But, some studies did not show this benefit, and in some mouse strains, calorie restriction shortened lifespan rather than extending it. In the worm C.

elegans , a fasting diet increased lifespan by 40 percent. A study with fruit flies found that calorie restriction—but not intermittent fasting—was associated with living longer.

One study of male mice found that lifelong alternate-day fasting increased longevity, mainly by delaying cancer occurrence rather than slowing other aging processes. Two National Institute on Aging NIA -supported studies in rhesus monkeys sought to find out whether the benefits of calorie restriction are seen in longer-lived species.

In both studies, the monkeys were kept on a calorie-restriction diet 30 percent fewer calories than for monkeys in the control groups for more than 20 years. Although there were differences between the two studies—including monkey breed and type of food—both provided evidence that calorie restriction reduced the incidence of age-related conditions, such as cancer, heart disease, and diabetes.

One study found an extension of lifespan, while the other did not. Many of the monkeys are still alive, so the full impact of calorie restriction on their maximum lifespan has yet to be determined. Some study results suggest that calorie restriction may have health benefits for humans, but more research is needed before we understand its long-term effects.

There are no data in humans on the relationship between calorie restriction and longevity. Some people have voluntarily practiced extreme degrees of calorie restriction over many years in the belief that it will extend lifespan or preserve health.

Studies on these individuals have found markedly low levels of risk factors for cardiovascular disease and diabetes. The studies have also found many other physiologic effects whose long-term benefits and risks are uncertain, as well as reductions in sexual interest and the ability to maintain body temperature in cold environments.

These people generally consume a variety of nutritional supplements, which limits knowing which effects are due to calorie restriction versus other factors. To conduct a more rigorous study of calorie restriction in humans, NIA supported a pioneering clinical trial called Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy CALERIE.

In CALERIE, young and middle-aged, normal-weight or moderately overweight adults were randomly divided into two groups. People in the experimental group were told to follow a calorie-restriction diet for 2 years, while those in the control group followed their usual diet.

The study was designed to have participants in the experimental group eat 25 percent fewer calories per day than they had regularly consumed before the study. Although they did not meet this target, they reduced their daily caloric intake by 12 percent and maintained, on average, a 10 percent loss in body weight over 2 years.

A follow-up study 2 years after the intervention ended found that participants had sustained much of this weight loss. It's important to note that calorie-restriction regimens are not starvation diets. The weight loss achieved with calorie restriction in the CALERIE trial resulted in body weights within the normal or overweight range.

Compared to participants in the control group, those in the calorie-restriction group had reduced risk factors lower blood pressure and lower cholesterol for age-related diseases such as diabetes , heart disease , and stroke. They also showed decreases in some inflammatory factors and thyroid hormones.

There is some evidence that lower levels of these measures are associated with longer lifespan and diminished risk for age-related diseases. Moreover, in the calorie-restricted individuals, no adverse effects and some favorable ones were found on quality of life, mood, sexual function , and sleep.

The calorie-restriction intervention did cause slight declines in bone density, lean body mass, and aerobic capacity the ability of the body to use oxygen during exercise. However, these declines were generally no more than expected based on participants' weight loss.

Other short-term studies have found that combining physical activity with calorie restriction protects against losses of bone, muscle mass, and aerobic capacity. Some CALERIE participants also experienced brief episodes of anemia diminished number of circulating red blood cells that carry oxygen through the body.

Overall, these findings indicate that while the degree of calorie restriction in CALERIE is safe for normal-weight or moderately obese people, clinical monitoring is recommended. Most research to date has focused on the weight-loss aspect of fasting, primarily in obese people, and only a few small clinical trials have been conducted.

More work is needed to determine which, if any, types of fasting diets have long-term benefits. Observational studies have been conducted in people who practice fasting in one form or another. In an observational study, the investigator does not determine the treatment to offer and does not randomize subjects into a control group or experimental group.

Instead, the investigator records data from real-life situations. For example, one observational study compared people who routinely fasted as part of a religious practice or for another reason to those who did not fast.

It found that those who routinely fasted were less likely to have clogged arteries or coronary artery disease. However, the study did not control for other factors that could have affected the results, such as the kind of diet, quality of food consumed, or use of nutritional supplements.

