Caloric restriction (CR), the significant decrease in calorie intake, is a strategy for improving health and increasing lifespan. Increased lifespan has been observed in many types of calorie-restricted animals, including rhesus monkeys. Significantly, restricting calories does not only lengthen lifespan, but also healthspan (the period of healthy living before the onset of age-related diseases, such as diabetes, cardiovascular disease, and some cancers).
Lifespan studies in humans are difficult; however, CR studies in humans can measure biomarkers of aging (eg, dehydroepiandrosterone sulfate [DHEA-S], the levels of which decrease with age).
CR in humans has been shown to improve heart function, reduce markers of inflammation, and reduce risk factors for cardiovascular disease and diabetes.
CR has been shown to increase risk of diminishing muscle strength, aerobic capacity, and bone mineral density; therefore, proper exercise in addition to a CR diet is crucial. Never attempt a drastic change in diet or exercise regimen without consulting a doctor.
Maintaining a long-term CR diet can be very difficult and demanding. Therefore, calorie restriction mimetics (CRMs), or compounds that mimic the effects of CR, are desirable. CRMs mimic the metabolic, hormonal, or physiological effects of CR without reducing long-term food intake, while stimulating maintenance and repair processes, and producing CR-like effects on longevity and reduction of age-related disease.
Caloric restriction (CR) is a general strategy for improving wellbeing and lifespan. It is more than a simple limitation of calories for maintenance of body weight; CR is the dramatic reduction of caloric intake to levels that may be significantly (up to 50% in some cases) below that for maximum growth and fertility, but nutritionally sufficient for maintaining overall health (“undernutrition without malnutrition.”1). It remains one of the most researched and successful approaches to life extension in laboratory settings. Although the effects of CR on health are diverse, its mechanisms are not fully understood, and are thought to involve the activation of survival mechanisms that have been evolutionarily conserved to protect organisms from stress.
The idea of extending healthspan (the period of healthy living before the onset of age-related disease) and lifespan by lowering food intake is not a new one. Louis Caranaro’s 16th century best-selling anti-aging book suggested that longevity would come to those who ate only enough to sustain life; Benjamin Franklin supported the concept of abstinence as a defense against disease two centuries later.2 But it was the work of McCay in the 1930’s that first demonstrated that reducing calories below the level required for maximum fertility, while avoiding malnutrition, could extend the mean and maximum lifespan of laboratory rats by 40% or more.3 In the years following that seminal work, the health and longevity effects of CR have been observed in a wide range of organisms, ranging from single-celled Saccharomyces, to primates and man.
The practical challenge of long-term or lifetime CR has recently generated interest in caloric restriction mimetics (CRMs), an alternative to CR which may provide the pro-longevity benefits without an actual reduction in caloric intake.4 CRMs are a broad class of compounds and interventions that may promote life- and health-span by a diversity of mechanisms, ranging from induction of genes that protect against stress, to antioxidation and anti-inflammation.
CR in Animals and Non-human Primates
Seventy-five years of research have determined that the longevity and health effects of CR are a broad biological phenomenon that has been observed in species from three kingdoms of life (Animalia, Fungi, and Protoctista).5 Both the mean and maximum life spans of yeast (Saccharomyces cerevisiae), rotifers, nematodes (Caenorhabditis elegans), fruit flies (Drosophila melanogaster) and medflies, spiders, fish (guppies, zebrafish), rodents (hamsters, rats, mice), and dogs have been extended significantly by decreasing normal caloric consumption by 30 to 40 percent.6 Recently, the effects of CR on lifespan have been observed in non-human primates. The rhesus monkey (Macaca mulatta) is an excellent model for the study of human aging, exhibiting many physiological and biochemical similarities to humans.7 Unlike other animal aging models, the rhesus monkey also allows the study of brain atrophy, a characteristic of human aging that does not occur in smaller mammals8. With an average lifespan of about 27 years in captivity9, the rhesus also is suitable for determining the effects of CR on maximal lifespan.
