Calorie Restriction or Caloric Restriction (CR) is the practice of limiting dietary energy intake, while insuring sufficient quantities of vitamins, minerals and other important nutrients are eaten, in the hope of improving health and retarding aging. The central, radical premise of Caloric Restriction dieting is that the less you eat, the longer you'll live. CR is the only known dietary measure scientifically proven to extend maximum lifespan, as opposed to increasing average lifespan.
In human subjects, CR has been shown to lower cholesterol, fasting glucose, and blood pressure. Some consider these to be biomarkers of aging, since there is a correlation between these markers and risk of diseases associated with aging. Except for houseflies, animal species tested with CR so far, including primates, rats, mice, spiders, Drosophila, C. elegans and rotifers, have shown lifespan extension. In CR, energy intake is minimized, but sufficient quantities of vitamins, minerals and other important nutrients must be eaten. To emphasize the difference between CR and mere "FR" (food restriction), CR is often referred to by a plethora of other names such as CRON or CRAN (calorie restriction with optimal/adequate nutrition), or the "high-low diet" (high in all nutrients aside from calories, in which it is "low"). Other names for the diet emphasize the goal of the diet, such as CRL (calorie restriction for longevity), or simply The Longevity Diet, as in a recently published book by that name.
While modern innovations and improvements to safety can decrease the annual mortality percentage, caloric restriction can demonstratably raise maximum life expectancy itself.
How Does Caloric Restriction Work?:
There have been many hypotheses put forward as to the exact method of action behind CR, however it is still not completely understood.
The Hormesis Hypothesis of CR proposes that the diet imposes a low-intensity biological stress on the organism, which elicits a defense response that helps protect it against the causes of aging. In other words, CR places the organism in a defensive state so that it can survive adversity, and this results in improved health and longer life. This switch to a defensive state may be controlled by longevity genes.
Sir2 or "silent information regulator 2" is a longevity gene in baker's yeast cells that extends lifespan by suppressing DNA instability. Sirtuins are hypothesized to play a key role in an organism's response to stresses (such as heat or starvation) and to be responsible for the lifespan-extending effects of CR. Certain small molecules such as resveratrol may also activate the SIRT1 gene (a Sir2 homolog in mamals), and extend lifespan.
Free Radicals and Glycation:
Two very prominent theories of aging are the free radical theory and the glycation theory, both of which can explain how CR could work. With high amounts of energy available, mitochondria do not operate very efficiently and generate more superoxide. With CR, energy is conserved and there is less free radical generation. A CR organism will be less fat and require less energy to support the weight, which also means that there does not need to be as much glucose in the bloodstream. Less blood glucose means less glycation of adjacent proteins and less fat to oxidize in the bloodstream to cause sticky blocks resulting in atherosclerosis.
In examining Calorie Restriction with Optimal Nutrition (CRON), it is observed that with less food, and equal nutritional value, there is a higher ratio of nutrients to calories. This may lead to more ideal essential and beneficial nutrient levels in the body. Many nutrients can exist in excess to their need, without side effects as long as they are in balance and not beyond the body's ability to store and circulate them. Many nutrients serve protective effects as antioxidants, and will be at higher levels in the body as there will be lower levels of free radicals due to the lower food intake.
Calorie Restriction with Optimal Nutrition has not been tested in comparison to Calorie Excess with Optimal Nutrition. It may be that with extra calories, nutrition must be similarly increased to ratios comparable to that of Calorie Restriction to provide similar antiaging benefits.
Stated levels of calorie needs may be biased towards sedentary individuals. Calorie restriction may be more of adapting the diet to the body's needs.
Although aging can be conceptualized as the accumulation of damage, the more recent determination that free radicals participate in intracellular signaling has made the categorical equation of their effects with "damage" more problematic than was commonly appreciated in years past.
CR diet is not a protective strategy for patients with amyotrophic lateral sclerosis (ALS), as low energy intake for such patients hastens death. Hence CR is contraindicated.