Does dieting slow your metabolism?

A little. Most of the drop is simply a smaller body: each pound lost lowers your daily burn by about 10 calories. Dieting adds a small extra dip on top, about 90 calories a day after a 30-pound loss in one study, and it shrank by more than half once people held their weight steady. Your metabolism has been resized for a smaller you, and that’s all.

The fear comes mostly from one famous study. Six years after The Biggest Loser, 14 contestants burned about 500 calories a day less at rest than their size predicted. But they had lost an average of 128 pounds in a 30-week TV competition, far from any ordinary diet.

The simple version

Shrink a house and the heating bill drops. The furnace still works fine. There’s just less house to heat.

The studies

• 2020, 71 people with obesity. They ate 1,000 calories a day for 8 weeks and lost about 30 pounds. Their resting burn was about 90 calories a day below what their new size predicted. After four weeks at a steady weight it was about 40, and in the 45 people measured every time it was gone at one year. The dip didn’t predict who regained.
• 2016, The Biggest Loser. 14 contestants lost an average of 128 pounds in 30 weeks. Six years on they had regained about 90 of it, and their resting burn was still about 500 a day below what their size predicted. It’s the far end of what’s been measured.
• The size effect. NIH scientists’ model of weight change has a rule of thumb that every 10 calories a day you cut for good takes off about a pound in the end. Turned around, each pound lost lowers your burn by about 10 calories a day.

What to do

• Lose at a pace you can hold, with enough protein and some lifting.
• After a long diet, spend a few weeks at your new maintenance before cutting again. Measure it on the real TDEE calculator.