Your TDEE: what the number means, and how wrong it can be

What the number is made of

TDEE stands for total daily energy expenditure, and it has two parts. The first is your resting metabolic rate, the energy you would burn lying still all day, which is most of the total for most people. The second is a multiplier for everything else: moving, working, training, digesting.

The resting part is estimated from your sex, age, height and weight. The most accurate general equation is Mifflin-St Jeor, published in 1990, and a comparison of predictive equations found it performed better than the older Harris-Benedict for non-obese and obese adults alike.

The activity multiplier is where most of the error creeps in. It usually runs from about 1.2 for sedentary to somewhere near 1.9 for very active, and the calculator on this site stops at 1.725, and people are famously bad at placing themselves on it. Most of us think we are one step more active than we are.

How wrong it can be, honestly

Mifflin-St Jeor predicts resting metabolic rate within 10 percent of measured values in about 82 percent of people who are not obese, and about 70 percent of people who are. That is good for an equation using four inputs, and the gap between those two figures matters if you are reading this in order to lose weight.

It also means somewhere between one person in five and one in three is off by more than 10 percent, and the equation cannot tell you which one you are. Those figures are for resting rate alone, so once the activity multiplier is applied the error on a full-day estimate is wider still. For someone with a 2,000 calorie TDEE, a 10 percent error is 200 calories a day, which is the difference between losing weight steadily and not losing it at all.

Body composition is the main reason. The equations assume a typical ratio of muscle to fat for your height and weight. If you carry considerably more muscle than average, or considerably less, your real rate sits outside the prediction.

The right way to use it

Use the estimate as a hypothesis and let your own data correct it. Eat at roughly the estimated number for two to three weeks, weigh yourself consistently, and watch the trend rather than any single reading.

If your weight is stable, that is your real maintenance level and it is worth more than any equation. If it is drifting up or down when you expected stability, adjust by 100 to 200 calories and give it another fortnight.

There is a floor to this loop. If it is pushing you below roughly 1,500 calories a day as a man or 1,200 as a woman, stop adjusting and talk to a dietitian. Below that it becomes genuinely hard to meet your nutrient needs, and the answer is usually more activity rather than less food.

Two weeks is the minimum because the first week of any change is dominated by water and glycogen, which can swing two or three kilograms without a gram of fat moving either way.

What this has to do with fasting

Fasting does not change your TDEE in any way the equations capture. A shorter eating window does not burn more calories; it usually helps you eat fewer, which is a different and more useful thing.

Short fasts do not slow your metabolism either, at least not in the timeframe people worry about. The two studies that looked disagree: one found resting energy expenditure about 14 percent higher by day three of a fast, another about 8 percent lower at the same point. Both had fewer than a dozen participants. The honest summary is that nobody knows the direction, not that it stays put.

What does lower your metabolic rate is weight loss itself, and that applies to every method equally. In the most extreme case anyone has followed, fourteen contestants from a televised weight-loss competition who lost around 58 kg each, resting metabolic rate was still roughly 700 calories a day below baseline six years later, even though most of the weight had returned. That is an outlier group rather than a forecast for a 10 kg loss, but the direction is worth knowing before you set an aggressive target.