Fasting lowered the hunger hormone instead of raising it

Whether a diet succeeds long term often comes down to hormones rather than willpower. Leptin, adiponectin, ghrelin and resistin all feed into how the body regulates weight and appetite, and ghrelin in particular is known as the hormone that drives the urge to eat. A common worry about fasting regimens is that going without food for long stretches will send hunger signalling into overdrive and make the approach unsustainable. This review set out to test what fasting actually does to these weight-regulating hormones across the randomised trial evidence.

The researchers searched four databases up to June 2024 for randomised clinical trials assessing fasting regimens in healthy, overweight and obese adults. The regimens covered intermittent fasting, time-restricted feeding, alternate day fasting and the fasting-mimicking diet. Sixteen documents met the criteria and were included. The team pooled effect sizes for four hormones: leptin, adiponectin, ghrelin and resistin, expressing results as weighted mean differences against comparison conditions.

Fasting regimens significantly lowered leptin, by 2.65 ng/ml on average, and also lowered ghrelin, by 0.57 ng/ml. The drop in ghrelin is the surprise, since many assume fasting would push the hunger hormone upward. Adiponectin, a hormone linked to metabolic health, increased by 0.41 µg/ml, but this only reached marginal significance. Resistin showed no significant change at all. The authors describe the overall pattern as a beneficial impact on weight-regulating hormone levels.

The leptin result fits comfortably with existing knowledge, since leptin tends to fall as fat mass falls. The ghrelin finding is the more interesting contribution, because it pushes against the intuition that fasting inevitably ramps up hunger signalling. The evidence is not the final word: the trials were pooled from varied fasting styles and populations, and the authors explicitly note that the long-term effects of these dietary approaches still need evaluating in future studies.

For anyone who has avoided fasting on the assumption that it would leave them fighting a hormonal tide of hunger, this is a reason to reconsider that assumption. The pooled evidence points to lower, not higher, ghrelin across these trials. It says nothing about how any individual will feel day to day, and it cannot yet speak to what happens over years, but it does suggest the hormonal case against fasting is weaker than commonly believed.