19th Nov 2013

CAN caloric restriction help individuals live longer?

Or does it just feel like it…

Two themes in nutrition have recently come together. The first, calorie restriction (CR), involves permanently reducing total energy intake by up to 30%.

CR has been shown consistently to increase the longevity of a number of different species of animal, as well as reduce weight.

The second theme, intermittent dieting (ID), or reducing energy intake on some days but not others, has spawned yet another diet craze that is dominating discussion at the dinner parties of middle suburbia.

Being battered and bruised by the numerous false starts in the dieting game, it’s tempting to pass off both of these as fads.

But the interest of some hard-nosed nutrition scientists makes a second look warranted.

Dr Eric Ravussen from the Pennington Institute in Arizona, is a world expert in energy metabolism and obesity.

For some time, he and several postgraduate students (including several Australians) have studied the mechanisms involved in CR.

The two forms of ageing

Speaking at a recent Australian and New Zealand Obesity Society (ANZOS) conference, Dr Ravussen described two forms of ageing: primary ageing, determined by genetics and natural factors; and secondary ageing, which is related to lifestyle and environmental factors.

Together these determine one’s maximum lifespan.

From animal studies it’s known that rats are able to run daily, live longer than those deprived of exercise.

But when a CR diet of about 30% total energy restriction is introduced, they live even longer.

Possible explanations for this are the reduced cellular oxidative stress from food, decreased DNA damage, decreased inflammation and auto-immunity, and increased metabolic efficiency.

For obvious reasons, such a study over a lifetime in humans has not yet been done.

Those that have been carried out opportunistically for short periods (such as during wars and in the human biosphere study) show a negative bounce-back in weight gain and health after the CR period.

Molecular changes

Physiological studies carried out by Dr Ravussen’s group, however, show molecular changes that are reflective of potential longevity advantages.

There’s also no doubt that weight loss follows such a regime — if it can be maintained.

A different way of restricting calories is through intermittent dieting popularised by the 5:2 diet and TV doctor Michael Mosley.

ID involves two days each week of energy restriction of 500 calories for women and 600 for men, with ad-lib intake over the remaining five days.

Exponents claim not just weight loss but reduced chronic disease risk.

And while there are not a lot of data to support the latter, there is good support for the former — strangely even with an increased overall food intake.

Dr Amanda Sainsbury-Salis from Sydney University’s Boden Institute of Obesity, Nutrition, Exercise & Eating Disorders and author of The Don’t Go Hungry Diet, is currently doing the tests in mice.

While the results are not yet published, she does believe there might be something in the 5:2 diet and that the approach could be improved by using different degrees of energy restriction.

So far, studies with humans suggest weight loss may be more (and easier) with an ID plan like this.

A panacea in middle-age?

Given that most people won’t have the opportunity to calorie reduce (at least in a healthy fashion) for life, the question becomes, what advantage is there for someone trying the process in middle life?

There’s little doubt that (short-term) weight loss at least will be an outcome, but adverse effects, according to Dr Ravussen, include cold intolerance, decreased libido, constant hunger and reduced desire to exercise.

Reversion could also lead to increased difficulties with weight.

Based on animal studies, Dr Ravussen has calculated that a 50-year-old human could be expected to live a measly two months longer! So is it really all worth it?

If not weight, then what?

The use of BMI in diagnosis of metabolic disorder has come under question. Weight over height squared measures mass only and doesn’t take account of body fat. This then discriminates against mesomorphic body shapes – like some short male athletes – and the aged, whose height may decrease with age while weight remains stable. On top of this, BMI is not a consistent measure of ill health, as illustrated by the ‘obesity paradox’.

Garvan Institute researcher Dr Dorit Samocha-Bonet has shown that almost 50% of expected risk can be explained by other, easily measurable factors. The cumulative of risk for each is:

HDL cholesterol	26%
HbA1c	35.5%
Systolic blood pressure	43.2%
Triglycerides (Tg)	46.7%

According to Canadian lipidology expert J-P Despres, a Tg of >2.0 and a waist circumference greater than that recommended for ethnic groups (usually around 100cm for men and 90cm for women – called the ‘triglyceride-waist’), adds even more to diagnostic value. It may all make BMI less relevant at the clinical level.