TDEE (total daily energy expenditure)

The full calorie picture

If BMR is what you burn lying in bed, TDEE is what you burn living your actual life, including walking, working, exercising and even digesting food. Total daily energy expenditure has four parts, and the proportions surprise most people:

• Basal metabolism, 60 to 75 % of the total. Breathing, circulation, temperature, cell upkeep
• Thermic effect of food, about 10 %. Digesting and storing what you eat costs energy; protein costs the most (20 to 30 % of its calories), fat the least
• Non-exercise activity, 15 to 30 % and the most variable part. Walking to the car, standing, fidgeting, carrying shopping. Between two people of the same size it can differ by 2 000 kcal a day (Levine 2004)
• Exercise, usually 5 to 10 % unless you train for hours. A 45-minute run is 400 to 600 kcal, which is a smaller share of the day than the effort suggests

Typical multipliers

The classic shortcut multiplies BMR by an activity factor:

• Sedentary (desk job, little exercise): BMR x 1.2
• Lightly active (exercise 1 to 3 days a week): BMR x 1.375
• Moderately active (exercise 3 to 5 days a week): BMR x 1.55
• Very active (hard exercise 6 to 7 days a week): BMR x 1.725
• Extremely active (athlete or physical job plus training): BMR x 1.9

The trap is the checkbox. People overestimate their category, and the multipliers were built for the whole day, not for the workout: a desk worker who trains hard four times a week but sits for the remaining 23 hours is closer to 1.4 than to 1.55. Doubly labeled water studies, the reference method for free-living energy expenditure, put most adults in developed countries between 1.5 and 1.8 times BMR, with 1.6 to 1.7 the common range (Black 1996).

What moves TDEE and what does not

Body size and lean mass set the floor through BMR. Daily movement sets most of the rest, and it is the part that quietly changes: a new job, a longer commute, an injury, a winter. Exercise adds less than the display on the treadmill claims, partly because people move less for the rest of the day after a hard session and partly because the body reduces other expenditure when activity is very high, the constrained model of energy expenditure (Pontzer 2016).

What does not change it much: eating small frequent meals, most supplements, and "fat-burning" heart-rate zones, which change the fuel mix in the moment but not the day's total.

Using it

• For weight maintenance, TDEE is your intake target; for a slow, sustainable loss subtract 300 to 500 kcal, which is about 0.25 to 0.5 kg a week
• Estimates are starting points. Track weight for three or four weeks at a fixed intake and let the trend correct the number; a stable weight means the intake equals TDEE, whatever the calculator said
• Do not chase the daily number. Expenditure swings by several hundred calories between a rest day and a long ride; the weekly average is what matters
• Treat the calorie readout on a watch as an estimate with a wide error band. In a Stanford validation of seven wrist devices, the best one was off by a median of 27 % on energy expenditure and the worst by more than 90 %, while heart rate on the same devices was within 5 % (Shcherbina 2017)

Why wearable data beats a checkbox

The multipliers above are rough averages. A wearable measures the inputs the multiplier was guessing at: steps, active minutes, heart rate across the day and the sessions you actually did. MyBodyAI reads active and basal calories from your connected devices and reconciles the two against the device's own daily total, so a day is never scored as the workout alone. When a day's total from the device differs from basal plus active by more than the model can explain, about 15 % of basal, the total wins, because the device saw the whole day; a measured zero is never overwritten. That is still an estimate, but it is an estimate of your day rather than of an average person's.