hours for any caloric intake – and could be combined with overall caloric restriction and variants of the Mediterranean diet which usually has benefits of long-term cardiovascular health and longevity. Activity levels Calorie restriction preserves muscle tissue in nonhuman primates and rodents. Mechanisms include reduced muscle cell apoptosis and inflammation; protection against or adaptation to age-related mitochondrial abnormalities; and preserved muscle stem cell function. Muscle tissue grows when stimulated, so it has been suggested that the calorie-restricted test animals exercised more than their companions on higher calories, perhaps because animals enter a foraging state during calorie restriction. However, studies show that overall activity levels are no higher in calorie restriction than ad libitum animals in youth. Laboratory rodents placed on a calorie restriction diet tend to exhibit increased activity levels (particularly when provided with exercise equipment) at feeding time. Monkeys undergoing calorie restriction also appear more restless immediately before and after meals. Sirtuin-mediated mechanism Preliminary research indicates that sirtuins are activated by fasting and serve as "energy sensors" during metabolism. Sirtuins, specifically Sir2 (found in yeast) have been implicated in the aging of yeast, and are a class of highly conserved, NAD+-dependent histone deacetylase enzymes. Sir2 homologs have been identified in a wide range of organisms from bacteria to humans. Hormesis Some research has pointed toward hormesis as an explanation for the benefits of caloric restriction, representing beneficial actions linked to a low-intensity biological stressor such as reduced calorie intake. As a potential role for caloric restriction, the diet imposes a low-intensity biological stress on the organism, eliciting a defensive response that may help protect it against the disorders of aging. In other words, caloric restriction places the organism in a defensive state so that it can survive adversity. Intensive care , current clinical guidelines recommend that hospitals ensure that the patients get fed with 80–100% of energy expenditure, the normocaloric feeding. A systematic review investigated whether people in hospitals' intensive care units have different outcomes with normocaloric feeding or hypocaloric feeding, and found no difference. However, a comment criticized the inadequate control of protein intake, and raised concerns that hypocaloric feeding safety should be further assessed with underweight critically ill people. See also CR Society International Fasting Intermittent fasting List of diets Luigi Cornaro Mitohormesis Okinawa diet Very low calorie diet References Bibliography Diets Eating behaviors Life extension Senescence