Studies of the effects of CR on three separate rhesus colonies are currently underway; results from two have been published. A 20-year study conducted at the Wisconsin National Primate Research Center suggests that CR of baseline diet by 30 percent may slow aging in the rhesus, as gauged by two indicators of aging retardation: delays in mortality, and in the onset of age-associated diseases (particularly diabetes, cancer, cardiovascular disease, and neurological impairment; the most prevalent age-related diseases in humans).10 At study entry, the animals (46 males and 30 females) were at adult age (7-14 years); at the time the study was published (twenty years later), nearly three times as many control monkeys had died of age-related causes than CR monkeys (37% vs. 13%). The CR monkeys appeared to be biologically younger than their normal-fed counterparts, and not surprisingly, had lower body and fat mass. Sarcopenia (age-related muscle loss) was attenuated in the CR group. CR monkeys were also free of diabetes (compared to 5/38 of control animals) or glucose intolerance (compared to 11/38 of control animals). Incidence of cardiovascular disease, all cancers, and adenocarcinoma of the GI tract (the most common cancer in rhesus monkeys11) was reduced by half in the CR group. Calorie restriction resulted in preservation of brain volume in the caudate, putamen and insula, areas that are classically involved in regulation of motor and executive function. The effects of CR on maximum lifespan have yet to be determined for this colony, as animals in both groups are still living.
A smaller study at the University of Maryland tracked 8 CR rhesus monkeys and 109 ad-libitum (free-fed) controls over a period of 25 years, and produced many of the same observations.12,13 Ad libitum fed animals died at 25 years of age compared to a median survival of 32 years in the CR group. Calorie restriction also reduced hyperinsulinemia (elevated circulating insulin) and the frequency of age-associated diseases.
Although the available data is limited, these two studies do implicate that the healthspan and lifespan benefits of CR that have been observed in rodents and lower animals may also extend to primates and possibly man.
CR in Humans and Increased Lifespan
Assessing the effects of dietary interventions on human lifespan is a difficult endeavor; with average life expectancies of 75 and 80 years for men and women, respectively14 , any prospective study would likely necessitate several generations of researchers to carry out. Therefore, human aging studies must rely on surrogate measures (biomarkers) of aging. Reduced body temperature and lowered fasting insulin levels are robust markers of CR and slowed aging in rodents and rhesus monkeys.15
Dehydroepiandrosterone sulfate (DHEA-S), which declines in both rhesus monkeys and humans during normal aging, may be important in health maintenance and may serve as another potential longevity marker.16 DHEA-S, a product of the adrenal glands and the most abundant circulating steroid hormone, serves as the precursor to the sex steroids (androgens and estrogens). Increased DHEA-S levels in monkeys on CR are associated with survival17. Similarly, data from the Baltimore Longitudinal Study of Aging (BLSA)18 suggests that long-lived humans exhibit some of the same physiological and biochemical changes that accompany caloric restriction in animals. In the study, human survival rates were highest in those with low body temperatures, low levels of circulating insulin; and high DHEA-S levels.19
While there is yet no direct evidence of human lifespan extension by CR, there has been limited observational and clinical data that suggests a connection. In the 1970s, the Japanese island of Okinawa was reported to contain up to 40 times as many centenarians as other Japanese communities, which was suggested to result from CR (The caloric intake of adults and children in Okinawa was 20- and 40 percent lower than their mainland counterparts, respectively)20 Two decades earlier, a small study revealed that 60 healthy seniors receiving an average of 1500 kcal/day for a period of 3 years had significantly lowered rates of hospital admissions and a numerically lowered death rate than an equal number of control volunteers.21
CR in Humans Mitigates Disease Risk
There is a growing body of evidence suggesting that CR may reduce disease risk factors, which may have a direct influence on healthspan (and indirectly increase lifespan). Several observational studies have tracked the effects of CR on lean, healthy individuals, and have demonstrated that moderate CR (22-30% decreases in caloric intake from normal levels) improves heart function, reduces markers of inflammation (C-reactive protein, tumor necrosis factor (TNF)), reduces risk factors for cardiovascular disease (elevated LDL cholesterol, triglycerides, blood pressure) and reduces diabetes risk factors (fasting blood glucose and insulin levels).22,23,24,25 CR in healthy individuals has also been associated with reductions in circulating insulin-like growth factor - 1 (IGF-1), and cyclooxygenase II (COX-2) 26, all of which may be indicative of a decreased risk of certain cancers. Epidemiological data shows an association between higher plasma IGF-1 concentrations and a greater risk of breast27, prostate28, and colon cancers.29 COX-2, in addition to its role in inflammation, can promote the growth and spread of tumors.30 31 32
