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. "Low" can be defined relative to the subject's previous intake before intentionally restricting calories, or relative to an average person of similar body type. In a number of species, including yeast, fish, rodents and dogs, calorie restriction without malnutrition has been shown to slow the biological aging process, resulting in longer maintenance of youthful health and an increase in both median and maximum lifespan. However, the life-extending effect of calorie restriction is not shown to be universal.
In humans, the long-term health effects of moderate caloric restriction with sufficient nutrients are unknown.
Using rhesus monkeys, a collaboration of the United States National Institute on Aging and the University of Wisconsin found that caloric restriction without malnutrition extended lifespan and delayed the onset of age-related disorders; older age, higher diet quality, and female gender were positive factors affecting the benefits realized from lower caloric intake.
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Health effects
In humans the long-term health effects of moderate caloric restriction with sufficient nutrients are unknown.
Risks of malnutrition
As noted above, the term "calorie restriction" as used in biogerontology 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 90% of their diet with carbohydrates. 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, lower extremity edema, muscle wasting, weakness, neurological deficits, dizziness, irritability, lethargy, and depression.
Musculoskeletal losses
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 fractures 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."
Lower-than-normal body mass index, high mortality
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-aged 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 of the more famous of such studies 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."
Triggering binge eating
In those who already suffer from a binge-eating disorder, calorie restriction can precipitate an episode of binge eating, but it does not seem to pose any such risk otherwise.
Young or pregnant
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.
Miscellaneous concerns
It has also been noted that people losing weight on such diets risk developing cold sensitivity, menstrual irregularities, and even infertility and hormonal changes.
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Mechanisms
Even though there has been research on caloric restriction for over 70 years, the mechanism by which caloric restriction works is still not well understood. Some explanations include reduced core body temperature, reduced cellular divisions, lower metabolic rates, reduced production of free radicals, reduced DNA damage and hormesis.
Temperature
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.
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, resulting in improved health and longer life. This switch to a defensive state may be controlled by longevity genes (see below).
Mitochondrial hormesis
Mitochondrial hormesis was a purely hypothetical concept until late 2007, when work on a small worm named Caenorhabditis elegans suggested that the restriction of glucose metabolism extends life span primarily by increasing oxidative stress to stimulate the organism into having an ultimately increased resistance to further oxidative stress. This is probably the first experimental evidence for hormesis being the reason for extended life span following caloric restriction.
Although aging can be conceptualized as the accumulation of damage, the more recent determination that free radicals participate in intracellular signaling has made the categorical equation of their effects with "damage" more problematic than was commonly appreciated in the past. It was previously proposed on a hypothetical basis that free radicals may induce an endogenous response culminating in more effective adaptations that protect against exogenous radicals (and possibly other toxic compounds). Sublethal mitochondrial stress with an attendant augmentation of reactive oxygen species may precipitate many of the beneficial alterations in cellular physiology produced by caloric restriction.
Evolution
It has been recently argued that during years of famine, it may be evolutionarily desirable for an organism to avoid reproduction and to up-regulate protective and repair enzyme mechanisms to ensure that it is fit for reproduction in future years. This argument seems to be supported by recent work studying hormones. Prolonged severe CR lowers total serum and free testosterone while increasing sex hormone binding globulin concentrations in humans; these effects are independent of adiposity.
Lowering of the concentration of insulin and substances related to insulin, such as insulin-like growth factor 1 and growth hormone, has been shown to up-regulate autophagy, the repair mechanism of the cell. A related hypothesis suggests that caloric restriction works by decreasing insulin levels and thereby up-regulating autophagy, but caloric restriction affects many other health indicators, and it is still undecided whether insulin is the main concern. Calorie restriction has been shown to increase DHEA in primates, but it has not been shown to increase DHEA in post-pubescent primates. The extent to which these findings apply to humans is still under investigation.
Chromatin and PHA-4
Evidence suggests that the biological effects of caloric restriction are closely related to chromatin function. One study determined that the gene PHA-4 is responsible for the longevity behind calorie restriction in roundworms, "with similar results expected in humans".
