Energy balance

Energy balance is the basic rule behind every body weight: if, over a longer period, you take in more energy than you use, your body stores the surplus – mainly as fat. If you take in less, it draws the difference from its reserves. On one side is everything you eat and drink; on the other are your basal metabolic rate, digestion, everyday movement and exercise.

The key words are “over a longer period”. A single day decides nothing – one festive meal won't make you fat, and one day of fasting won't make you slim. The body balances things out over days and weeks, and your weight follows the average. That's why a week's balance tells you much more than a single day's, and taking the occasional blip in your stride isn't just allowed – it's smart.

Energy balance isn't a rigid calculation: the two sides influence each other. If you eat less, you often move less without realising it and burn slightly less energy at rest too; if you eat more, you give off some of it as heat and through extra movement. That's why a permanent surplus of 100 kcal doesn't lead to ever-increasing weight, but to a new, slightly higher equilibrium – and the same applies in reverse.

What the balance doesn't explain is the why: whether you feel full or hungry depends on protein, fibre, sleep, stress and habits. Hardly anyone can keep up a negative balance that feels like deprivation for long; almost everyone can keep up one that comes from filling food and more everyday movement.

The balance applies to children in exactly the same way – except that it includes growth. Children need a slight surplus, and their bodies usually regulate it themselves, as long as sugary drinks and constant snacking don't get in the way.