Calorie restriction

From Longevity Wiki

The physiological decline of an organism, known as aging, is a process highly conserved across the evolutionary tree[1]. External stressors such as excessive food intake, poor fitness or certain diseases can accelerate biological aging. Reducing calorie intake significantly below the levels of ad libitum (feeding without restriction) without malnutrition is commonly referred to as calorie restriction (CR) or dietary restriction (DR).[2]

A number of studies have indicated that CR can increase the lifespan (50-300%) and reduce the onset of age-related diseases in a variety of organisms (e.g. rats, mice, flies, worms, and yeast).[3] There is some evidence from human epidemiological and clinical trial data suggesting that CR could increase healthy lifespan by 1 to 5 years.[3]

Care should be taken when using CR as a means to increase lifespan and prevent age-related diseases. It is important to recognize that scientists point to the benefits of CR only when avoiding malnutrition and when performed under adequate nutrition.[4] Nutrient deficiencies are associated with various health deficits, and consuming less calories than recommended can also be detrimental. There is also concern that reductions in body fat mass could affect muscle bone and tissue functionality.[3] Thus, it is important to have sufficient quality macronutrient intake along with CR.

Additionally, there are risks associated to impaired immune function during CR interventions, an example of a potential trade-off.[3] There may be utility in combining CR with other interventions to maximize healthy longevity, but more data is needed from both animal and human studies.

Several mice studies have shown that different genetic backgrounds may substantially influence the response to CR.[5] This means that while some mice strains obtain lifespan benefits, others may attain no benefit or even experience harmful consequences.

Evidence

CR is the most widely researched intervention for slowing aging and preventing age-related diseases. Clive McCay first published his groundbreaking research in 1935 with the observation that rats with restricted diets experienced a 33% increase in lifespan.[6]

Similar survival experiments have shown that DR can increase the median and maximum lifespan of a variety of other organisms. Below we discuss in more details findings in each species:

Worms

Caenorhabditis elegans is a roundworm nematode widely used as an aging animal model.[7] Mutations in "eat" genes disrupt the function of the pharynx and the feeding behaviour of the worm, leading to partial starvation. Eat mutations are therefore considered CR-mimetics and can lengthen the lifespan of worms by up to 50%.[8] The most studied "eat" gene in C. elegans, eat-2, extends lifespan through a mechanism independent of the insulin-signalling pathway, as it does not require the transcription factor daf-16/FOXO (a central component of the insulin signalling pathway) to extend lifespan. Eat-2 mutants, as well as wild-type worms under CR, require the transcription factor pha-4/FOXA for the associated lifespan extension phenotype. More specifically, pha-4/FOXA is required in the intestinal tissue, but not in other tissues such as the nervous tissue, muscle or hypodermis.[9]

In another study, it was found that when C. elegans experiences dietary restriction early during development, proteostasis is enhanced and adult lifespan is increased.[10] Similarly, both dietary restriction and dietary deprivation complete removal of food) in adulthood is reported to increase lifespan and to enhance thermotolerance and resistance to oxidative stress.[11]

Mice

A caloric-restriction experiment was conducted on wild mice to see if they would experience similar results as genetically bred lab mice.[12] Whilst the 8.1% of longest-lived wild mice belonged to the CR test group, there was no robust longevity difference in mean lifespan between the groups. However, there was an anticancer effect in the CR group, as seen in other experiments with laboratory bred mice. Authors argued that strong differences in longevity were not noted possibly because wild animals have a higher genetic variation than inbred mice, which could affect CR strength.

In another study, it was noted that caloric restriction increased working memory in mice.[13] Male mice that experienced long periods of fasting between meals were found to live longer and healthier lifespans, regardless of the types of food they ate.

Inbred mice have shown to benefit significantly less from CR interventions than non-inbred mice, with some inbred mice strains not benefiting at all from CR.[14] Therefore, this suggests rodent studies might be potentially biased when conducting experiments in laboratory inbred mice and encourages the diversification of CR studies in a wider genetic background.

