Calorie restriction

Calorie restriction (CR, also known as caloric restriction or energy restriction) is a dietary regimen that reduces the energy intake from foods and beverages below energy requirements, without incurring malnutrition. The possible effect of calorie restriction on body weight management, longevity, and aging-associated diseases has been an active area of research.

Calorie restriction is recommended for people with diabetes and prediabetes, For overweight or obese individuals, calorie restriction may improve health through weight loss, although a gradual weight regain of per year may occur. for six 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.

Possible side effects

People losing weight during calorie restriction risk developing side effects, such as cold sensitivity, menstrual irregularities, infertility, or hormonal changes.

Research

Humans

Decreasing caloric intake by 20-30%, while fulfilling nutrient requirements, has been found to remedy diseases of aging, including cancer, cardiovascular disease, dementia, and diabetes in humans, and result in an average loss of in body weight, but because of the long lifespan of humans, evidence that calorie restriction could prevent age-related disease in humans remains under preliminary research. While calorie restriction leads to weight and fat loss, the precise amount of calorie intake and associated fat mass for optimal health in humans is not known.

Minnesota Starvation Experiment

The Minnesota Starvation Experiment examined the physical and psychological effects of extreme calorie restriction on 32 young and lean 24-year-old men during a 40% reduction in energy intake for 6 months. The study was designed to mimic dietary conditions during World War II. Participants could only eat 1800 kcal per day, but were required to walk 5 km per day and expend 3000 calories. The men lost about 25% of their body weight of which 67% was fat mass and 17% fat-free mass. Emotional distress, confusion, apathy, depression, hysteria, hypochondriasis, suicidal thoughts, and loss of sex drive were among the abnormal psychological behaviors that occurred within six weeks. 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.

Non-human primates

A calorie restriction study started in 1987 by the National Institute on Aging showed that calorie restriction did not extend years of life or reduce age-related deaths in non-obese rhesus macaques. It did improve certain measures of health, however. These results were publicized as being different from the Wisconsin rhesus macaque calorie restriction study, which also started in 1987 and showed an increase in the lifespan of rhesus macaques following calorie restriction. 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.

Activity levels

Calorie restriction preserves muscle tissue in nonhuman primates and rodents. 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.

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<sup>+</sup>-dependent histone deacetylase enzymes. Sir2 homologs have been identified in a wide range of organisms from bacteria to humans.