Health is thought to be improved by fasting interventions, which involve alternating periods of fasting and feeding. But this doesn’t work as well in old animals. By studying the short-lived killifish, researchers at the Max Planck Institute for Biology of Ageing in Cologne have shown that older fish deviate from a youthful fast and refeed cycle, and instead enter a state of perpetual fasting, even when feeding. However, the benefits of refeeding after fasting in old killifish can be restored by genetically activating a specific subunit of AMP kinase. These fish experienced improved health and longevity, indicating that both fasting and refeeding are needed to confer health benefits.