Energy Balance

The regulation of energy taken in (as food) against energy spent (on metabolism, activity, and heat) — the system that, over the long term, governs body weight.

How it is regulated

The hypothalamus integrates signals reporting both immediate hunger and long-term energy stores:

• Hunger signals — chiefly ghrelin from the stomach, which rises before meals.
• Fullness signals — gut hormones released during a meal (such as GLP-1, PYY, and CCK).
• Long-term store signals — leptin from fat tissue, which reports how much energy is banked.

An asymmetric defense

The system resists weight loss more strongly than weight gain — an inheritance from an evolutionary past in which famine was the greater threat. This is part of why deliberate weight loss is difficult to sustain.

Key idea

Body weight is a regulated variable, not simply a running total — the body actively defends an energy setpoint. See the gut–brain & energy-balance axis.