Reduced DNA damage
Calorie restriction reduces production of reactive oxygen species.
Sohal et al. observed that caloric restriction decreased 8-OHdG damages in the DNA of mice heart, skeletal muscle, brain, liver and kidney. The levels of 8-OHdG in the DNA of these organs in 15 month old mice were reduced to an average of 81% of that in the DNA of mice fed an unrestricted diet. Kaneko et al. observed that, in rats, dietary restriction retarded the onset of age-related increases in 8-OHdG in nuclear DNA of brain, heart, liver and kidney. The level of 8-OHdG in these organs of the calorie restricted rats at 30 months averaged 65% of the level in rats fed an unrestricted diet.
In rodents, calorie restriction slows aging, decreases ROS production and reduces the accumulation of oxidative DNA damage in multiple organs. These results link reduced oxidative DNA damage to slower aging. The consistent observation that calorie restriction reduces oxidative DNA damage lends support to the possibility that oxidative DNA damage is associated with aging.
Sirtuin-mediated mechanism
Sirtuins, specifically Sir2 (found in yeast) has been implicated in the aging of yeast and is a highly conserved, NAD+ - dependent histone deacetylase. Sir2 homologs have been identified in a wide range of organisms from bacteria to humans. Yeast has 3 SIR genes (SIR2, SIR3, and SIR4) that are responsible for silencing mating type loci, telomeres, and rDNA. Although all three genes are required for the silencing of mating type loci and telomeres, only SIR2 has been implicated in the silencing of rDNA. In addition, SIR2 related genes also regulate formation of some specialized survival forms, such as spores in yeast and daher larvae in C. elegans. A study done by Kaeberlein et al. (1999) in yeast found that deletions of Sir2 decreased lifespan, and additional copies increased lifespan.
In many calorie restriction studies, it is believed that Sir2 mediates the longevity effects from calorie restriction for several reasons. First, it was found that in yeast without SIR2, calorie restriction did not impart longevity in yeast; second, in Sir2 mutants an abundance of extra-chromosomal ribosomal DNA circles (that typically limit lifespan) has been observed, and that mitigation of these circles restore regular life span, but still are resistant to calorie restriction-mediated longevity; third, that caloric restriction increases the activity of Sir2 in vivo. Although Sir2 has been implicated in calorie restriction-mediated longevity, the method by which Sir2 is regulated under caloric restriction is still debated.
Two hypotheses of Sir2/caloric restriction -mediated longevity is the NADH mechanism and the NAD salvage pathway mechanism. In the NADH hypothesis, it is believed that caloric restriction causes an increase in respiration, which in turn causes a reduction in the levels of NADH. This decrease in the concentration of NADH would up regulate SIR2, since NADH functions as a competitive inhibitor of SIR2. In addition, it has been shown that overexpression of NADH hydrogenase increased longevity and knocking out the electron transport chain blocked caloric restriction-mediated longevity. The NAD salvage pathway mechanism relies on a study by Anderson et al., in which they showed that caloric restriction causes an up regulation of PNC1 (an enzyme responsible for synthesizing NAD from nicotinamide and ADP-ribose) decreases the levels of nicotinamide, which in turn upregulates SIR2 and thus increases lifespan. Although these models are not mutually exclusive, no experiment has been conducted linking the two.
Caloric restriction mimetics
Work on the mechanisms of caloric restriction has given hope to the synthesizing of future drugs to increase the human lifespan by simulating the effects of calorie restriction. In particular, the large number of genes and pathways reported to regulate the actions of caloric restriction in model organisms represent attractive targets for developing drugs that mimic the benefits of CR without its side effects.
Sir2, or "silent information regulator 2", is a sirtuin, discovered in baker's yeast cells, that is hypothesized to suppress DNA instability. In mammals, Sir2 is known as SIRT1. One proponent of the view that the gene Sir2 may underlie the effect of calorie restriction in mammals by protecting cells from dying under stress. It is suggested that a low-calorie diet that requires less Nicotinamide adenine dinucleotide to metabolize may allow SIRT1 to be more active in its life-extending processes, possibly by a mechanism of SIRT1 releasing fat from storage cells.