Preliminary results from the Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy (CALERIE)33 are reproducing many of the metabolic and physiologic responses to CR observed in rodents and monkeys.34 To better elucidate the effects of CR in humans, The National Institute on Aging (NIA) is sponsoring a multi-site randomized human clinical study to assess the safety and efficacy of 2 years of CR in non-obese but overweight healthy individuals. Researchers of the Pennington CALERIE group have followed 48 overweight (average BMI 27.5) middle-aged (average age 37) individuals for 6 months adopting one of 4 protocols: 1) 25% caloric restriction (CR group), 2) 12.5% CR with an additional 12.5% caloric expenditure from exercise (CREX group), 3) very low calorie diet (890 kcal/day) until 15% weight reduction, followed by a diet of sufficient calories to maintain this weight (VLCD group), or 4) control. Not surprisingly, all three intervention groups demonstrated reduced body weight, visceral (abdominal) fat, and fat cell size35,36, as well as reduced liver fat deposits.37 Fat loss was not significantly different between the CR and CREX groups (24% total fat, 27% visceral fat).38 All three intervention groups also demonstrated reductions in DNA damage.39 Only the CR and CREX groups, however, were able to improve two markers of longevity (reduced body temperature and reduced fasting plasma insulin), as well as reduce cardiovascular risk factors (LDL-C, triglycerides, and blood pressure. C-reactive protein was reduced only in the CREX group.40 Circulating thyroid hormone (T3) concentrations were lower in the CR and CREX groups. 41 Under conditions of CR, reduction in circulating thyroid hormone and body temperature suggests the normal adaptation of the body to lower energy intake and expenditure; similar reductions in T3 and metabolic rate have been observed in other human and animal CR studies.42 The CR groups also exhibited increases in the amount of mitochondria (the cellular sites of energy production), and increased expression of two genes (TFAM and PGC-1α) that are indicative of mitochondrial biogenesis, the formation of new mitochondria.43 Mitochondrial loss and dysfunction may be responsible for some of the most potent effects of the aging process.44
Similar results have been observed from the CALERIE studies at Washington University on a separate group of 50-60 year old non-obese overweight (average BMI 27) volunteers after 1 year of either CR (3 months of 16% CR followed by 9 months of 20% CR) or exercise training of equivalent energy expenditure (ie. expending 20% of daily caloric intake).45 CR improved cardiovascular parameters (left ventricular diastolic function, diastolic and systolic blood pressure) 46, lowered C-reactive protein and insulin resistance 47, and lowered circulating thyroid hormone T348 and fasting plasma insulin.49
CR in this second, older volunteer population was not without some negative consequences: Compared to the exercise-only group, CR demonstrated decreases in muscle mass, strength, and aerobic capacity.50,51 The CR group also demonstrated significantly more loss of bone mineral density (BMD) at the spine, hip, and femur (interochanter) than either the exercise-only or control groups, which was observable by month 3 of the study.52 It should be noted that in the younger CALERIE study group, there was no significant differences in BMD in any of the groups at month 6.53 The potential of losses in aerobic capacity and BMD stress the importance of exercise in CR protocols.
The CALERIE group at the Jean Mayer-USDA Human Nutrition Center on Aging at Tufts University compared the effects of CR diet composition (high glycemic vs. low glycemic load) in 29 healthy overweight adults provided with 30% calorie restricted meals for 6 months, followed by self-monitored restriction for an additional 6 months. Clinical indicators (fasting serum triglycerides, cholesterol, insulin) were significantly reduced in both groups at 6 and 12 months, but were not different between groups.54 There was no significant difference in weight loss or energy expenditure between the high glycemic (HG; 60% of calories from carbohydrates) and low glycemic (LG; 40% of calories from carbohydrates) groups, but the LG group lost significantly more fat mass, and retained more fat-free mass.55 The LG group also demonstrated greater declines in CRP during the first 6 months of the CR protocol.56 While these data indicate that the overall reduction in energy intake, and not diet composition, may be a more important determinant of weight loss and its associated CR health benefits, it does suggest additional benefits of LG diets. By their very nature, LG diets can limit postprandial (“post-meal”) elevations in blood glucose; aiding in the maintenance of the target 2-hour postmeal level of <140 mg/dL, which the International Diabetes Federation suggests may lower the risk of several diseases, including cancer, cognitive impairment, cardiovascular disease, and retinopathy.57
A randomized clinical trial examined the effects of two years of calorie restriction on metabolism and oxidative stress. The 53 participants who completed the trial were healthy and either normal-weight or slightly overweight at the time of enrollment; 34 were in a calorie restriction group, given detailed instructions and support to help them achieve a 25% reduction in daily caloric intake while maintaining adequate nutritional intake; 19 were in a control group, instructed not to substantially change their diet.
