Energy Balance Basics: How Calories In and Calories Out Really Work

Every food tracker is built on one idea: if you eat less energy than you use, your body makes up the difference from its stores, and over time your weight falls. The idea is right. The details are where most of the frustration comes from.

Where your daily calories go

Total daily energy expenditure (TDEE) has three main parts:

Component	Roughly	What it is
Basal metabolic rate (BMR)	60–70%	The energy to keep you alive at rest: heart, brain, breathing, temperature
Thermic effect of food	About 10%	The energy used to digest and process what you eat; higher for protein
Activity	The rest, and the most variable	Exercise, plus everyday movement such as walking, standing and fidgeting

The everyday-movement part, called non-exercise activity thermogenesis (NEAT), varies a great deal between people and can change without your noticing.

How apps estimate your needs

Most trackers estimate BMR with an equation, commonly the Mifflin-St Jeor equation, using your weight, height, age and sex, then multiply it by an activity factor. The result is a reasonable starting point, not a measurement. Two people with identical numbers can burn noticeably different amounts.

That is why the best guide to your real expenditure is your own data: if your weekly average weight stays flat over several weeks while you log carefully, the calories you are eating are close to your maintenance level. Some apps now do this calculation for you, adjusting your target as your intake and weight trend come in.

The 3,500-calorie rule, and why it misleads

A pound of body fat stores roughly 3,500 calories (about 7,700 per kilogram). From that came the rule that a 500-calorie daily deficit loses a pound a week. In the short term it is a fair rough guide. Over months it overestimates, because:

• a smaller body burns less, so the same intake becomes a smaller deficit;
• everyday movement often drops a little when eating less;
• early weight loss includes water, which makes the first weeks look faster than the rest.

Research modelling weight change, notably by Kevin Hall and colleagues at the US National Institutes of Health, shows weight loss slowing gradually rather than continuing in a straight line.

Water hides the trend

Day-to-day weight can swing by a kilogram or more with salt, carbohydrate, digestion, hormones and exercise. None of that is fat. Weigh at the same time each morning, look at weekly averages, and judge any change over three or four weeks.

Practical rules

1. Start with an estimate, then let your own trend correct it.
2. Recalculate your target after every few kilograms lost or gained.
3. Keep protein up during a deficit to protect muscle.
4. Protect everyday movement, for example with a step count you keep steady.
5. Prefer modest deficits; very low intakes are hard to sustain and make nutrient needs harder to meet.

The bottom line

Energy balance works, but it moves. Use an equation to start, your weight trend to adjust, and weekly averages to judge, and expect progress to slow gently as you get closer to your goal.