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To restrict calories or not to restrict calories, that is the question

by Sasha Crowe Published on 23rd Jun 2015

by Sasha Crowe Published on 23rd June 2015

Before contemporary, come-and-go diet fads became all the rage, previous cultures have long revered another form of diet control: fasting. In and among remarkably differing places and beliefs, from Tibet to Japan to religious observations such as Islamic Ramadan, fasting practitioners can be found. This type of behaviour calls into question whether it actually does bring about any actual health benefits. Are they onto something or not?

Scientists have been studying the effects of calorie restriction, the reduction of calorie intake, for quite some time now. The earliest study on the topic was published by McCay et al. in 1935 and found for the first time, that when compared to ad libitum feeding, rats whose diet was calorie restricted (without reaching malnutrition) had a longer mean and maximal lifespan.

Calorie restriction (CR) usually refers to an approximate 10%-40% reduction of ad libitum feeding whilst maintaining adequate nutritional intake. There are many variations of CR, such as: CE, which means calorie expenditure (CR plus exercise); ADF, which is alternate day fasting; and DR, dietary restriction, which restricts one or more specific components of usual daily intake without changing the total calorie intake. CR (and its alternatives) is so far the only demonstrated non-genetic method shown to prolong lifespan and delay age-related functional decline, and has arguably contributed more to the overall understanding of the aging process than any other model.

Since McCay et al. released their seminal study, many CR studies have since been conducted, with most being done on rodents and primates. A 20 year long research project by the Wisconsin National Primate Research Center in 2009 established that CR delayed disease onset and mortality in rhesus monkeys. A follow up study published in 2014 by the WNPRC further backed up this claim. Another longitudinal study conducted by the National Institute on Aging in 2012 did not find that CR improved survival outcome when implemented on younger and older rhesus monkeys, but did find beneficial health effects. A 2014 study on mice and a phase 1 trial on humans by Cheng et al., found that prolonged cycles of fasting cleared out old, damaged cells and triggered immune system stem cells  from a dormant state to an active one.

The possibility that CR does not universally exercise the same longevity benefits has been raised. In 2009 Liao et al. found in a study of 41 different mice strains, DR actually shortened more lifespans than it lengthened, suggesting that the effects of CR may be influenced by genetic background (and possible other contributing factors). Further research by Schleit et al. has used yeast to further study the relationship between CR and genotype, with some strains experiencing an increase in lifespan and others not.

In order to include as much of the CR benefits while minimizing as many adverse effects as possible, a recent study by Brandhorst et al. developed a 'fast mimicking diet' (FMD), which induced low glucose levels and high levels of ketone bodies that are associated with prolonged fasting. This low-protein, high in healthy-fat diet not only boosted better overall health and extended lifespan in the mice undergoing the treatment diet, but they also experienced fat loss (without losing mean body mass), increased better learning and memory levels, and lowered rates of cancer incidence. The FMD was also tested on a human sample, who also saw a reduction in associated age-related risk factors. The researchers are planning on a future clinical-use release of the diet.

Any form of CR is not recommended to be undertaken by pregnant mothers, children, and young adults under 21, as CR may possibly interfere with mental and physical development. It is also advisable against older people as well as women attempting to become pregnant, since CR might affect reproductive function.

Despite the fact that research into CR has been around for over 80 years, the mechanisms behind why CR would impact lifespan still remain unclear and need further investigation. There are so many different variables involved with each individual person, that CR is not something that should be attempted without first consulting a medical expert. When done incorrectly CR could lead to near-starvation and malnutrition, which would completely undermine any potential benefits. Ultimately CR holds a lot of potential health benefits that one day might be widely made available for human application, but at present, research has still not reached a point where we can conclusively say that such practices are certain to extend healthy lifespan. So to answer the question of whether we should follow a calorie-restrictive diet, well, at best it’s a maybe.