At the end of two years, the calorie restriction group had achieved an average calorie reduction of 14.8% and sustained an average weight loss of 19.1 pounds, mostly due to loss of fat. In addition, a significant drop in resting energy output, particularly during sleep, was measured in tests done after one and two years of calorie restriction. While some of this reduction could be attributed to loss of metabolically active tissue due to weight loss, most of it—equivalent to 80–120 calories per day—exceeded this expected outcome. This phenomenon of decreased energy output in excess of what could be accounted for by weight loss alone has been noted in previous weight loss studies and is known as metabolic adaptation. Metabolic adaptation is thought to represent a shift toward more efficient energy use.
Importantly, measures of oxidative stress were improved in the calorie restriction group. This effect was noted after one year and was sustained at two years. The drop in oxidative stress correlated with the degree of calorie restriction and metabolic adaptation attained. It has been proposed that reduced production of harmful free radicals resulting from more efficient metabolic activity is the link between calorie restriction and extended lifespan. This theory is supported by the current findings.185
The mechanism(s) of CR has not been definitively determined, although theories abound. Possible mechanisms include protection from oxidative damage, increased cellular repair, reduction in the production of catabolic cytokines, such as the inflammatory molecules tumor necrosis factor (TNF) and interleukin-6 (IL-6), and increases in energy (ATP) production.58
The free-radical theory of aging proposes that cumulative oxidative damage during to the course of normal metabolism compromise cellular function and cause aging59,60 the observation that CR inhibits oxidative damage to lipids, DNA, and protein supports a role of antioxidation as a CR mechanism.61,62,63,64 Levels of endogenous antioxidants (glutathione) and antioxidant enzymes (superoxide dismutase, catalase, glutathione-S-transferase) are also protected by CR from age-related decline in animal models. 65,66,67 CR also stimulates DNA repair.68
While inflammation is a complex, well-orchestrated process that is designed to limit injury and promote repair, uncontrolled or chronic inflammation can have the opposite effect; chronic inflammation has been implicated in a range of age-related diseases. Age-related increases in the production of pro-inflammatory enzymes, cytokines, and adhesion molecules may also accelerate aging through the increase in reactive oxygen and nitrogen species (ROS and RNS) and subsequent oxidative damage. In cell culture and animal models, CR has been shown to attenuate the inflammatory response by suppressing the production of pro-inflammatory proteins (interleukins 1B, 6 and, TNF) and prostaglandins (E2, I2) (reviewed in 69). CR has reduced the activity of the inflammatory enzyme COX-2 in rats70 and humans71, and has suppressed COX-derived free-radical production in rats.72
Autophagy is a major repair process for cellular damage73, one which has been associated with positive effects on longevity.74 During autophagy, intracellular components such as damaged or unnecessary cellular machinery or aggregated proteins are engulfed by organelles called autophagosomes and degraded within lysosomes (organelles that digest cellular wastes). Autophagy also represents an important mechanism for cell survival during nutrient deprivation.75 Recent studies have revealed that age-related reductions in autophagy in rats are slowed by CR.76,77
CR has been shown to increase efficiency of the mitochondrial energy production while decreasing the generation of reactive oxygen species, the undesirable by-product of this process.78,79
At the genetic level, CR has been shown to stimulate the production of several factors that are involved in nutrient sensing and insulin signaling, notably the proteins PGC-1α and SIRT1. PGC-1α (peroxisome proliferator-activated receptor γ coactivator-1α) is often described as the master regulator of mitochondrial biogenesis. Amongst its many functions, PGC-1α turns up (up-regulates) the expression of genes in the cell nucleus that encode mitochondrial enzymes.80 Additionally, PGC-1α stimulates the replication of mitochondrial DNA, a necessary step in mitochondrial biogenesis.81,82 The enzyme SIRT1, the founding member of the sirtuin gene family, has been of considerable interest in the last decade: acting as a “metabolic sensor”, SIRT1 may increase mitochondrial activity83, improve glucose tolerance84, and extend lifespan in experimental models.85 CR also reduces the production of mTOR (mammalian target of rapamycin), an enzyme that responds to levels of insulin and IGF-1, to control cell growth and division. mTOR is abnormally elevated in many cancers86, and its inhibition has been found to slow aging in yeast, nematodes, and mice.87
