Calorie restriction is a dietary regimen that reduces food intake without incurring malnutrition. "Reduce" can be defined relative to the subject's previous intake before intentionally restricting food or beverage consumption, or relative to an average person of similar body type. Calorie restriction is typically adopted intentionally to reduce body weight. It is recommended as a possible regimen by US dietary guidelines and scientific societies for body weight control. Mild calorie restriction may be beneficial for pregnant women to reduce weight gain, and reduce perinatal risks for both the mother and child. For overweight or obese individuals, long-term health improvement may result from calorie restriction, although a gradual weight regain may occur.
Health effects
Recommendations
Caloric intake control, and reduction for overweight individuals, is recommended by US dietary guidelines and science-based societies. Calorie restriction is recommended for people with diabetes and prediabetes, in combination with physical exercise and a weight loss goal of 5-15% for diabetes and 7-10% for prediabetes to prevent progression to diabetes. and mild calorie restriction may be beneficial for pregnant women to reduce weight gain and reduce perinatal risks for both the mother and child. For overweight or obese individuals, calorie restriction may improve health through weight loss, although a gradual weight regain of 1–2 kg per year may occur.
Risks of malnutrition
The term "calorie restriction" as used in the study of aging refers to dietary regimens that reduce calorie intake without incurring malnutrition. If a restricted diet is not designed to include essential nutrients, malnutrition may result in serious deleterious effects, as shown in the Minnesota Starvation Experiment. This study was conducted during World War II on a group of lean men, who restricted their calorie intake by 45% for 6 months and composed roughly 77% of their diet with carbohydrates. As expected, this malnutrition resulted in metabolic adaptations, such as decreased body fat, improved lipid profile, and decreased resting heart rate. The experiment also caused negative effects, such as anemia, edema, muscle wasting, weakness, dizziness, irritability, lethargy, and depression. Typical low-calorie diets may not supply sufficient nutrient intake that is typically included in a calorie restriction diet. Nutritionally complete meal replacements can be used to minimize malnutrition.
The long-term effects of calorie restriction are unknown. Effects of calorie restriction in humans over multiple years or decades may be small in comparison to conventional medical and public health interventions.
Primates
In a 2017 report on rhesus monkeys, caloric restriction in the presence of adequate nutrition was effective in delaying the effects of aging. Older age of onset, female sex, lower body weight and fat mass, reduced food intake, diet quality, and lower fasting blood glucose levels were factors associated with fewer disorders of aging and with improved survival rates. Specifically, reduced food intake was beneficial in adult and older primates, but not in younger monkeys. The study indicated that caloric restriction provided health benefits with fewer age-related disorders in elderly monkeys and, because rhesus monkeys are genetically similar to humans, the benefits and mechanisms of caloric restriction may apply to human health during aging.
Activity levels
Calorie restriction preserves muscle tissue in nonhuman primates and rodents. Mechanisms include reduced muscle cellapoptosis 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 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 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.
