The effect of caloric restriction on longevity in animals has been observed as far back as 1935 when McCay observed extended lifespan in underfed rats. In that experiment, the rodents were weighed weekly and those on the restricted diet received calories based on weekly weight gain, they were allowed to gain 10g per week. The animals placed on caloric restriction in this manner “attained extreme ages beyond those of either sex that grew normally” (1).
Rate Of Living
McCay believed that longevity was attained by the slowing of growth, possibly influenced by the Rate Of Living Theory, proposed a few years before this experiment (*). The calorically restricted animals were smaller than animals eating freely and McCay observed that they could continue to grow even late into their lives, if and when calories were increased (2).
Caloric Restriction And Body Fat
Later theories proposed that it was a reduction in body fat or the reduction in caloric intake itself that extended life. Restriction of dietary macronutrients pointed toward a better understanding. Carbohydrate restriction and/or dietary fat restriction did not provide significant life extension. Protein restriction, however, does extend lifespan in some lifeforms (3). Further experiments showed that restricting one amino acid provided similar life extending properties to that of broad protein or amino acid restriction. Orentreich showed that cutting 80% of methionine from the diet increased the lifespan of rats by 30% (4). Methionine is essential for growth and rats placed on a restricted methionine diet from birth were smaller than average much like their calorically restricted predecessors.
Restriction In The Real World
Having tested various types of intermittent starvation, fasting and methionine restriction (veganism) on myself, I’m not convinced that the longevity effects (if translated to humans) are worth the costs. Caloric restriction, protein restriction, or even methionine restriction seem, well, too restrictive and all of those experiments were detrimental to my health, eventually. Many authors have noted the very limited compliance humans demonstrate regarding restrictive diets of this kind.
Methionine And Glycine
I’ve been reading about some of the benefits of glycine and it seems to confer similar life extending properties to methionine/protein/calorie restriction. An experiment providing different amounts of supplemental glycine in the diet of rats showed that addition of 12% glycine could increase lifespan by 30% (compared with rats given only 2.3% glycine). Both of these experimental diets provided 0.43% methionine. In the prior experiment on methionine restriction 0.86% was given in the control diet and the restriction group was provided with 0.17% methionine. So you could argue that this experiment included mild methionine restriction alongside glycine supplementation. At any rate, glycine supplementation significantly increased lifespan without any alteration in dietary methionine (6). Fasting glucose, insulin, and triglycerides were all lowered in the high glycine group. The team conducting these experiments believe that glycine’s benefits may be due to the removal of excess methionine from the liver, which is glycine -dependent.
Another experiment looked at the effect of glycine on methionine toxicity. Rats were fed a 2% methionine diet which led to accumulation of methionine in the liver. The addition of 2% dietary glycine eliminated the buildup of methionine in the liver and restored glycine levels (7). A similar study showed this effect of glycine could mitigate hyperhomocysteinemia caused by excess methionine (9). Hyperhomocysteinemia is associated with Alzheimer’s, cardiovascular disease and kidney disease. Glycine removes methionine from the liver, where excess methionine may be stored.
There seem to be a number reasons why methionine restriction or glycine supplementation may increase lifespan in some animals. One reason may be related to an interaction between reactive oxygen species (free radicals) and the mitochondria. The mitochondrial free radical theory of aging (MFRTA) posits that the damage caused by reactive oxygen species on the mitochondria is a cause of aging.
“Electrons may escape from metabolic processes in the mitochondria like the Electron transport chain, and these electrons may in turn react with water to form ROS such as the superoxide radical, or via an indirect route the hydroxyl radical. These radicals then damage the mitochondria’s DNA and proteins, and these damage components in turn are more liable to produce ROS byproducts. Thus a positive feedback loop of oxidative stress is established that, over time, can lead to the deterioration of cells and later organs and the entire body.
This theory has been widely debated and it is still unclear how ROS induced mtDNA mutations develop.” From Wikipedia:(*)
Methionine, Glycine, And Mitochondria
There are some studies looking at the effect of methionine and glycine on mitochondrial free radicals (mtROS) and on mitochondrial DNA integrity, which is threatened by mtROS. A 40% reduction in methionine decreases mtROS in rats and mice. The same effect was seen with 40% protein restriction, with no effect from carbohydrate or fat restriction (3).
Reversing Aging With Glycine?
