Calorie restriction (CR, also known as caloric restriction or energy restriction) is a dietary regimen that reduces the energy intake from foods and beverages without incurring

malnutrition Malnutrition occurs when an organism gets too few or too many nutrients, resulting in health problems. Specifically, it is a deficiency, excess, or imbalance of energy, protein and other nutrients which adversely affects the body's tissues a ...

. The possible effect of calorie restriction on body weight management,

longevity Longevity may refer to especially long-lived members of a population, whereas ''life expectancy'' is defined Statistics, statistically as the average number of years remaining at a given age. For example, a population's life expectancy at birth ...

, and aging-associated diseases has been an active area of research.

Dietary guidelines

Caloric intake control, and reduction for

overweight Being overweight is having more body fat than is optimally healthy. Being overweight is especially common where food supplies are plentiful and lifestyles are sedentary. , excess weight reached epidemic proportions globally, with more than ...

individuals, is recommended by US dietary guidelines and science-based societies. Calorie restriction is recommended for people with

diabetes Diabetes mellitus, commonly known as diabetes, is a group of common endocrine diseases characterized by sustained high blood sugar levels. Diabetes is due to either the pancreas not producing enough of the hormone insulin, or the cells of th ...

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. Mild calorie restriction may be beneficial for pregnant women to reduce weight gain (without weight loss) 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 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

. 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 World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

on a group of lean men, who restricted their calorie intake by 45% for six months and composed roughly 77% of their diet with carbohydrates.Keys A, Brozek J, Henschels A & Mickelsen O & Taylor H. The Biology of Human Starvation, 1950. University of Minnesota Press, Minneapolis 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 Anemia (also spelt anaemia in British English) is a blood disorder in which the blood has a reduced ability to carry oxygen. This can be due to a lower than normal number of red blood cells, a reduction in the amount of hemoglobin availabl ...

edema Edema (American English), also spelled oedema (British English), and also known as fluid retention, swelling, dropsy and hydropsy, is the build-up of fluid in the body's tissue (biology), tissue. Most commonly, the legs or arms are affected. S ...

, muscle wasting,

weakness Weakness is a symptom of many different medical conditions. The causes are many and can be divided into conditions that have true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, ...

dizziness Dizziness is an imprecise term that can refer to a sense of disorientation in space, vertigo, or lightheadedness. It can also refer to Balance disorder, disequilibrium or a non-specific feeling, such as giddiness or foolishness. Dizziness is a ...

irritability Irritability is the excitatory ability that living organisms have to respond to changes in their environment. The term is used for both the physiological reaction to stimuli and for the pathological, abnormal or excessive sensitivity to stimul ...

lethargy Lethargy is a state of tiredness, sleepiness, weariness, fatigue, sluggishness, or lack of energy. It can be accompanied by depression, decreased motivation, or apathy. Lethargy can be a normal response to inadequate sleep, overexertion, overw ...

, and depression. Typical low-calorie diets may not supply sufficient nutrient intake that is typically included in a calorie restriction diet.

Possible side effects

People losing weight during calorie restriction risk developing

side effect In medicine, a side effect is an effect of the use of a medicinal drug or other treatment, usually adverse but sometimes beneficial, that is unintended. Herbal and traditional medicines also have side effects. A drug or procedure usually use ...

s, such as cold sensitivity, menstrual irregularities,

infertility In biology, infertility is the inability of a male and female organism to Sexual reproduction, reproduce. It is usually not the natural state of a healthy organism that has reached sexual maturity, so children who have not undergone puberty, whi ...

, 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. Moderate amounts of calorie restriction may have harmful effects on certain population groups, such as lean people with low body fat.

