Does fasting lower appetite over time?

Two alternate-day trials

In 2010, Monica Klempel, Surabhi Bhutani, Marian Fitzgibbon, Sally Freels, and Krista Varady followed 16 adults with obesity for 10 weeks. After 2 weeks of their usual eating, they spent 8 weeks on alternate-day modified fasting. The fast day still included a meal. There was no comparison group. On each fast day they rated hunger in the evening, a few minutes before bed. Hunger was higher in the first week of that pattern, then lower by week 2, and it stayed lower for the rest of the trial. Fullness stayed low for the whole 8 weeks.

In 2021, Faiza Kalam, Kelsey Gabel, Sofia Cienfuegos, and colleagues reported on 31 adults with obesity who finished a 6-month alternate-day fast. The background diet was meant to be low in carbohydrate and higher in protein. The fast day still included a meal, given as meal-replacement shakes. There was no comparison group. Hunger and fullness were rated on an eating-day morning. They were not rated in the evening on the fast day. Hunger did not change from the start to month 3, or from month 3 to month 6. Fullness did not change either. On the fast day, people ate more than the plan asked.

Put side by side, the findings differ. The Klempel trial found evening hunger on the fast day lower after about 2 weeks, in 16 people, while fullness stayed low. The Kalam trial found no change in hunger or fullness around a morning meal on the eating day, over 6 months, in the 31 people who finished. Both trials still fed people on the fast day, and neither had a comparison group.

Protein at one meal

In 2023, James Stubbs, Graham Horgan, Eric Robinson, Mark Hopkins, Clarissa Dakin, and Graham Finlayson reviewed how protein, carbohydrate, and fat affect appetite and how much people eat. Even when foods were matched for energy density, protein was more filling than carbohydrate or fat. Most of the appetite studies they discuss last hours or days, and people differ.

An earlier review by Thomas Halton and Frank Hu looked at higher-protein meals set against meals higher in carbohydrate or fat. Of 14 comparisons, 11 found the higher-protein meal increased fullness. Three did not. Their summary is that meals higher in protein tend to increase fullness compared with meals lower in protein, at least in the short term. The clearest results come from these short, single-meal studies.

A ketogenic low-carbohydrate diet

In 2015, A. A. Gibson, R. V. Seimon, and colleagues pooled studies of ketogenic low-carbohydrate diets. People rated appetite before the diet, while eating enough to hold their weight, and again during the diet. Hunger fell by 5.5 points on a 100-point scale. That figure comes from three studies. Desire to eat fell by 8.9 points on a 100-point scale in two of them. Fullness did not clearly rise, and only two studies rated it. The authors call the changes small. Each person was compared with themselves before the diet, and the ratings were taken during a calorie deficit.

People differ, and noticing your own pattern (for example, hunger the day after a large carbohydrate meal) is reasonable, but nothing here promises it.