Why deficit is needed for weight loss and surplus for weight gain?

Your body burns a certain amount of energy to perform all the activities you do throughout the day. This is called Total Daily Energy Expenditure (TDEE), also known as maintenance calories. It includes all energy you spend, including lifting, cardio, talking, walking, breathing everything!!!

When you eat more than this amount of energy, the excess is stored in the body as fat to be used later as a reserve source of energy.

But what happens if you eat less than your TDEE? The body taps into stored energy reserves, primarily body fat, to meet the energy shortfall.

The unit we use for measuring this energy is “calorie” or “kcal”. In nutrition, they are effectively used interchangeably.

When you eat more than your TDEE, this is called a caloric surplus. When you eat less than your TDEE, this is called a caloric deficit.

So, weight gain or loss is largely driven by the balance between energy intake and energy expenditure.

Let’s say your TDEE or Maintenance Calories is 2,500 kcal (a fairly typical value for an average adult male).

If you eat 2,000 kcal per day, you are in a 500 kcal deficit. In that case, your body must supply the missing energy from stored reserves, primarily body fat.

1 kg of stored body fat is often estimated to provide about 7,700 kcal of energy.

So, to lose 1 kg of body fat per week, you would need a total deficit of about 7,700 kcal over 7 days, which averages to roughly 1,100 kcal per day.

7,700 kcal ÷ 7 ≈ 1,100 kcal/day deficit

Typically, people aim for around 0.5 kg loss per week, which corresponds to about a 550 kcal daily deficit.

Very simple math. Right?

If your body burns more calories than you eat → you lose weight (deficit)

If your body burns fewer calories than you eat → you gain weight (surplus)

If your body burns calories equal to the calories you eat → your weight stays the same (maintenance)

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