CR may attenuate some of the detrimental changes in gene expression that accompany the aging process. Aging in rats is accompanied by changes in expression of genes associated with increased inflammation and stress, and decreased apoptosis and DNA replication; CR reversed many of these changes.88 CR reduces the expression of nuclear factor kappa beta (NF-kB), a key mediator of inflammation. NF-kB senses cellular threats (such as free radicals or pathogens) and responds by activating other inflammatory genes. NF-kB activity is enhanced in many tissues during the aging process.89 By reducing NF-kB, CR in turn reduces the expression of other pro-inflammatory genes, including IL-1B, IL-6, TNFa, COX-2, and inducible nitric oxide synthase (iNOS).90
An attempt to resolve the seemingly disparate mechanisms of CR on life extension and health promotion has suggested a unified process, called hormesis, may also be at work.91 Hormesis is classically described as a phenomenon in which the response to a chemical or physical agent is different depending in the degree of its intensity92; for example, a cell might respond positively to caloric restriction (low intensity) but negatively to frank starvation (high intensity). In the context of aging, hormesis is characterized by the beneficial effects of cellular responses to the mild stress of caloric restriction, which stimulates maintenance and repair processes.93 In this manner, a significant, sustained reduction of calories below a certain threshold may activate several genes that sense the nutrient deprivation (such as sirtuins, PGC-1α, or mTOR), which turn off cell growth, and switch on processes that protect or repair the cell (which, in turn, may increase antioxidant capacity and attenuate inflammation).
Practicing Caloric Restriction with Optimum Nutrition (CRON)
Although CR has in the past been defined as a 30 to 40 percent reduction in calorie intake (as determined by daily energy expenditure) there is no “official” definition of caloric restriction,94 and newer investigations have revealed CR benefits can still occur at less-restrictive caloric intakes. Based on our current knowledge of CR, its definition may someday be not simply based on a restriction “value”, but rather a combination of anticipated gene expression patterns and physiological changes. As demonstrated in the examples above, CR protocols that have demonstrated significant results over a range of caloric intakes and durations, with and without the inclusion of exercise. Extremely low caloric intakes (only 550 kcal/day) have been used for very short durations (6 weeks) with dramatic results in obese individuals, insulin sensitivity increased by 35%; CRP decreased by half, and liver triglycerides decreased by 60%.95,96 However, maintenance of extreme CR for longer periods of time, for instance 45% CR for 6 months has resulted in several negative side effects including anemia, muscle wasting, neurologic deficits, edema.97 Although the comprehensive CALERIE studies were designed for CR of 16-25% and have demonstrated short-term success; when compliance is considered, the actual degree of CR in the groups may have been closer to 11.5%98
The frequency of meals is not important for CR, at least in animal models. Lifespan extensions in rodents have been observed at meal frequencies ranging from 6 times per day to 3 times per week.99,100 “Every-other-day-feeding” (EOD), which was initially thought to be distinct from CR, may actually function as a mild CR , and demonstrate a lower incidence of diabetes, lower fasting blood glucose and insulin concentrations.101 It is unclear whether meal frequency affects the benefits of CR in humans. While reduced meal frequency to 1 meal per day consuming sufficient calories to maintain body weight in healthy, normal-weight, middle-aged adults demonstrated significant increases in blood pressure and LDL-C 102, this effect was not observed in non-obese overweight individuals following an EOD approach to CR. 103
The duration of a CR plan depends on its anticipated outcomes. Although controlled longevity data is unavailable for humans, one could imagine that, based on human observational data and the wealth of animal studies, that life extension through CR requires a lifetime commitment. However, reduction in body fat mass, cardiovascular disease and diabetes risks are observable even within the abbreviated timescales of the CALERIE studies (6-12 months), as are certain markers of slowed aging, such as mitochondrial biogenesis and reduced DNA oxidative damage. Even short (21-48 day) periods of fasting or caloric/dietary restriction (such as religious fasts) can have favorable effects on blood lipids, insulin sensitivity, and biomarkers of oxidative stress.104,105 Short term CR has also been validated by gene expression data, in which alterations in the expression of age-related genes including those involved in inflammation, apoptosis, and DNA expression could be observed after only 4 weeks of CR in mice.106