Sirtuins
Attempts are being made to develop drugs that act as CR mimetics, and much of that work has focused on a class of proteins called sirtuins. Resveratrol has been reported to activate SIRT1 and extend the lifespan of yeast, nematode worms, fruit flies, vertebrate fish, and mice consuming a high-caloric diet. However, resveratrol does not extend life span in normal mice and the effect of resveratrol on lifespan in nematodes and fruit flies has been disputed.
There are studies that indicate that resveratrol may not function through SIRT1 but may work through other targets. A clinical trial of the resveratrol formulation SRT501 was suspended.
History
The first person to promote calorie restriction as a means of prolonging life was Luigi Cornaro, a 15th-century Venetian nobleman who adopted a calorie restricted diet at age 35 to address his failing health. It is said his restricted diet cured him of all of his ailments in less than a year, and he went on to live to be 102 years old. His book Discorsi della vita sobria (Discourses On the Temperate Life) described his regimen, which centered on the "quantifying principle" of restricting himself to only 350 grams of food daily (including bread, egg yolk, meat, and soup) and 414 milliliters of wine. The book was extremely successful, and "was a true reconceptualization of old age. As late as the Renaissance it was largely the negative aspects of this phase of life which were emphasized ... Cornaro's method offered the possibility for the first time not only of a long but also a worthwhile life."
The findings have since been accepted and generalized to a range of other animals. Researchers are investigating the possibility of parallel physiological links in non-human primates and humans. In response to these results, a small number of people have independently adopted the practice of calorie restriction in some form as a potential anti-aging intervention.
Through 2017, the US National Institute on Aging is conducting human clinical trials on caloric restriction to determine potential for reducing the risks of age-related disorders.
Research
Primates
In a 2017 collaborative report on rhesus monkeys by scientists of the US National Institute on Aging and the University of Wisconsin, caloric restriction in the presence of adequate nutrition was effective in delaying the effects of aging. Older age of onset, female gender, 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.
Rodents
It has been known since the 1930s that reducing the number of calories fed to laboratory rodents increases their life spans. The life extension varies for each species, but on average there was a 30-40% increase in life span in both mice and rats. In late adulthood, acute CR partially or completely reverses age-related alterations of liver, brain and heart proteins, and mice placed on CR at 19 months of age show an increases in life span.
Yeast
Fungi models are very easy to manipulate, and many crucial steps toward the understanding of aging have been made with them. Many studies were undertaken on budding yeast and fission yeast to analyze the cellular mechanisms behind increased longevity due to calorie restriction. First, calorie restriction is often called dietary restriction because the same effects on life span can be achieved by only changing the nutrient quality without changing the number of calories. Data from Guarente and others showed that genetic manipulations in nutrient-signaling pathways could mimic the effects of dietary restriction. In some cases, dietary restriction requires mitochondrial respiration to increase longevity (chronological aging), and in some other cases not (replicative aging). Nutrient sensing in yeast controls stress defense, mitochondrial functions, Sir2, and others. These functions are all known to regulate aging. Genes involved in these mechanisms are TOR, PKA, SCH9, MSN2/4, RIM15, and SIR2. Importantly, yeast responses to CR can be modulated by genetic background. Therefore, while some strains respond to calorie restriction with increased lifespan, in others calorie restriction shortens it
Humans
Studies have been conducted to examine the effects of calorie restriction with adequate intake of nutrients in humans; however, long-term effects are unknown.
