclude your body is not working, measure. There is a calculator here on the site that tells you how much you burn in a day. Write down what you eat for a week, weighed, and compare it with that number.

When the scale really gets stuck

Then there is the other group. People who weigh their food, stick to it, cut calories hard, lose weight in the first weeks and then stop. They cut a bit more and everything stays still. I have coached cases like these and it is not lack of effort.

What happens there has a name: metabolic adaptation. When you give the body much less food than it burns, it notices and starts saving. It works like a phone in battery-saving mode: it keeps going, but dims the screen and switches off whatever is not urgent.

That saving has several parts:

• You move less without noticing. You gesture less, sit longer, take the lift. This happens on its own, driven by hormones, with no decision involved.
• Muscles become more economical. People who lost 10 per cent of their weight started burning less energy pedalling at the same pace, on the same bike.
• Leptin, a hormone that comes out of fat and tells the brain how much reserve you have, drops. Thyroid hormones drop with it and hunger goes up.
• A lighter body burns less to move, because it carries fewer kilos.

What the studies show

Rudolph Leibel's group at Columbia University did the most careful measurements there are on this, with people staying at the hospital and every meal controlled. After losing 10 per cent of their weight, their expenditure sat 10 to 15 per cent below what the new body predicted (Leibel and colleagues, 1995). When they were given leptin, the muscles went back to burning as before and part of that saving disappeared (Rosenbaum and colleagues, 2005).

In the Minnesota experiment, in 1944, 36 men ate half their calories for six months. Resting expenditure fell about 40 per cent. Most of it came from being smaller and moving less, but a slice was left over, a few dozen calories a day, that the lost tissue did not explain (Müller and colleagues, 2015).

And there is the extreme case of the Biggest Loser contest. Six years after losing dozens of kilos in a few months, the contestants were burning at rest about 500 calories a day less than expected for the body they had (Fothergill and colleagues, 2016). Those are cuts and training loads nobody should copy.

In athletes the picture goes further. With a lot of training and little food, the body switches functions off to save: thyroid hormones drop, recovery gets worse and in women the menstrual cycle can stop. The International Olympic Committee gave it a name, relative energy deficiency in sport (Mountjoy and colleagues, 2018).

But the body does not run on air

This adaptation has a ceiling. In the studies it sits between a few dozen and some 300 calories a day, and reached 500 in the extreme case of the contest. That is enough to slow down an aggressive diet. It is not enough to make someone gain weight on 1200 calories.

The studies also show this saving appears while you are in a deficit and fades once your weight stabilises (Martins and colleagues, 2020). The body does not stay broken forever.

And when the scale gets stuck for weeks on end, there is almost always more in the mix: water retention hiding the fat you lost, and forkfuls that never make it into the numbers, because hunger went up.

What to do with this

If you are on your own, measure first. Calculator, food weighed for a week, numbers done. And be wary of brutal cuts: they give fast results at the start and end in a body in saving mode.

If you are my client, your job is to follow the plan and tell me how you feel. Hunger, sleep, strength in training, menstrual cycle. Adjusting the plan is my job and the nutritionist's, and that is why we track your numbers.

References

• Lichtman SW and colleagues. Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. New England Journal of Medicine, 1992.
• Leibel RL, Rosenbaum M, Hirsch J. Changes in energy expenditure resulting from altered body weight. New England Journal of Medicine, 1995.
• Rosenbaum M and colleagues. Low-dose leptin reverses skeletal muscle, autonomic, and neuroendocrine adaptations to maintenance of reduced weight. Journal of Clinical Investigation, 2005.
• Müller MJ and colleagues. Metabolic adaptation to caloric restriction and subsequent refeeding: the Minnesota Starvation Experiment revisited. American Journal of Clinical Nutrition, 2015.
• Fothergill E and colleagues. Persistent metabolic adaptation 6 years after The Biggest Loser competition. Obesity, 2016.
• Martins C and colleagues. Metabolic adaptation is an illusion, only present when participants are in negative energy balance. American Journal of Clinical Nutrition, 2020.
• Mountjoy M and colleagues. IOC consensus statement on relative energy deficiency in sport (RED-S). British Journal of Sports Medicine, 2018.