Is my weight gain because I have a slow metabolism?

A large 2021 study that measured energy expenditure with doubly labelled water in more than 6,400 people overturned a common belief: metabolism does not slowly decline through adult life. Once the researchers accounted for body size, expenditure stayed stable between the ages of 20 and 60, and only began to fall after that. The feeling that your metabolism 'slowed down' at 35 rarely matches the measurements.

Individual differences exist, but they are small. A review of what drives obesity concluded that variation in resting metabolism and energy turnover, on its own, is rarely enough to explain why someone gains weight. Body size and muscle mass account for most of the difference between two people's expenditure.

A classic study shows where the misunderstanding usually starts. People convinced they had a sluggish metabolism and could not lose weight turned out to have entirely normal expenditure, within five percent of what was predicted. The gap was in the reporting: they underestimated their food intake by an average of 47 percent and overestimated their physical activity.

One caveat: some medical conditions, such as an underactive thyroid, genuinely affect metabolism. If you suspect one, that assessment belongs with a doctor, not a table.

The explanation is comforting. Blaming weight gain on biology lifts the fault from everyday choices, and shame is a poor foundation to change anything from. At the same time, an entire market profits from the story: supplements and programs that promise to 'boost your metabolism' are selling exactly that feeling. And we all carry a measurement error that feeds the myth. We systematically underestimate how much we eat and overestimate how much we move, so a real energy surplus feels inexplicable and gets blamed on metabolism instead.

Your metabolism is mostly set by your body size and how much muscle you carry, not by luck. Start with a realistic estimate of your actual energy needs, and track your intake honestly for a week before drawing conclusions. If the numbers don't add up, a small adjustment to portions will usually do more than any supplement. And if you suspect a medical cause, raise it with a doctor.