Life extension

As of 2021,

intermittent fasting Intermittent fasting is any of various meal timing schedules that cycle between voluntary fasting (or reduced calorie intake) and non-fasting over a given period. Methods of intermittent fasting include alternate-day fasting, periodic fasting, ...

and calorie restriction remain under preliminary research to assess the possible effects on disease burden and increased lifespan during aging, although the relative risks associated with long-term fasting or calorie restriction remain undetermined. Intermittent fasting refers to periods with intervals during which no food but only clear fluids are ingested – such as a period of daily time-restricted eating with a window of 8 to 12 hours for any caloric intake – and could be combined with overall calorie restriction and variants of the

Mediterranean diet The Mediterranean diet is a concept first proposed in 1975 by the American biologist Ancel Keys and chemist Margaret Keys. The diet took inspiration from the eating habits and traditional food typical of Crete, much of the rest of Greece, and s ...

which may contribute to long-term cardiovascular health and longevity.

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

calorie The calorie is a unit of energy that originated from the caloric theory of heat. The large calorie, food calorie, dietary calorie, kilocalorie, or kilogram calorie is defined as the amount of heat needed to raise the temperature of one liter o ...

s. The men lost about 25% of their body weight of which 67% was fat mass and 17% fat-free mass. The quality of the diet was insufficient, as it was intended to accurately represent the diet during war due to the inadequate consumption of protein, and a lack of fruits and vegetables. Despite the extreme calorie restriction, the experiment was not representative of true calorie-restrictive diets, which adhere to intake guidelines for macronutrients and

micronutrient Micronutrients are essential chemicals required by organisms in small quantities to perform various biogeochemical processes and regulate physiological functions of cells and organs. By enabling these processes, micronutrients support the heal ...

s. Chronic weakness, decreased aerobic capacity, and painful lower limb edema was caused by the malnourished calorie restrictive diet. Emotional distress, confusion,

apathy Apathy, also referred to as indifference, is a lack of feeling, emotion, interest, or concern about something. It is a state of indifference, or the suppression of emotions such as concern, excitement, motivation, or passion. An apathetic i ...

, depression,

hysteria Hysteria is a term used to mean ungovernable emotional excess and can refer to a temporary state of mind or emotion. In the nineteenth century, female hysteria was considered a diagnosable physical illness in women. It is assumed that the bas ...

hypochondriasis Hypochondriasis or hypochondria is a condition in which a person is excessively and unduly worried about having a serious illness. Hypochondria is an old concept whose meaning has repeatedly changed over its lifespan. It has been claimed that th ...

, suicidal thoughts, and loss of sex drive were among the abnormal psychological behaviors that occurred within six weeks.

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

intensive care unit An intensive care unit (ICU), also known as an intensive therapy unit or intensive treatment unit (ITU) or critical care unit (CCU), is a special department of a hospital or health care facility that provides intensive care medicine. An inten ...

s 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.

Non-human primates

A calorie restriction study started in 1987 by the

National Institute on Aging The National Institute on Aging (NIA) is a division of the U.S. National Institutes of Health (NIH), located in Bethesda, Maryland. The NIA itself is headquartered in Baltimore, Maryland. The NIA leads a broad scientific effort to understand ...

showed that calorie restriction did not extend years of life or reduce age-related deaths in non-obese

rhesus macaque The rhesus macaque (''Macaca mulatta''), colloquially rhesus monkey, is a species of Old World monkey. There are between six and nine recognised subspecies split between two groups, the Chinese-derived and the Indian-derived. Generally brown or g ...

s. 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. 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 Fasting is the act of refraining from eating, and sometimes drinking. However, from a purely physiological context, "fasting" may refer to the metabolic status of a person who has not eaten overnight (before "breakfast"), or to the metabolic sta ...

blood glucose The blood sugar level, blood sugar concentration, blood glucose level, or glycemia is the measure of glucose concentrated in the blood. The body tightly blood sugar regulation, regulates blood glucose levels as a part of metabolic homeostasis ...

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. 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 In music and other performing arts, the phrase (; or 'as you desire'), often shortened to "ad lib" (as an adjective or adverb) or "ad-lib" (as a verb or noun), refers to various forms of improvisation. The roughly synonymous phrase ('in acc ...

animals in youth.

Sirtuin-mediated mechanism

Preliminary research indicates that sirtuins are activated by fasting and serve as "energy sensors" during

metabolism Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...

. 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

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s. Sir2 homologs have been identified in a wide range of organisms from bacteria to humans.