While there is no defined composition of the CR diet, the potentially significant reduction in caloric intake necessitates the consumption of nutrient-dense foods, and the avoidance of “empty” calories from foods such as white flour and refined sugar. It is also imperative that the intake of essential micronutrients, such as vitamins, minerals, essential fatty acids and essential amino acids, are carefully monitored, and added back to the diet if necessary. Even a carefully chosen CR diet may not be nutritionally complete; in studies of 4 popular, published diet plans that limited calories to 1100-1700 per day including the NIH and American Heart Association-recommended “DASH diet”, all were found to be on average only 43.5% sufficient in RDIs for 27 essential micronutrients values, and deficient in 15 of them.107 While hunger cannot realistically be eliminated during a dedicated CR diet, there are dietary strategies to reduce hunger such as sufficient fiber consumption (increasing fiber intake to 35 grams/day had a significant effect on satiation and adherence to the CR protocol in the CALERIE study108) and consumption of “fast” proteins, like whey, that are rapidly absorbed and quickly signal satiety.109,110
Maintaining a dramatically reduced caloric intake over the long-term can be very demanding. Few people are willing to reduce their caloric consumption by the 30 to 40 percent to meet the classic CR definition,111 and even the less restrictive protocols (16-25%) used in human interventions have not been met with full compliance.112 The search for an alternative or complement to CR has involved the identification or development of compounds that mimic some of the physiological or gene-expression changes associated with CR, without the requirement of lowered caloric intake or loss of body weight. While many compounds can be broadly interpreted as CRMs, a more focused definition of CRM would be a compound or intervention that mimics the metabolic, hormonal, or physiological effects of CR without reducing long-term food intake, while stimulating maintenance and repair processes, and producing CR-like effects on longevity and reduction of age-related disease.113
Several compounds have been investigated as CRMs, with encouraging preliminary results in animal models. Tetrahydrocurcumin (a curcumin metabolite) and green tea polyphenols have both demonstrated increases in average and maximum lifespans in mice.114 The effects were observed when the mice received treatments by month 13 (if given later in life, the treatments had no effect on lifespan), and in the case of green tea extract, the treatment had no effect on body weight. An investigation of ginkgo biloba on cognitive behavior in male Fischer rats revealed an unexpected, statistically significant increase in average lifespan when compared to controls (26.4 vs. 31.0 months).115 The NIA Aging Intervention Testing program,116,117 a multi-center study on longevity-enhancing compounds, has already identified life-extending or CR mimetic activities in rapamycin118,119 and aspirin120 in rodents, and is currently testing other potential compounds including medium chain triglycerides, caffeic acid esters, and curcumin.121
Stress-induced plant compounds can stimulate stress responses in other species, this cross-species hormesis is called xenohormesis.122 Xenohormesis may have evolved as an early warning in animals about impending changes in the environment (such as scarcities in the food supply), allowing them to adapt accordingly. The most familiar of these stress-inducing compounds is resveratrol, well-known for its presence in grape skin, but present at detectable levels in several plant species. Resveratrol simulates caloric restriction123 in the absence of actual nutrient deficiency by activating sirtuins (SIRT1 is the human homolog), and has been shown to increase lifespan in fungi, nematodes, flies, fish, and mice124 SIRT1 also suppresses NF-kB (and the inflammatory cytokines and enzymes NF-kB activates), lending resveratrol anti-inflammatory activity in cell culture and animal models.125 ,126, 127 High-dose resveratrol reduced IGF-1 levels in healthy human volunteers, a chemopreventative activity that is also associated with CR.128 Pterostilbene, a methylated analogue of resveratrol from blueberries, similarly attenuates inflammation in a CR-like manner, reducing NF-kB signaling and COX-2 activities in cell culture.129, 130
Other plant-derived polyphenolic compounds (such as catechins, curcumin, or flavonoids) may also have xenohormesis activities as well; it has been suggested that the majority of health benefits from plant phytochemical consumption might not be from their antioxidant properties, but rather by a CR-like modulation of stress-response pathways.131 Fisetin, quercetin, proanthocyanidins, and theaflavins are examples of compounds that have inherent chain-breaking antioxidant chemistries, but appear to exert profound health effects unrelated to their ability to quench free radicals. Fisetin and quercetin have both been shown to stimulate SIRT1132, a central activity of CR. In vitro, fisetin, like CR, reduced mTOR signaling133, Nf-kB activation and COX-2 gene expression134, and activated antioxidative and detoxifying gene pathways (Nrf2).135 Fisetin has also been shown to increase lifespan in Saccharomyces 136 and Drosophila.137 Quercetin, in addition to proanthocyanidins from grape seed, have also been shown to reduce the production of inflammatory cytokines, and the expression of vascular endothelial growth factor (VEGF)138, which may prevent tumors from recruiting blood vessels. This same chemoprotective activity has been observed in rats under CR.139 Theaflavins are flavan-3-ols from black tea that are produced during the oxidation (fermentation) of tea leaves. Aside from their suppression of NF-kB and inflammatory cytokines in vitro and in mice140 and their induction of apoptosis in cancer cells141, theaflavins also stimulate the longevity factor Forkhead box 1 (FOXO1) in invertebrate and mammalian cells.142