After decades of research, scientists still don't know why calorie restriction extends lifespan and delays age-related diseases in laboratory animals. Do these results come from consuming fewer calories or eating within a certain timeframe? Are the results affected by the diet's mix of nutrients?

Several studies have focused on what occurs inside the body when caloric intake is restricted. In laboratory animals, calorie restriction affects many processes that have been proposed to regulate the rate of aging. These include inflammation, sugar metabolism, maintenance of protein structures, the capacity to provide energy for cellular processes, and modifications to DNA.

Another process that is affected by calorie restriction is oxidative stress, which is the production of toxic byproducts of oxygen metabolism that can damage cells and tissues.

Several of these processes were similarly affected by calorie restriction in the human CALERIE trial. However, we do not yet know which factors are responsible for calorie restriction's effects on aging or whether other factors contribute. Research supported by NIA has also focused on the effects of intermittent fasting.

During fasting, the body uses up glucose and glycogen, then turns to energy reserves stored in fat. This stored energy is released in the form of chemicals called ketones.

These chemicals help cells—especially brain cells—keep working at full capacity. Some researchers think that because ketones are a more efficient energy source than glucose, they may protect against aging-related decline in the central nervous system that might cause dementia and other disorders.

Ketones also may inhibit the development of cancer because malignant cells cannot effectively obtain energy from ketones. In addition, studies show that ketones may help protect against inflammatory diseases such as arthritis. The body may change the rate at which it burns calories depending on how many calories a person eats.

Therefore, an individual on a 1,calorie diet may burn fewer of them. This can slow weight loss. It is possible that some people might not lose any weight or will need to remain on a restricted calorie diet for a significant time before experiencing results.

Weight loss will also vary between people. For some people, an alternative approach, such as getting more exercise or consuming less sugar , might be a more suitable option.

It is more difficult to get essential macronutrients , such as proteins and fats , and micronutrients, including vitamins and minerals, on a low calorie diet.

Therefore, it is important to read nutritional labels and talk with a healthcare professional before trying this method of eating. A dietitian can help people create a personalized eating plan. A 1,calorie diet can potentially be unhealthy for some people. For example, someone who receives all their calories from sweetened cereal, potato chips, or cookies may experience malnourishment.

A major risk of long-term calorie restriction is malnutrition. This can affect the body in the following ways:. People on a 1,calorie diet need to eat nutrient-dense food to avoid feeling hungry and prevent malnourishment.

Lean proteins tend to be very dense in nutrients and low in calories. By contrast, white carbohydrates , such as white bread, contain more calories and fewer nutrients. The National Heart, Lung, and Blood Institute provides menu plans and other information about which foods to choose when following a 1,calorie diet.

Following a 1,calorie diet does not necessarily mean a person has to avoid any specific foods entirely. Allowing an occasional small treat may make it easier to stick to this restrictive diet. However, very high calorie foods, such as large muffins or slices of cake, can make it difficult to stay under 1, calories for the day while maintaining nutritional needs.

Eliminating empty calories from the diet can free up more space for treats. Some sources of empty calories include:. A calorie deficit is one of the most important factors for weight loss. However, other factors, such as meal timing and the type of food someone eats, will also contribute to weight loss.

Weight loss may not happen the same way for every person. Some people may lose weight faster than others or benefit from different methods. It may also be difficult to consume all necessary nutrients on very low calorie diets. People can work with a healthcare professional to ensure they are eating a balanced diet while still achieving a safe calorie deficit.

Reducing the number of calories a person consumes can help them lose weight. Low calorie diets may also reduce the risk of certain diseases, such as diabetes. However, weight is not the only or best measure of health.

A person will not necessarily become healthier merely by losing weight. A doctor or dietitian can help a person plan a custom diet and safe nutrition plan that offers a balance of nutrient-dense food alongside occasional treats. A person on a calorie diet eats only about a quarter of the recommended daily intake.

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However, these declines were generally no more than expected based on participants' weight loss. Other short-term studies have found that combining physical activity with calorie restriction protects against losses of bone, muscle mass, and aerobic capacity. Some CALERIE participants also experienced brief episodes of anemia diminished number of circulating red blood cells that carry oxygen through the body.