A study on human fibroblast cells revealed down-regulation of a glycine synthesizing enzyme in cells taken from older people. Treating the cells with glycine reversed the aging signs, the cells were able to produce energy like much younger cells (8).
Glycine reverses aging in human cells. Will this translate into humans? We’ll probably be waiting a very long time for human studies, but there are pretty clear indicators that most people are not getting adequate glycine. A 2009 paper from The Journal of Biosciences argues that dietary glycine, plus what we can synthesize from other amino acids, comes far short of our requirements for just collagen production. They estimate most people get/make 4.5 to 6g per day total, with a minimum requirement of 10g for collagen synthesis in a 70kg individual (10). In the past humans likely ate a lot more glycine in the form of gelatinous parts of animals. Gelatin is about 25% glycine. Current diets are more skewed toward methionine rich and glycine poor meats. Just how much glycine is good or optimal isn’t clear, but a study using .8g/kg, or 56g of glycine per day in a 70kg individual, was well tolerated. The study showed a significant reduction in schizophrenia metrics (11). I take about 15/20g free glycine per day. I haven’t noticed much any difference in different brands of glycine, unlike most supplements. Recently I’ve been using myprotein from the UK. Bulksupplements Glycine Powder looks good in the US. I’ve also started using collagen as there are some benefits to this above what is had with free glycine.
“Rats were retarded in growth and not allowed to attain maturity until after periods of 766 and 911 days. The rat body still retains the power to grow at these extreme ages. After such periods of retardation the rat cannot attain a body size equal to that of an animal that grows to maturity younger. This conclusion is based upon the smaller size of the entire body, the weight of such organs as the heart, and the size of the bones represented by the femur. Even after these long periods of suppressed growth the male rat retains a growth potential greater than the female although the males of the retarded groups grow no larger than the normal females of this species. The hearts of all these animals dying in old age were larger than normal while the livers were smaller. The kidneys corresponded in weight at the time of death to the maximum weight attained by the body. The femurs of members of the retarded groups were less dense than those that matured normally.
In both retarded groups individuals of both sexes attained extreme ages beyond those of either sex that grew normally. The mean age of the males of both retarded groups was greatly increased in comparison with ‘rapid growth’ males while the mean age for the females was about the same in all three groups. The males of the retarded groups exceeded the females in age in contrast to the ‘rapid-growth’ group.
At a constant weight level in the course of retarded growth the female requires more calories for maintenance than the male. In the course of retarded growth, the diameter of the hair as well as the growth of the body reflects the retardation.(*)”
“McCay and colleagues proposed that the basis of this life extension was the slowing of growth. This hypothesis went unchallenged until 1960 when Berg and Simms proposed that the life extension was due to the reduction in body fat content. Starting in the 1970s a series of different hypotheses have been proposed to be the mechanism underlying the life extension. However, to date there is not a general agreement regarding the biological basis. In the years following the seminal 1935 paper, it was shown that the nutritional factor underlying the life extension is the reduction in energy intake; thus the term caloric restriction (CR) has been widely used when referring to this phenomenon. Over the years, CR has been reported to retard a spectrum of age-associated diseases and the aging of many physiological processes. Also, hypotheses on the evolution of the life extending action have been proposed. Prior to the 1990s, morphological pathology and physiology were the primary tools used to explore CR. However, during the past two decades, molecular biology has been increasingly put to use in the effort to understand the basis of the CR’s actions.(*)”
“Dietary restriction (DR), around 40%, extends the mean and maximum life span of a wide range of species and lowers mtROSp and oxidative damage to mtDNA, which supports the mitochondrial free radical theory of aging (MFRTA). Regarding the dietary factor responsible for the life extension effect of DR, neither carbohydrate nor lipid restriction seems to modify maximum longevity. However protein restriction (PR) and methionine restriction (at least 80% MetR) increase maximum lifespan in rats and mice. Interestingly, only 7weeks of 40% PR (at least in liver) or 40% MetR (in all the studied organs, heart, brain, liver or kidney) is enough to decrease mtROSp and oxidative damage to mtDNA in rats, whereas neither carbohydrate nor lipid restriction changes these parameters. In addition, old rats also conserve the capacity to respond to 7weeks of 40% MetR with these beneficial changes. Most importantly, 40% MetR, differing from what happens during both 40% DR and 80% MetR, does not decrease growth rate and body size of rats. All the available studies suggest that the decrease in methionine ingestion that occurs during DR is responsible for part of the aging-delaying effect of this intervention likely through the decrease of mtROSp and ensuing DNA damage that it exerts.(*)”