Nicotinamide riboside is another naturally-occurring compound that may act as a CRM. It is a source of vitamin B3 and a precursor for nicotinamide adenine dinucleotide (NAD+), a molecule involved in a wide array of biological processes. NAD+, one of the important biologically active forms of NAD, is necessary for the activation of proteins called sirtuins, including SIRT1, that regulate cellular metabolism and DNA transcription.175-177 NAD+ levels are known to decrease with age, resulting in lower sirtuin activity. This may contribute to dysfunction in cell nuclei and mitochondria, and to a range of age-related disorders.177,178 Like calorie restriction and exercise, nicotinamide riboside can increase NAD+ levels and SIRT1 activation, and may be able to prevent or reverse age-related mitochondrial and metabolic dysfunction and disease.177-180 In cultured yeast cells, nicotinamide riboside supplementation raised NAD+ levels and increased lifespan without calorie restriction.181 Even in mice on a high-fat diet, nicotinamide riboside supplementation was found to raise NAD+ levels and SIRT1 activity, and was associated with positive metabolic effects, including less weight gain, improved exercise performance, and decreased liver fat.179
The glucoregulatory agent metformin can produce many of the gene expression changes found in mice on long-term caloric restriction, in particular, it can decrease the expression of chaperones; a set of proteins which, in addition to their other functions, can reduce apoptosis (self-destruction of damaged or malignant cells) and promote tumorgenesis.143 Metformin has increased mean lifespan in the worm C elegans.144 Along with the related anti-diabetic biguanide drugs phenformin and buformin, metformin extended the mean life span of mice by up to 37.9 percent and their maximum life span by up to 26 percent in multiple studies (reviewed in145) while significantly decreasing the incidence and size of mammary tumors.146 These effects on spontaneous tumor incidence, however, were limited to female animals.147 Metformin's CR-like effects are possibly due to influence on insulin or IGF-1 signaling. This mechanism may also explain the lifespan extension properties of the glucoregulatory herb Cinnamomum cassia (cinnamon bark) in the C. elegans 148
Numerous studies have found that metformin, which can induce a calorie restriction-like state, activates a critical enzyme called adenosine monophosphate-activated protein kinase (AMPK). This enzyme, which affects glucose metabolism and fat storage, has been called a “metabolic master switch” because it controls numerous pathways related to extracting energy from food and storing and distributing that energy throughout the body.144,157-162
Gynostemma pentaphyllum (G. pentaphyllum) is used in Asian medicine to promote longevity.163 Its longevity effects appear to be due, in part, to its ability to activate AMPK.161 Studies of G. pentaphyllum supplementation in humans demonstrate effects also found in calorie restriction, such as improved glucose metabolism, and reduced body weight, abdominal fat, and overall fat.112,162,164-165 Other studies found that G. pentaphyllum significantly improves insulin sensitivity, a mechanism also observed in studies of caloric restriction.35,104,166
Hesperidin and related flavonoids are found in a variety of plants, but especially in citrus fruits, particularly their peels.167,168 Digestion of hesperidin produces a compound called hesperetin along with other metabolites. These compounds are powerful free radical scavengers and have demonstrated anti-inflammatory, insulin-sensitizing, and lipid-lowering activity.169,170 Findings from animal and in vitro research suggest hesperidin’s positive effects on blood glucose and lipid levels may be related in part to activation of the AMP-activated protein kinase (AMPK) pathway.171-173 Accumulating evidence suggest hesperidin may help prevent and treat a number of chronic diseases associated with aging.169
Hesperidin may protect against diabetes and its complications, partly through activation of the AMPK signaling pathway. Coincidentally, metformin, a leading diabetes medication, also activates the AMPK pathway. In a six-week randomized controlled trial on 24 diabetic participants, supplementation with 500 mg of hesperidin per day improved glycemic control, increased total antioxidant capacity, and reduced oxidative stress and DNA injury.174 Using urinary hesperetin as a marker of dietary hesperidin, another group of researchers found those with the highest level of hesperidin intake had 32% lower risk of developing diabetes over 4.6 years compared to those with the lowest intake level.182
