How the estimate is calculated
The calculator estimates basal metabolic rate, the energy your body uses at rest, then multiplies it by an activity factor to give total daily energy expenditure. The equation used here is Mifflin-St Jeor, which performs better than the older Harris-Benedict formula in most validation studies.
Every equation of this kind is a population average applied to an individual. Measured resting metabolic rate can differ from a predicted figure by several hundred calories in either direction, which is why the number is a starting point to test rather than a fact about your body.
Why the number drifts at midlife
Resting metabolic rate tracks lean mass more closely than it tracks age. Muscle is metabolically active tissue, and as muscle declines through the forties and fifties, resting expenditure falls with it. The often-repeated claim that metabolism collapses at menopause overstates the case: large studies of total energy expenditure find it stays relatively stable through midlife and declines later, mostly in step with body composition.
What changes more is where weight is stored and how insulin sensitivity behaves. Our guide to menopause weight gain covers what the evidence actually shows, and resistance training covers the intervention that protects the lean mass this number depends on.
Using a deficit without wrecking the things that matter
A deficit of roughly 500 calories a day is the conventional starting point for gradual loss. Two constraints matter more for women at midlife than the size of the deficit.
- Protein. Intake in the region of 1.2 to 1.6 grams per kilogram of body weight supports muscle retention during weight loss, and that requirement rises rather than falls with age. Our protein calculator works out your range.
- Resistance training. Weight lost without it comes disproportionately from muscle and bone, which lowers your resting expenditure further and raises fracture risk. See menopause and bone loss.
Very low intakes accelerate loss of lean mass, disrupt sleep, and are difficult to sustain. Slower loss that preserves muscle beats faster loss that does not.
What the number cannot account for
Thyroid function alters resting expenditure directly, and an underactive thyroid is common enough in women at midlife that it belongs in the explanation before willpower does. See thyroid and weight.
Several medicines cause weight gain independently of intake, including some antidepressants, beta-blockers and corticosteroids. Sleep loss raises appetite hormones measurably. And GLP-1 medications change the equation entirely by altering appetite rather than expenditure, which is a different mechanism from anything this calculator models.
How to use the result
Take it as a hypothesis. Eat to that figure for two to three weeks, track weight and how you feel rather than reacting to daily fluctuation, and adjust from what actually happened rather than from the equation. Weight moves with fluid, cycle phase and sodium far more than daily energy balance would suggest, which is why a fortnight of data beats a single morning reading.