“Dietary energy restriction has been a widely used means of experimentally extending mammalian life span. We report here that lifelong reduction in the concentration of a single dietary component, the essential amino acid L-methionine, from 0.86 to 0.17% of the diet results in a 30% longer life span of male Fischer 344 rats.(*)”
“Dietary methionine (Met) restriction (MR) extends lifespan in rodents by 30–40% and inhibits growth. Since glycine is the vehicle for hepatic clearance of excess Met via glycine N-methyltransferase (GNMT), we hypothesized that dietary glycine supplementation (GS) might produce biochemical and endocrine changes similar to MR and also extend lifespan. Seven-week-old male Fisher 344 rats were fed diets containing 0.43% Met/2.3% glycine (control fed; CF) or 0.43% Met/4%, 8% or 12% glycine until natural death. In 8% or 12% GS rats, median lifespan increased from 88 weeks (w) to 113 w, and maximum lifespan increased from 91 w to 119 w v CF. Body growth reduction was less dramatic, and not even significant in the 8% GS group. Dose-dependent reductions in several serum markers were also observed. Long-term (50 w) 12% GS resulted in reductions in mean (±SD) fasting glucose (158 ± 13 v 179 ± 46 mg/dL), insulin (0.7 ± 0.4 v 0.8 ± 0.3 ng/mL), IGF-1 (1082 ± 128 v 1407 ± 142 ng/mL) and triglyceride (113 ± 31 v 221 ± 56 mg/dL) levels compared to CF. Adiponectin, which increases with MR, did not change in GS after 12 w on diet. We propose that more efficient Met clearance via GNMT with GS could be reducing chronic Met toxicity due to rogue methylations from chronic excess methylation capacity or oxidative stress from generation of toxic by-products such as formaldehyde.(*)”
“The addition of glycine to the high methionine diet effectively suppressed the enhancement of the hepatic methionine level and almost completely restored the glycine level, but it only partially restored the serine level and further decreased the threonine level. From these results, it is suggested that the alleviating effect of dietary glycine on methionine toxicity is primarily elicited by the restoration of the hepatic glycine level rather than by an increase in hepatic enzyme activity.(*)”
“These results indicate that glycine and serine were effective for suppressing methionine-induced hyperhomocysteinemia: serine and its precursor glycine are considered to have elicited their effects mainly by stimulating cystathionine synthesis by supplying serine, another substrate for cystathionine synthesis.(*)”
“Age-associated accumulation of somatic mutations in mitochondrial DNA (mtDNA) has been proposed to be responsible for the age-associated mitochondrial respiration defects found in elderly human subjects. We carried out reprogramming of human fibroblast lines derived from elderly subjects by generating their induced pluripotent stem cells (iPSCs), and examined another possibility, namely that these aging phenotypes are controlled not by mutations but by epigenetic regulation. Here, we show that reprogramming of elderly fibroblasts restores age-associated mitochondrial respiration defects, indicating that these aging phenotypes are reversible and are similar to differentiation phenotypes in that both are controlled by epigenetic regulation, not by mutations in either the nuclear or the mitochondrial genome. Microarray screening revealed that epigenetic downregulation of the nuclear-coded GCAT gene, which is involved in glycine production in mitochondria, is partly responsible for these aging phenotypes. Treatment of elderly fibroblasts with glycine effectively prevented the expression of these aging phenotypes.(*)”
“In a previous paper, we pointed out that the capability to synthesize glycine from serine is constrained by the stoichiometry of the glycine hydroxymethyltransferase reaction, which limits the amount of glycine produced to be no more than equimolar with the amount of C 1 units produced. This constraint predicts a shortage of available glycine if there are no adequate compensating processes. Here, we test this prediction by comparing all reported fluxes for the production and consumption of glycine in a human adult. Detailed assessment of all possible sources of glycine shows that synthesis from serine accounts for more than 85% of the total, and that the amount of glycine available from synthesis, about 3 g/day, together with that available from the diet, in the range 1.5–3.0 g/day, may fall significantly short of the amount needed for all metabolic uses, including collagen synthesis by about 10 g per day for a 70 kg human. This result supports earlier suggestions in the literature that glycine is a semi-essential amino acid and that it should be taken as a nutritional supplement to guarantee a healthy metabolism.(*)”