Does your metabolism slow down with age?

In 2021 a team published an analysis in Science of daily energy expenditure measured by doubly labelled water, across ages 8 days to 95 years. Doubly labelled water matters here: it measures what people actually burn while living their normal lives, rather than asking them what they did or estimating from a formula. It is the reference method.

Once expenditure was adjusted for fat-free mass, the picture came out in four stages:

The middle row is the surprising one. Between roughly 20 and 60 there is no gradual decline. Not at 30, not at 40, not through the decade most people blame for the weight they gained.

Read that finding carefully, because the headline version of it is wrong in a way that matters. What stays stable is expenditure adjusted for fat-free mass. That is not the same as the number of calories you personally burn.

Fat-free mass is muscle, organs, bone and water. Most adults lose muscle across middle age, especially if they stop training and move less. Less muscle means less tissue to keep running, so your actual daily expenditure does fall. What the study shows is that the tissue you still have is not burning energy any more slowly than it did at 25.

So the honest version is this: your metabolism is not slowing down, but you may be carrying less of the thing that does the burning. One of those is out of your control. The other largely is not.

A fair question, and worth answering directly rather than hoping nobody notices. Mifflin-St Jeor includes an age term: it subtracts 5 kcal for every year. For the same 175 cm, 70 kg man at a moderate activity level, the calculator produces:

That is 150 kcal of resting metabolism across thirty years, at a constant weight. Two things explain why the equation does this without contradicting the study.

First, the equation predicts from total weight, not from lean mass. It has no way to see your body composition, so the age term is standing in for the muscle that an average person loses over those years. It is a population-level correction for something the formula cannot measure directly.

Second, 150 kcal over three decades is small. It is less than the difference between choosing one activity level and the next, which moves the result by roughly 290 kcal in a single step. Age is in the equation, but it is nowhere near the biggest thing in it.

Not this one, and not really any of the common ones. Mifflin-St Jeor was derived from 498 healthy adults aged 19 to 78. Nobody under 19 was in the sample it was built from, so applying it to a 14-year-old is using an equation outside the range anyone has checked it against.

The 2021 data shows why that matters rather than being a technicality. Adjusted expenditure is well above adult levels in early childhood and only settles to adult values at around 20. A growing body is running a different problem: it is building tissue, not just maintaining it, and an equation fitted to adults has no term for that.

The practical answer is that a teenager who wants a calorie target should get it from a doctor or a registered dietitian, not from any calculator on the internet, including ours. That is not us being cautious for the sake of it. It is that the number would be produced by a formula that was never tested on anyone their age.

If the tissue is not slowing down, the weight has to come from somewhere. Usually a combination of ordinary things:

• Less muscle. The main lever, and the one resistance training addresses directly.
• Less incidental movement. Desk jobs, driving, fewer sports. This is often a larger change than people realise, and it shows up in your activity level rather than in your metabolism.
• Eating the same as before. Habits formed at 25 do not automatically adjust when the rest of life does.

None of these is a metabolism problem, which is genuinely good news, because a metabolism problem would not be something you could do much about.

Recalculate rather than assume. If your weight has changed, your maintenance figure has changed with it, and the number you were using at 30 is not the number you need at 45. Run it again in the TDEE Calculator and be honest about the activity level, since that is where most of the real change hides.

Then treat it as a starting point and confirm it against the scale over two or three weeks, the same as at any age. Your real maintenance level is measured, not predicted, and that has not changed either.

And if you want the single highest-leverage thing: keep the muscle. It is the only item on the list that directly defends the number the equation is trying to estimate.