Overall, these findings indicate that while the degree of calorie restriction in CALERIE is safe for normal-weight or moderately obese people, clinical monitoring is recommended. Most research to date has focused on the weight-loss aspect of fasting, primarily in obese people, and only a few small clinical trials have been conducted.

More work is needed to determine which, if any, types of fasting diets have long-term benefits. Observational studies have been conducted in people who practice fasting in one form or another. In an observational study, the investigator does not determine the treatment to offer and does not randomize subjects into a control group or experimental group.

Instead, the investigator records data from real-life situations. For example, one observational study compared people who routinely fasted as part of a religious practice or for another reason to those who did not fast. It found that those who routinely fasted were less likely to have clogged arteries or coronary artery disease.

However, the study did not control for other factors that could have affected the results, such as the kind of diet, quality of food consumed, or use of nutritional supplements.

After decades of research, scientists still don't know why calorie restriction extends lifespan and delays age-related diseases in laboratory animals. Do these results come from consuming fewer calories or eating within a certain timeframe?

Are the results affected by the diet's mix of nutrients? Several studies have focused on what occurs inside the body when caloric intake is restricted.

In laboratory animals, calorie restriction affects many processes that have been proposed to regulate the rate of aging. These include inflammation, sugar metabolism, maintenance of protein structures, the capacity to provide energy for cellular processes, and modifications to DNA.

Another process that is affected by calorie restriction is oxidative stress, which is the production of toxic byproducts of oxygen metabolism that can damage cells and tissues.

Several of these processes were similarly affected by calorie restriction in the human CALERIE trial. However, we do not yet know which factors are responsible for calorie restriction's effects on aging or whether other factors contribute.

Research supported by NIA has also focused on the effects of intermittent fasting. During fasting, the body uses up glucose and glycogen, then turns to energy reserves stored in fat. This stored energy is released in the form of chemicals called ketones.

These chemicals help cells—especially brain cells—keep working at full capacity. Some researchers think that because ketones are a more efficient energy source than glucose, they may protect against aging-related decline in the central nervous system that might cause dementia and other disorders.

Ketones also may inhibit the development of cancer because malignant cells cannot effectively obtain energy from ketones. In addition, studies show that ketones may help protect against inflammatory diseases such as arthritis.

Ketones also reduce the level of insulin in the blood, which could protect against type 2 diabetes. But too many ketones in the blood can have harmful health effects. This is one of the reasons researchers want to understand more about how calorie restriction diets work before recommending them.

Most calorie-restriction and fasting-diet studies have been in younger people, but researchers are beginning to study older adults.

A clinical trial conducted by NIA is testing the diet in obese people, age 55 to 70, with insulin resistance. This is a condition in which cells do not respond normally to the hormone insulin. This can lead to serious diseases such as diabetes.

People in the experimental group can eat at will for 5 days, and then for 2 consecutive days are restricted to to calories per day. The experiment is designed to find out how 8 weeks of the diet, compared to a regular diet, affects insulin resistance and the brain chemicals that play a role in Alzheimer's disease.

In the coming years, researchers will continue to explore many unresolved questions. What are the long-term benefits and risks of the various eating patterns?

Which diets are feasible as a long-term practice? What specific biological effects on aging and disease are triggered by a particular eating pattern? If a specific way of eating is recommended, at what age is it best to start, and is it safe to continue as you get older? There's insufficient evidence to recommend any type of calorie-restriction or fasting diet.

A lot more needs to be learned about their effectiveness and safety, especially in older adults. You may be tempted to try one of these eating patterns.

It's important to make sure that whatever you try provides you with a safe level of nutrition. Talk with your healthcare provider about the benefits and risks before making any significant changes to your eating pattern. An official website of the National Institutes of Health.

Share: Print page Facebook share Linkedin share X social media share. Research Highlights Calorie restriction and fasting diets: What do we know? August 14, On this page: What Are the Different Forms of Calorie Restriction and Fasting?

What's the Evidence from Animal Studies? What's the Evidence from Human Studies of Calorie Restriction? What's the Evidence from Human Studies of Fasting? How Does Calorie Restriction or Fasting Work? What's Next in Research? However, these studies are not without controversy.

Many scientists are concerned that calorie restriction will not benefit humans, or that it could even result in long-lasting harm.

