How it's calculated
The starting point is basal metabolic rate: energy use at complete rest. The most common estimation formula comes from Mifflin and St Jeor in 1990 and combines weight, height, age and sex. An activity factor is applied to this value – and the result is your daily need.
Rhythmus uses exactly this formula. Not because it's particularly exact, but because it performs comparatively well among simple estimation formulas and needs no measuring equipment.
Why the number is only a starting point
A formula with four inputs can't capture what differs between two people: muscle mass, thyroid, how much you move without noticing, how efficiently you use food. Two women with identical details can differ in actual energy use by several hundred kilocalories.
A sensible approach is therefore: take the number as a starting point, eat roughly to it for two to three weeks and watch what happens. Your observation beats the formula – the same idea as with training.
What the cycle has to do with it
There are indications of a slightly higher resting metabolic rate in the second half of the cycle. The size is small and the data are inconsistent – small enough to disappear in the formula's inaccuracy. So we don't turn it into a separate number.
Of more practical relevance is something else: appetite noticeably changes across the cycle. If you're hungrier in the luteal phase, you don't have a discipline problem.
More important than the calorie number
- Eat enough at all. Too little energy over a long time has consequences far beyond weight – see When your period stops.
- Enough protein. For people who strength train, recommendations are well above what general reference values for adults state.
- Consistency. A roughly suitable intake you can keep up beats an exactly calculated one you give up after two weeks.