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Why calorie-restricted diets could be a bad idea

An early study says cutting calories for rats, in otherwise good diet, nearly doubled lifespan - but it's unproven for humans

The nearest animal-relative to humans studied on increasing lifespan is the monkey. Two US research teams have now reported that calorie-restricted monkeys are living longer than controls

I assume that most people, if offered an easy formula to lengthen their lifespan while retaining good health, would grab the opportunity with both hands. A formula is now potentially available to lengthen human lifespan in the form of a calorie-restricted diet, although this is not yet proven.

The calorie-restricted diet is also difficult to tolerate. However, the good news is that significant health benefits can be realised by adopting more moderate diets and healthy habits.

In 1935, CM Crowell, MP McCay and LA Maynard reported (Journal of Nutrition, Vol 10) that reducing the calorie intake of rats, in a diet otherwise adequate in vitamins and minerals, nearly doubled their lifespan.

It was later shown that a calorie-restricted diet beginning in early adulthood not only increased rats’ lifespans but also reduced the incidence of spontaneous cancers arising by more than 50 per cent.

Much of the biochemistry that underpins the effects of calorie-restricted diets has yet to be discovered

We now know that calorie-restricted diets lengthen the lifespan of a wide variety of organisms, including yeast, worms, flies, rodents, fish and primates.

The body needs energy to carry out its various activities and it derives this energy from food. The energy is measured in units called calories. In a healthy balanced diet a man needs about 2,500 kilocalories (kcal) a day and a woman about 2,000 kcal to maintain body weight. A calorie-restricted diet typically limits daily calories to 1,800kcal (men) and 1,500kcal (women).

Ingested food is burned in the body to release the energy required to do essential work. This food-burning process generates a class of chemicals called “reactive oxygen species” that can damage the cell.

You can visualise reactive oxygen species as maniacal scissors that, unless neutralised, fly about randomly snipping and damaging cell structures. A class of chemical in the cell called antioxidants can neutralise reactive oxygen species, but do so less effectively when levels of these chemicals are high – “oxidative stress”.

Biochemical studies (eg Huiru Tang and others, Journal of Proteome Research, 2016) have shown that low calorie diets reduce oxygen stress and disturbance of energy metabolism. However much of the biochemistry that underpins the effects of calorie-restricted diets has yet to be discovered.

Because calorie-restricted diets lengthen the lifespan of such a wide range of organisms, it is reasonably anticipated that they will also lengthen human lifespan, but science has yet to determine this.

The nearest animal-relative to humans studied in this regard is the monkey. Macaque monkeys live about 26 years in captivity. Two trials began in the US in the 1980s but not all the monkeys have died yet. Both research teams have now reported that calorie-restricted monkeys are living longer than controls.

Calorie-restricted males are living two years and females living six years longer on average than controls, as well as having lower rates of cancer and heart disease (Julie Mattison and others, Nature Communications, published online, January 17th). The longevity improvements in the calorie-restricted monkeys are very modest compared to the effects seen in mice.

The monkey results extrapolate to a calorie-restricted diet induced human lifespan lengthening of about nine years.

Caloric restriction delays ageing of model organisms, but whether it works in nonhuman primates has been controversial.

Here, the authors pool and reanalyse data from two long-running primate studies, concluding that moderate caloric restriction indeed improves health and survival of rhesus monkeys.

There are difficulties associated with this diet

A few thousand people worldwide are now on calorie-restricted diets, hoping for longer and healthier lives. However there are difficulties associated with this diet. It must be carefully formulated to ensure it is nutritionally adequate in all respects, apart from energy calories.

Unpleasant side effects are not uncommon – feeling hungry and cold, reduced libido, low energy, reduced inclination to exercise and more. One wonders why anyone would want an extra nine years of that.

I strongly advise against going on a calorie-restricted diet given current limited knowledge of its effects in humans, but, if you are determined to go ahead, it is essential that you first consult your GP, who will refer you to a specialist in nutrition for expert advice on formulating the diet.

My advice on diet/lifestyle is – eat a wide variety of whole foods in moderation, mostly plants, and take plenty of aerobic exercise and light resistance exercise (weights). This lifestyle has no unpleasant side effects, is relatively easily sustained and there is extensive evidence that it is good for your health.

William Revile is an emeritus professor of biochemistry at UCC,