Dogs

Purina Lifespan Study

The Purina Lifespan Study was performed on Labrador retrievers randomly assigned to either 25% caloric restriction or to control feeding, offering the same diet and only differing by quantity.[15][16] Over 14 years of follow up, there was a 1.8 year extension in median lifespan along with several improved health markers, such as delayed osteoarthritis.[15][16] Whilst various measures of immune function are expected to decline with age,[17] the study showed that total lymphocytes, T-cells and CD8 cells did not decline in the CR group, in contrast to observed declines in the immune system function of the control diet group.[15]

Dog Aging Project

The Dog Aging Project is an initiative that is studying thousands of dogs over their lifetimes to understand which environmental and genetic factors influence healthy aging.[18] In an early study of 10.474 companion dogs, one-time daily feeding versus more frequent feeding was associated with better measures across multiple domains of health.[18] This included lower scores on the Canine Cognitive Dysfunction Rating Scale, and lower odds of having gastrointestinal, dental, orthopedic, kidney, urinary, liver and pancreas disorders. The authors suggest that while this preliminary data is not sufficient to support recommendations for meal timing in pet dogs, this data might help guide future research into dietary variables that affect health.[18]

Primates

Restrikal study (2006)

The Restrikal study, initiated in 2006, studied the effect of chronic 30% CR in the grey mouse lemur primate, Microcebus murinus.[19] Results of the study indicated that CR prolonged lifespan by 50%, from 6.4 to 9.6 years, but affected brain structural integrity.[19] It was observed that gray matter integrity in the cerebrum was compromised by CR, yet importantly, this did not result in any apparent changes to cognitive function.

NIA (2012) & Wisconsin NPRC (2014) studies controversy

The National Institute on Aging (NIA) study in Maryland, USA, performed CR in rhesus monkeys and saw no differences between survival of monkeys fed control versus calorie-restricted diets.[20] The diet of controls in this study was not reported as fully ad libitum, but rather control monkeys were subject to a slight dietary restriction to prevent obesity.

On the other hand, in the Wisconsin National Primate Research Centre (WNPRC) study, rhesus monkeys subjected to long-term 30% dietary restriction showed a significantly reduced risk of all-cause mortality and age-related mortality compared to control group monkeys. This suggested the benefits of CR on aging might be conserved in primates.[21]

Given that the Wisconsin and NIA primate studies found contradictory results, researchers have attempted to determine why slowed aging was only demonstrated in the Wisconsin study. The observed differences between these two studies is particularly controversial because the control primates in the NIA study lived longer than the CR group in the Wisconsin study, suggesting differences in methodology played an important role.

Some have suggested that diet composition is important, due to clear differences in feeding quality and composition between the Wisconsin and NIA studies. A key difference is certainly the fact that the Wisconsin study subjected monkeys to strict ad libitum in the control group, whilst the NIA study did not in order to prevent obesity. The latter is generally considered a better controlled experiment.

Humans

There is currently no definite evidence that calorie restriction extends healthy human lifespan.[5] However, there is early clinical evidence suggesting that CR without malnutrition may lead to various health benefits related to aging, based on several randomized controlled trials. In many human studies, CR is defined as a restriction of calories by ≥10% compared to feeding without restriction (ad libitum).[2]

The population of Okinawa

Studies into certain populations known for their exceptional longevity, such as in Okinawa - a small island of Japan - have provided some insights into potential lifestyle determinants of longevity. Okinawans have long been recognized as one of the longest-lived populations on the planet, and this is typically attributed to their diet (fish and vegetables). However, more recently, some attention in the scientific community has deviated from the contents of Okinawan’s diets and focused, instead, on their caloric deficits. Six generations of Okinawans aged 65+ were studied; their diet composition, energy intake and expenditure, and survival patterns were analyzed, among many other factors. The results lent support to the wide-ranging health benefits of caloric restriction in humans. Some researchers have speculated that the introduction of Westernized diets may in part explain recent decreases in Okinawan population lifespan.[3]

Biosphere-II

The Biosphere II experiment was an ecological investigation that provided an unexpected opportunity to measure the effects of CR.[22] Eight volunteers were kept in an ecological ecosystem for two years and allowed to harvest 85% of their food. The food consisted mainly of fruits, vegetables, grains and minimal protein. During the experiment, because of food scarcity, the energy intake of the volunteers decreased by 38% for 6 months. After leaving the experiment the volunteers had a 6% slowing of metabolism which lasted for another 6 months.

