«Calorie restriction, or caloric restriction, or energy restriction, is a dietary regimen that reduces calorie intake without incurring malnutrition or a reduction in essential nutrients. "Reduce" can be defined relative to the subject's previous intake before intentionally restricting calories, or relative to an average person of similar body type. Commonly consumed food components containing calories are carbohydrate, proteins and fat.
In preliminary research, some non-human species on calorie restriction diets without malnutrition may exhibit slowing of the biological aging process, resulting in an increase in both median and maximum lifespan, but this effect is not universal. In humans, the long-term health effects of moderate caloric restriction with sufficient nutrients are unknown.
The term "calorie restriction" as used in gerontology 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 carbohydrate. As expected, this malnutrition resulted in many positive metabolic adaptations (e.g. decreased body fat, blood pressure, improved lipid profile, low serum T3 concentration, and decreased resting heart rate and whole-body resting energy expenditure), but also caused a wide range of negative effects, such as anemia, edema, muscle wasting, weakness, neurological deficit, dizziness, irritability, lethargy, and depression.
Short-term studies in humans report a loss of muscle mass and strength and reduced bone mineral density. However, whether or not the reduction in bone mineral density actually harms bone health is unclear. In a study in premenopausal women, bone mineral density after weight loss was higher when normalized for body weight; reduced bone mineral density is also observed in humans undergoing long-term calorie restriction with adequate nutrition, but no fracture have been reported and the reduction in bone mineral density was not associated with deleterious changes in bone microarchitecture.
The authors of a 2007 review of the caloric restriction literature warned that "[i]t is possible that even moderate calorie restriction may be harmful in specific patient populations, such as lean persons who have minimal amounts of body fat."
Caloric restriction diets typically lead to reduced body weight, yet reduced weight can come from other causes and is not in itself necessarily healthy. In some studies, low body weight has been associated with increased mortality, particularly in late middle-age or elderly subjects. Low body weight in the elderly can be caused by pathological conditions associated with aging and predisposing to higher mortality (such as cancer, chronic obstructive pulmonary disorder, or depression) or of the cachexia (wasting syndrome) and sarcopenia (loss of muscle mass, structure, and function). One study linked a body mass index lower than 18 in women with increased mortality from noncancer, non−cardiovascular disease causes. The authors attempted to adjust for confounding factors (cigarette smoking, failure to exclude pre-existing disease); others argued that the adjustments were inadequate.
Such epidemiological studies of body weight are not about caloric restriction as used in anti-aging studies; they are not about caloric intake to begin with, as body weight is influenced by many factors other than energy intake, Moreover, "the quality of the diets consumed by the low-body mass index individuals are difficult to assess, and may lack nutrients important to longevity." Typical low-calorie diets rarely provide the high nutrient intakes that are a necessary feature of an anti-aging calorie restriction diet. As well, "The lower-weight individuals in the studies are not a caloric restriction because their caloric intake reflects their individual ad libitum set-points and not a reduction from that set-point."
In those who have a binge-eating disorder, calorie restriction can precipitate an episode of binge eating, but it does not seem to pose any such risk otherwise.
Long-term caloric restriction at a level sufficient for slowing the aging process is generally not recommended in children, adolescents, and young adults (under the age of approximately 21), because this type of diet may interfere with natural physical growth, as has been observed in laboratory animals. In addition, mental development and physical changes to the brain take place in late adolescence and early adulthood that could be negatively affected by severe caloric restriction.Pregnant women and women trying to become pregnant are advised not to practice calorie restriction, because low BMI may result in ovulatory dysfunction (infertility), and underweight mothers are more prone to preterm delivery.
It has also been noted that people losing weight on such diets risk developing cold sensitivity, menstrual irregularities, infertility, and hormonal changes.
Even though there has been research on caloric restriction for over 70 years, the mechanism by which caloric restriction works is not well understood. Some explanations include reduced core body temperature, reduced cellular divisions, lower metabolic rates and hormesis, according to one review of laboratory research.
Caloric restriction lowers the core body temperature, a phenomenon believed to be an adaptive response to reduce energy expenditure when nutrients availability is limited. Lowering the temperature may prolong the lifespan of cold blooded animals. Mice, which are warm blooded, have been engineered to have a reduced core body temperature which increased the lifespan independently of calorie restriction.» (wikipedia)
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