Biomarkers for cardiovascular risk
A review of the effects of calorie restriction on the aging heart and vasculature concluded that "Data from animal and human studies indicate that [beyond the effects of "implementation of healthier diets and regular exercise"], more drastic interventions, i.e., calorie restriction with adequate nutrition (CRAN), may have additional beneficial effects on several metabolic and molecular factors that are modulating cardiovascular aging itself (e.g., cardiac and arterial stiffness and heart rate variability)." Studies of long-term practitioners of rigorous calorie restriction show that their risk factors for atherosclerosis are substantially improved in a manner consistent with experimental studies in rodent models of atherosclerosis and nonhuman primates. Risk factors such as c-reactive protein; serum triglycerides, low-density lipoprotein, high-density lipoprotein; blood pressure; and fasting blood sugar, are substantially more favorable than persons consuming usual Western diets and comparable or better than long-term endurance exercisers. Similar effects were also seen during a "natural experiment" in Biosphere 2, and in subjects in the "Minnesota Starvation Experiment" during World War II. Cardiac "diastolic function was better in subjects who practiced strict calorie restriction for 3-15 years than that in healthy age- and sex-matched control subjects ... calorie restriction subjects had less ventricular stiffness and less viscous loss of diastolic recoil, both of which would be consistent with less myocardial fibrosis. "These effects, in combination with other benefits of calorie restriction, such as protection against obesity, diabetes, hypertension, and cancer, suggest that CR may have a major beneficial effect on health span, life span, and quality of life in humans."
Biomarkers for cancer risk
Long-term calorie restriction in humans results in a reduction of several metabolic and hormonal factors that have been associated with increased risk of some of the most common types of cancer in developed countries, consistent with similar shifts in calorie restriction rodents and nonhuman primates, in whom calorie restriction affords substantial protection against cancer morbidity and mortality. These include lower levels of total and abdominal fat, circulating insulin, testosterone, estradiol, and inflammatory cytokines linked to cancer. Long-term calorie restriction can also reduce levels of serum Insulin-like growth factor 1 in humans, and increase levels of IGFBP-3; however, unlike in rodents, this effect can be blocked if dietary protein is not reduced to the Dietary Reference Intake.
Concerns and confounders
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, while calorie restriction animals are more active at middle age and beyond due to a protective effect against the decline in activity observed in middle-aged and older animals.
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. Despite this brief daily period of increased activity overall activity is no higher in calorie restriction than ad libitum animals in youth after an initial period of adaptation to the diet. On the other hand, calorie restriction has been found to retard the decline in activity that occurs during normal aging: in one study, animals on a conventional diet "showed little activity" by early middle age, while those on calorie restriction "were observed to run around the cage and climb onto and hang from the wire cage tops throughout their life spans. In fact, the longest surviving calorie restriction mouse was observed hanging from the top of his cage only 3 days before he became moribund."
Larger organisms
Another objection to calorie restriction as an advisable lifestyle for humans is the claim that the physiological mechanisms that determine longevity are very complex, and that the effect would be small to negligible in our species. Several of the biological changes observed in animals with longer life spans on calorie restriction have already been observed in humans which proponents argue suggests that the life-extending properties of calorie restriction will extend to humans.
Stereotyped behavior
Observations in some accounts of animals undergoing calorie restriction have noted an increase in stereotyped behaviors. For example, monkeys on calorie restriction have demonstrated an increase in licking, sucking, and rocking behavior.
Aggression
A calorie restriction regimen may also lead to increased aggressive behavior in animals.
Obese controls
It has sometimes been suggested that the lives of calorie-restricted animals are only extended relative to control animals whose lives are artificially shortened by weight-gain from unnatural ad libitum feeding in the laboratory. However, studies designed to test this hypothesis suggest that reduced fat mass is not a major contributor to the longevity effects of calorie restriction. The strongest evidence comes from a study in which ob/ob mice, which are genetically prone to overeating and obesity, were compared to lean mice of the same genetic background but lacking the ob/ob mutation. When placed on calorie restriction, the ob/ob mice still have over twice the fat mass of ad libitum-fed congenic lean mice, and yet they have double the lifespan. Additionally, many modern calorie restriction studies restrict the control animals by 10-20% below their ad libitum intake in order to avoid confounding by obesity.
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