Energy needs on a GLP-1: what Mifflin-St Jeor estimates, what it misses, and why the number falls

The calculator on this page estimates how much energy you use in a day. It is worth knowing exactly what that estimate is made of before you act on it, and worth knowing why, on a GLP-1, it is the least important number the tool produces.

The equation

Mifflin-St Jeor, published in 1990 from measurements in 498 healthy adults (PubMed 2305711):

• Men: 10 x weight (kg) + 6.25 x height (cm) - 5 x age + 5
• Women: 10 x weight (kg) + 6.25 x height (cm) - 5 x age - 161

That gives resting energy expenditure (REE): what you burn lying still, awake, fasted, in a warm room. For a 100 kg, 170 cm, 45-year-old woman it is 1,000 + 1,062.5 - 225 - 161 = 1,676 kcal per day.

Why this equation and not Harris-Benedict? The Academy of Nutrition and Dietetics reviewed the candidates in 2005 and found Mifflin-St Jeor predicted measured REE within 10 percent in 82 percent of non-obese and 70 percent of obese adults, more than any other, with the others tending to overestimate in obesity (PubMed 15883556). Note the flip side: it misses by more than 10 percent in 30 percent of people with obesity. It is an estimate. Indirect calorimetry (breathing into a hood or mouthpiece for 20 minutes) is the measurement, and it is available in some clinics.

The activity multiplier

Total daily energy expenditure (TDEE) is REE times a physical activity level. The FAO/WHO/UNU expert consultation defines the bands: sedentary or light activity 1.40 to 1.69, active or moderately active 1.70 to 1.99, vigorous 2.00 to 2.40 (FAO 2004). The calculator offers points inside those bands. For the example above at 1.4, TDEE is about 2,350 kcal; at 1.75 about 2,930.

This multiplier is the biggest source of error in the estimate, larger than the equation's own error. People overestimate their activity, and a desk job with a gym session three times a week still sits in the sedentary band under the FAO scheme.

What the trials did with it

STEP 1 and SURMOUNT-1 counselled participants to eat 500 kcal per day below their estimated requirement (PubMed 33567185; see what the trials asked participants to do). The calculator shows that figure alongside the estimate so you can see what the published curves were built on. It is displayed as a fact about the trials, not as a target for you. Setting an intake target is a clinician's or dietitian's job, particularly on a medication that already reduces intake substantially.

Why the number falls, and why that is not failure

Resting expenditure is dominated by lean tissue and body size. Lose 15 percent of body weight and REE drops, partly because there is less tissue to run and partly because of adaptive thermogenesis, the extra drop beyond what body composition predicts (see why weight returns). Hall's 2011 model shows the consequence: a fixed reduction in intake produces a decelerating curve, not a straight line, because the gap between intake and expenditure closes as expenditure falls (PubMed 21872751). Re-run the calculator every 10 kg; the estimate at your new weight is lower, and the change is expected.

Why calories matter less than protein on a GLP-1

The medication does the part of the job that calorie counting was invented for: it lowers intake, in the trials by enough to produce a 12 to 18 point difference from placebo. Tracking calories on top of that mostly documents a deficit that is already happening. The failure mode on a GLP-1 runs the other way: intake falls so far, or so unevenly, that protein, fibre, fluids and micronutrients come up short while the scale looks fine. That is why the calculator leads with the protein split and treats energy as context. If you want to track one thing, track protein per meal (see protein targets). If you want to track two, add fluids (hydration and fibre).

When an energy number is worth having anyway

• If loss has stalled for two months or more and you want to check whether intake has crept back towards expenditure (see plateaus).
• If you train hard and suspect under-fuelling: fatigue, poor recovery, disturbed sleep, missed periods.
• If you are planning for maintenance, where the drug's effect on appetite may change and the expenditure estimate at your new weight is the number to plan around.

In each case take the estimate to a dietitian rather than acting on it alone. Compounded GLP-1 products, if that is what you take, are not FDA approved and not interchangeable with the brand products the trials used; the arithmetic here does not depend on which product you are on, but a clinician planning your intake should know.