Years later, a Biosphere-II participant founded the CR Society International, which consists of a group of volunteers that have chosen to restrict their calorie intake around 30% for a period of 3 to 15 years.[23] Individuals of the CR society appear leaner, have lower body fat, better cardiometabolic health and lower inflammation. However, this data is sparse and largely limited to self-reports.

CALERIE trials

The Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE) research network has produced one of the most rigorous clinical studies conducted in humans. Over a period of nine years, three pilot trials were conducted followed by a randomized study (CALERIE 2).[24][25]

During phase 1 of the trial, three differing degrees of CR (20%, 25%, and 30%) were tested in a variety of age groups with an overweight BMI status. The trial lasted for 6 – 12 months, and the studies were used to develop and advance the following Phase 2 trial.

In Phase 2 of CALERIE, participants were able to restrict caloric intake by 11.9% and experienced ~10% weight loss over two years, despite the identified target of 25% CR. It must be noted that the level of CR achieved in this study required intensive intervention, involving personalized treatments, algorithmic/computer tracking, and various educational initiatives.[25] Therefore, the feasibility of such a CR intervention in the real world is something that remains uncharacterized. However, despite participants in the CR group achieving a lower CR target than intended, various improvements to health were noted. The trial resulted in lower levels of T3 and TNF-ɑ, while also reducing certain cardiometabolic risk factors.[24]

Additional analyses suggested a slow down in the rate of biological aging and found that weight loss did not appear to account for these effects.[26] The authors highlighted that, based on prior knowledge that a divergence in biological aging trajectories can be observed as early as early adulthood, CR may be more effective in humans when started young.[27] Moreover, potential CR-related toxicities were posited to be better tolerated in younger adults.[26]

CR and immune function - randomized controlled trial

One clinical study investigated moderate CR versus ad-libitum feeding over 2 years. It was found that CR without malnutrition may induce health benefits without impairing cell-mediated immunity or increasing infection risk in non-obese humans.[28]

Underlying biological mechanisms

Consuming extra calories can lead to cellular glycotoxicity and lipotoxicity, which causes inflammation and oxidative stress and thus increases the risk of age-related diseases (e.g. cancer, diabetes, cardiovascular disorders).[3] Some proposed health benefits of CR include preservation of cognition, protection of colon health and reduced risk of arthritis, amongst others.

Evidence suggests that CR may lead to a variety of health benefits via the following biological pathways:[3]

  • Inhibition of mTOR pathway and consequent induction of autophagy, a specific process that recycles cellular waste.
  • Activation of known pro-longevity pathways such as FOXO/AMPK/SIRT, which are evolutionarily conserved across various species.
  • Increase in coenzyme Q10 (CoQ) dependent reductases within the plasma membrane, thus protecting phospholipids and preventing the lipid peroxidation reaction progression.
  • Reduction in oxidative damage due to a decreased production of Reactive Oxygen Species (ROS).
  • Decrease in the systemic risk factors for cardiovascular disease (glucose levels, blood pressure, plasma lipid levels).
  • Alteration in the sympathetic nervous system, as well as the neuroendocrine system in lab animals and, sometimes, humans.