Is calorie restriction a silver bullet against aging, or something that should be kept in the laboratory? At first glance, calorie restriction is a counter-intuitive way to approach longevity.

Consuming so much less food than a normal diet seems like it should reduce your lifespan, not extend it. There are two classic theories that could potentially shed some insight into this process: the rate of living theory, and the free radical theory. The rate of living theory arose from the observation that larger animal species tend to have longer lifespans than small ones.

This includes energy needed for processes like breathing, maintaining body temperature, and circulating blood. Since larger animals have also been observed to have lower metabolic rates, this theory suggests that slower metabolic rates — i.

Figure 1: The so-called rate of living theory posits that larger animals live longer than smaller animals due to their slower metabolic rates.

The free radical theory provides a potential explanation for the rate of living theory. Under this theory, aging is a result of the cells in the body accumulating damage over time.

This damage comes from so-called free radicals, which are highly reactive atoms or molecules naturally produced by our bodies. They can damage proteins, DNA, and fatty tissue, which in turn is theorized to eventually cause age-related diseases, such as heart conditions, neurodegenerative disorders, or cancers Figure 2.

Because free radicals are created by our metabolic processes, slower metabolisms could decrease the rate of free radical production and subsequently extend lifespans. Figure 2: According to the free radical theory, highly reactive atoms or molecules, called free radicals, can damage DNA, fatty tissues, and proteins in the body.

Accumulation of this damage may be what leads to aging. The exact mechanisms of aging remain an open question, but regardless of whether either of these theories are completely accurate, metabolic rates and free radicals seem likely to play some role in the aging process. Calorie restriction enters this equation because it is known that drastically reducing food intake will reduce metabolic rate.

If less food is being consumed, then there is less food that the body has to process. Moreover, since calorie restriction generally results in weight loss, less energy overall is needed to maintain the reduced body mass.

As a result of this reduction of metabolic rate, it is hypothesized that calorie restriction could extend lifespans by decreasing the rate of free radical damage. This idea is further supported by direct evidence that some species produce fewer free radicals under calorie restriction.

Although there is no consensus on the best way to specifically measure damage from free radicals, there is also some evidence that calorie restriction may result in lower levels of protein and DNA damage. This suggests that calorie restriction slows down the aging process, allowing the organism to live longer and with less risk of age-related diseases.

While many scientists are optimistic about the potential for calorie restriction to improve human longevity and quality of life, many others are skeptical of these studies and concerned that calorie restriction in humans could do more harm than good.

One of the largest critiques of calorie restriction studies is how the control group is treated , which is a common issue across the wide range of species tested. While the experimental group animals are placed on a highly restrictive diet, many studies allow the control group to eat as much as they want.

The control group often ends up consuming much more than they normally would in nature, which can lead to a number of weight-related diseases and poorer health overall.

Moreover, some studies examining rodents have found that benefits from calorie restriction are proportional to how excessively those animals would normally eat. In other words, a rodent that might ordinarily gain a lot of weight when eating freely would experience a larger improvement in health than a rodent that might naturally eat a more moderate diet Figure 3.

Hence, it is possible that the benefits from calorie restriction may only be due to how unhealthily those animals might otherwise live.

In animals that already eat healthy portions, it could be the case that calorie restriction may not yield any particular advantage.

Figure 3: Some studies suggest that animals who would normally overfeed are more likely to reap benefits from calorie restriction, compared to animals who would normally eat moderately.

Even putting the concerns with these studies aside, many scientists are skeptical of applying results from other species to humans.

It is much more challenging to carry out calorie restriction studies in humans, since we simply cannot and should not exert the same degree of control over human subjects as we might for rats. As a result, there have been very few studies in humans.

The most prominent such human study was the CALERIE trial , a randomized clinical trial in which healthy people were divided into a calorie reduction group and a control group. Because the trial only lasted for two years, the effects on lifespan could not be directly measured, and the goal was to instead investigate the effects on typical markers of age-related disease risk.

Nevertheless, there were still significant health benefits observed in this group. This included lower cholesterol and blood pressure levels, as well as improved insulin sensitivity index.

However, this trial alone is not enough evidence to conclude that calorie restriction should enter standard medical practice. Even though the participants were all healthy, many of them had BMIs that fall in the overweight category at the start of the trial.