Satiety

Satiety and fullness are two different mechanisms that everyday language mixes up. Fullness brings the current meal to an end; satiety is measured by how long passes before you feel hungry again. A food can easily take the edge off hunger at the time and hold up poorly two hours later.

The signal is first mechanical: the volume and stretching of the stomach trigger the first messages of fullness. Then come the hormonal signals, which are slower, carried notably by ghrelin (which triggers hunger) and leptin (which dampens it). This lag explains a familiar observation: eating fast means eating more, because the signals arrive after the fact.

Not all macronutrients are equal. Calorie for calorie, protein keeps you full the longest, ahead of complex carbohydrate and then fat. The fibre and water in a food play a major role: a whole apple keeps you going far better than apple juice with the same amount of sugar, because volume and chewing count as much as calories.

This is why energy density is a more useful lever than simple counting: foods that are low in calories but bulky (vegetables, soups, pulses) let you eat until you are satisfied while lowering your total intake, without relying on deprivation.

The hormonal detail is worth knowing. Beyond ghrelin and leptin, three gut hormones, GLP-1, PYY and CCK, signal the arrival of nutrients and curb food intake. Fibre contributes indirectly: fermented by the microbiota, it produces short-chain fatty acids that stimulate GLP-1 secretion.

This explains the effect of ultra-processed foods on satiety. A controlled study led by Kevin Hall in 2019 compared two diets with equivalent nutritional composition: participants spontaneously ate about 500 kcal more per day on the ultra-processed foods. Designed to be highly palatable and quick to eat, they bypass the satiety signals, which is known as “passive overconsumption”. Liquid calories pose the same problem: a juice is practically invisible to these mechanisms.