Caveats of caloric restriction

An exhaustive review of calorie restriction experiments in rodents highlighted some of the most common questions and caveats of CR:[29]

1. Until what age is CR effective?

One important aspect to consider is whether CR interventions are effective regardless of age of the individual. Whilst the question remains open as of today, available evidence suggests that performing CR in advanced age leads to a wide range of health benefits (especially in motor function) and is able to extend lifespan. However, it seems that late-life CR can extend lifespan to a significantly lesser degree than early-adulthood CR, although definitive evidence is missing.[29][30]

In humans, there is concern that late-life CR may exacerbate age-related muscle loss (sarcopenia) and potentiate the effects of falls.[31] While various pre-clinical studies have shown potential for treating or preventing sarcopenia, there is a lack of clinical evidence supporting its use in older, otherwise non-obese patients.[31][32][33]

2. Does CR improve cognitive decline?

CR has been shown to attenuate cognitive decline in mice models of Alzheimer's disease and to dramatically improve the behavioral phenotype of progeroid DNA-repair deficient mice.[34][35]

Despite this encouraging evidence, whether CR improves cognitive function in wild-type backgrounds appears largely inconsistent across experiments. A main issue might be the lack of systematic research and the range of differences in CR protocols, which has so far hindered extracting definitive conclusions. Similarly, several studies have shown inconclusive results in other species, such as in the fly Drosophila Melanogaster and the grey mouse lemur primate.[36][37]

In humans, the CALERIE trial included multiple tests of cognitive function and found that in non-obese, healthy adults, working memory was slightly improved after 2 years of CR.[38] However, this was a post-hoc analysis, which is not sufficient evidence to support causation.[38]

3. What are the deleterious effects of CR?
Bone health

In rodents, conventional 30% CR interventions have been associated to reduced body mass and reduced bone density, specially when CR is initiated in early life but also when initiated at mature or older ages.[39][40] More sophisticated studies have indicated that CR does not negatively impact bone material properties, despite its association to reduced bone size and decreased whole-bone strength.[41]

Wound healing

Experiments in both mice and rats has so far shown that animals under CR have a diminished capacity for skin wound healing than normally fed controls.[42][43] This appears to be consistent in vitro, where CR mice-derived cells were reported to have reduced proliferation compared to control cells.[44] Importantly, in vivo experiments found that wound healing capacity was restored to that of control levels a short period after rodents were normally fed again.[42][43]

Immune response

Several studies have shown that mice undergoing CR have an increased risk of infection than those on ad libitum diets. This is presumably due to a less efficient immune response, given that CR animals are not able to respond as well to the higher metabolic demands that an infection supposes.[45][46] However, similar to wound healing experiments, animals showed a recovered capacity to fight infection shortly after being re-fed. On the contrary, other studies have reported no deleterious effects of CR in response to infection or even beneficial ones.[47]

4. Are feeding times in CR important?
Time of feeding

The benefits of caloric restriction in mice appear to be affected by the timing of feeding during the day. As nocturnal animals, mice that underwent CR during their normally active feeding period (night time) showed increased health benefits compared to mice undergoing CR during their rest time (daylight), as measured by structural changes in the gut microbiota.[48] This showcases the important link between circadian clocks, CR interventions and potentially lifespan.

CR and Intermittent Fasting

See the full article on fasting.

Intermittent fasting (IF) has attained popular interest in recent years for its various potential health benefits, including for treating disease and extending lifespan.[49][50] Some preclinical evidence shows that certain IF regimens can prevent the onset of many age-related diseases. However, IF is not always associated with benefits in healthspan and may increase or decrease lifespan.[50] It has been proposed that at least part of the lifespan extending effect of CR in humans is related to fasting, and in rodents fasting is required for a CR diet to protect from frailty and extend lifespan.[50][51] In rodent models, the evidence for IF preventing cancer development or growth is ambiguous, with studies showing no effect or potential harm with IF. More studies are required to better understand IF, both for preclinical and clinical research.[52]

A recent study in China randomized 139 obese adults to either calorie restriction alone or to calorie restriction with time-restricted eating (a 16-hour intermittent fast and a 8-hour period for eating).[53] After one year, both groups had lost 7-10% of body weight and showed healthier markers for blood sugar, blood fat levels and insulin sensitivity. Importantly, there was no statistically significant difference between both groups, suggesting calorie restriction is responsible for the health-associated benefits and that intermittent fasting has no added benefits to CR diets.

