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locking in the benefits of dieting?

Saturday's NZ Herald carried a story under the headline, Locking in the benefits of dieting (along with the almost obligatory picture of someone carrying far too much weight round their middle). Nothing contentious in the research (& I went off & read the original paper too, since the Herald provided a reference) - but it's a good example of how not to talk about your science.

And also an example of how a story can be over-blown by the media - although, to be fair, in this case you get both sides of the scientific debate; it's just that the gee-whiz aspect gets the headline coverage, so that's what's likely to stick in most people's minds. The sub-heading, Dietary supplement widely sold in health stores also said to slow ageing, doesn't help.

So what's it all about? A research group (Merry et al. 2008) looked at the effect of a dietary supplement in maintaining the 'new' weight achieved by dieting, once the diet ended. First caveat: this was in rats i.e. they were looking at the impact of calorie-restricted diets in rats, & the effect of the supplement in maintaining the animals' new, post-diet weights. Here's the abstract from their paper:

Dietary restriction [DR] feeding extends survival in a range of species but a detailed understanding of the underlying mechanism is lacking. There is interest therefore in identifying a more targeted approach to replicate this effect on survival. We report that in rats dietary supplementation with alpha-lipoic acid, has markedly differing effects on lifetime survival depending upon the dietary history of the animal. When animals are switched from DR feeding to ad libitum feeding with a diet supplemented with alpha-lipoic acid, the extended survival characteristic of DR feeding is maintained, even though the animals show accelerated growth. Conversely, switching from ad libitum feeding a diet supplemented with alpha-lipoic acid to DR feeding of the non-supplemented diet, blocks the normal effect of DR to extend survival, even after cessation of lipoic acid supplementation. Unlike the dynamic effect of switching between DR and ad libitum feeding with a non-supplemented diet where the subsequent survival trajectory is determined by the new feeding regime, lipoic acid fixes the survival trajectory to that established by the initial feeding regime. Ad libitum feeding a diet supplemented with lipoic acid can therefore act as mimetic of DR to extend survival.

In other words, giving lipoic acid as a supplement in the rats' diet seems to mimic the effect of calorie-restricted diets in extending the animals' lifespan. (And there is some evidence from humans that this sort of diet extends human lives.) This effect was only seen in rats where they were first put on a restricted diet, & then given the supplement when they returned to a normal diet.

But - big but - one member of the research team went on to extrapolate these findings to humans. Dr Goyns is reported as saying that people who follow the same pattern of a restricted diet, followed by lipoic acid supplementation once they return to a normal diet, will experience the same extension of lifespan. Caveat #2: this was one study in rats: it hasn't yet been repeated & there have been no human trials. We cannot be sure that the effect will be the same in humans as in rats. (But I bet there'll be a rush on health stores selling the stuff!) And the research team is split on whether pushing the results in this way is really a Good Thing - look at the following comments from Dr Merry:

It is an unusual and interesting finding and it needs repeating in further research. That was as far as I was prepared to go, but [Dr Goyns] wanted to apply it to humans. I said I didn't agree with his interpretation and we had to wait for further studies... People have been buying this stuff and taking it for years as a dietary supplement. I don't think anyone knows what its effect is.

This is a much more measured approach - unfortunately, it doesn't make for such good headlines! 

B.J. Merry, A.J. Kirk & M.H. Goyns (2008) Dietary lipoic acid supplementation can mimic or block the effect of dietary restriction on lifespan. Mechanisms of Aging and Development 129(6): 341-348

 

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