In a randomized controlled trial, 24 adults with metabolic syndrome were treated with 500 mg of hesperidin per day or placebo for three weeks. After a washout period, the trial was repeated with hesperidin and placebo assignments reversed. Hesperidin treatment improved endothelial function, suggesting this may be one important mechanism behind its benefit to the cardiovascular system. Hesperidin supplementation also led to a 33% reduction in median levels of the inflammatory marker high-sensitivity C-reactive protein (hs-CRP), as well as significant decreases in levels of total cholesterol, apolipoprotein B (apoB), and markers of vascular inflammation, relative to placebo.172 In another randomized controlled trial in overweight adults with evidence of pre-existing vascular dysfunction, 450 mg per day of a hesperidin supplement for six weeks resulted in lower blood pressure and a decrease in markers of vascular inflammation.183 Another controlled clinical trial included 75 heart attack patients who were randomly assigned to receive 600 mg hesperidin per day or placebo for four weeks. Those taking hesperidin had significant improvements in levels of high-density lipoprotein (HDL) cholesterol and markers of vascular inflammation and fatty acid and glucose metabolism.184
Fish oil, while not a CRM, appears to increase the efficacy of CR at preventing free radical damage; fish oil feeding with 40% CR in mice demonstrated synergistic reductions in thiobarbituric acid reactive substances (TBARS, a marker of lipid peroxidation), and was more effective at reducing inflammatory markers (COX-2 and iNOS expression) that CR or fish oil alone. 149
Pyrroloquinoline quinone (PQQ), a bacterial electron carrier154 and cofactor for several bacterial enzymes (and at least one mammalian enzyme155) increased mitochondrial DNA content and stimulated oxygen respiration (both indicative of biogenesis) in cultured mouse hepatoma cells through the activation of the CR gene PGC-1α.156
What You Need to Know about Caloric Restriction
Caloric Restriction (CR), a significant, sustained reduction of caloric intake from baseline levels, is the most thoroughly and successfully researched method for lifespan and healthspan extension in a broad range of animals and non-human primates.
In many cases, the reduction of caloric intake by 30 to 40 percent in animal models has resulted in longevity increases by 40 percent or more.
Although there is not yet direct human evidence of lifespan extension in humans from CR, results of the NIA-funded CALERIE study have shown significant reductions in risk factors for disease (cardiovascular disease, diabetes, some cancers), from moderate CR.
CR in humans reduces fasting insulin levels and lowers resting body temperature, which are two biomarkers for aging reversal.
Although CR has classically been defined as a long-term 30 to 40 percent reduction in calories, some CR health benefits in humans have been observed at less-restrictive caloric reductions (16 to 25 percent) over short time periods (weeks to months).
CR may work by reducing oxidative damage, increasing cellular repair, lowering production of inflammatory cytokines, or by hormesis, a mild stress that may stimulate cellular protection.
Several compounds may mimic the effects of CR without requiring a reduction in calories; these include resveratrol, metformin, green tea polyphenols, aspirin, and pyrroloquinoline quinone (PQQ).
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This information (and any accompanying material) is not intended to replace the attention or advice of a physician or other qualified health care professional. Anyone who wishes to embark on any dietary, drug, exercise, or other lifestyle change intended to prevent or treat a specific disease or condition should first consult with and seek clearance from a physician or other qualified health care professional. Pregnant women in particular should seek the advice of a physician before using any protocol listed on this website. The protocols described on this website are for adults only, unless otherwise specified. Product labels may contain important safety information and the most recent product information provided by the product manufacturers should be carefully reviewed prior to use to verify the dose, administration, and contraindications. National, state, and local laws may vary regarding the use and application of many of the treatments discussed. The reader assumes the risk of any injuries. The authors and publishers, their affiliates and assigns are not liable for any injury and/or damage to persons arising from this protocol and expressly disclaim responsibility for any adverse effects resulting from the use of the information contained herein.
The protocols raise many issues that are subject to change as new data emerge. None of our suggested protocol regimens can guarantee health benefits. The publisher has not performed independent verification of the data contained herein, and expressly disclaim responsibility for any error in literature.
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