5. How long should CR interventions last?

Current evidence in rodents suggests that short-term CR (considered as interventions ranging from 1 day to a few months) can still have beneficial effects in health, similar to traditional long-term CR.[54] Short-term CR has also been associated to increased health biomarkers, to improve diseased-states in models of hypertensive rats and to enhance the benefits of chemotherapy in cancer mouse models.[55][56]

6. What are the genotype and gender effects on CR?

There is strong evidence that genotype and gender within the same species can have a dramatic effect on the efficiency of CR. A meta-analysis from mice studies performed between 1934 and 2012 revealed that lifespan extension could vary within gender of the same strain and also between different strains, with the degree of lifespan extension ranging between 4 to 27%.[57][58] Strains belonging to the lower end of percentage of lifespan extension are often recombinant inbred strains.[14] Other studies have demonstrated that certain mice genotypes are highly unresponsive to CR interventions, although it is argued that different genotypes might be responsive to different strengths of CR.[59][60] As previously discussed in primates, similar observations have been reported in rhesus monkeys undergoing 30% CR, where independent groups reported similar health benefits but largely dissimilar survival curves, highlighthing the complexity of genotypic and/or environmental factors in the efficiency of CR.[20][21]

7. Is diet composition important in CR?

See the full article on protein restriction.

Diet composition in CR is likely to be important. As mentioned previously, there is a general agreement that CR should reduce the total amount of calories ingested whilst avoiding malnutrition (ie. without reducing protein or macronutrient intake), in order to maximise health benefits.[2][4] In fact, an exhaustive review of literature suggested that most commonly adopted CR protocols led to malnutrition in rodents, which they argue might have added to the observed discrepancies in survival curves across CR interventions.[61]

Some scientists have proposed that protein restriction (PR) in particular is responsible for the benefits observed in CR.[62][63] However, more recent studies reviewing all published data since the early 1930s until 2016 concluded that lifespan extension in rodents is due to CR alone and not due to a reduction of protein or any other macronutrients.[64][65]

Controversies of calorie restriction research

There are several criticisms against CR, some of which are highlighted by Sohal and Forster (2014) in “Caloric Restriction and the Aging Process: A Critique”.[66] The authors highlight that there is a large disparity in CR-related longevity increases: namely, that longevity effects are not universal and sometimes are not shared across genetic strains of the same species. Moreover, control animals in widely-cited caloric restriction studies were mostly fed ad libitum, causing them to become overweight and vulnerable to disease and early death. Therefore the relative benefit in the CR group was exaggerated compared to control subjects. In other words, animals with CR diets may live relatively longer because the control animals were dying from complications of excess feeding.

Another challenge related to CR as an effective intervention for human aging is the difficulty in compliance over long periods of time.[3] Concerns over mental and sexual health have also been raised with more severe CR. There are concerns over the loss of weight and fat mass in younger people practicing CR. Exercising along with CR and good nutrition (high protein diet) appears to be highly beneficial for loss of free fat.[3] New nutritional approaches such as intermittent fasting have emerged. However, there is comparatively limited research on the topic, with CR being the most well-studied nutritional intervention for healthy aging. Furthermore, studies in worms showed that Allantoin, rapamycin, TSA and LY-294002 led to a slower decline in pharyngeal pumping, indicating a reduced aging rate.[67] Thus, the study uncovered that not only could drug treatments mimicking CR increase longevity, but they could also improve the organism’s healthfulness.[67]

It is important to note that researchers are increasingly aware that CR or strict intermittent fasting are not a “one size fits all”, but rather an efficient strategy for certain individuals in specific metabolic contexts. For instance, some studies have shown that two people's glucose responses are significantly different even after eating the same food.[68] Supporting these findings, companies like Lumen Metabolism and Levels are offering personalized dietary recommendations based on the measurement of an individuals’s metabolic flexibility.

Similar to non-human primates, the effects of CR on lifespan remain controversial in humans. However, what seems clear from obesity studies is that eating too much results in poor health and decreased longevity.[69]

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