Energy balance
The balance between calories consumed (food) and calories expended (metabolism + activity), the central determinant of weight gain, maintenance or loss.
Energy balance compares what comes in (what you eat) with what goes out (what you spend):
- Positive balance (in > out): weight gain
- Negative balance (out > in): weight loss
- Neutral balance: maintenance
Expenditure breaks down into: basal metabolic rate (~60-70%), physical activity (~15-30%), the thermic effect of food (~10%), and adaptive thermogenesis.
The original angle (a dynamic balance, not a fixed equation): the “calories in / calories out” model is real but often wrongly understood as static. In reality, the two sides of the equation influence each other
- When you eat less (restriction), the body reduces its expenditure (basal metabolic rate drops by 10-20%, through adaptive thermogenesis). Hence the stalling of diets
- When you eat more, the body can slightly increase its expenditure (fidgeting, thermogenesis)
- Hormones (leptin, ghrelin, thyroid) adjust hunger and expenditure according to your reserves
In other words, the body defends its weight (“set point” theory) by actively adjusting both sides. This is why losing weight for good is difficult: the body resists.
Practical implication: creating too aggressive a deficit is counter-productive (metabolism collapses, hunger soars). A moderate, sustainable deficit, combined with exercise (especially strength work to preserve metabolism) and enough protein, is more effective than a drastic diet. The quality of the diet also influences hunger and therefore the “intake”.
Energy balance still holds true, but it is adaptive and hormonal, not mechanical.