In a study The Max Planck Institute for Biology of Ageing in Cologne found that the alternation between fasting and eating is crucial for health and longevity, particularly in older animals. Research on killifish showed that older fish can no longer react to the alternation of fasting and eating, but instead remain in a permanent state of fasting. However, by genetically activating a specific subunit of AMP kinase, it was possible to restore the positive effect of feeding after fasting in old fish. This resulted in improved health and extended lifespan. The study suggests that the switch between fasting and eating takes place via AMP kinase and could also provide humans with relevant insights about the best time to fast.