Why Do Dieting Mice Live Longer?

Conventional wisdom said animals on strict diets exploit the nutrients they can get to groom the body, not the gonads. Two scientists from Australia think the very opposite

Ruth Schuster
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A male lab mouseCredit: AP
Ruth Schuster

The Passover feast behind us, some of us look back with regret, less for participating in the requisite family fight and more because we ate too much. We console ourselves with the gauzy fallacy that we'll go to the gym and work it off, but how many consider that we may be taking years off our lives?

Actually, we don't know that's true, but can definitively say it’s true of mice. The more modest the murine is in its diet, up to a point, the longer it will live. Now two scientists from the University of New South Wales have turned the theory of why that is on its tail.

First the empirical facts. Scientists have observed that along the evolutionary chain – from the primitive nematode worm to the more advanced fruit fly to the cute white laboratory mice – that if they eat less, they live longer. "Caloric restriction," which is what this enforced dieting is coldly called, may also apply to rhesus monkeys, though the data here is muddier.

This is actually fairly new science, with popular research exploding it only in the last decade, and the differences can be startling. Never mind the nematodes – who cares, really – among our fellow mammals, the murines, we find a difference in life span between semi-starved mice (they were fed a third less calories than the control group) and the sated of up to 40%.

Think about it. If we are to roughly extrapolate that into human terms, could we extend our life spans to well beyond 100?

Probably not, but meanwhile, while waiting for more definitive monkey data, we can mull why any of this makes evolutionary sense. Surely being well-fed would lead to a longer life span, assuming one stays away from the "bad" calories of junk foods, sugared sodas and so on? How could it be that opposite is true?

The answer is that life is a zero-sum game after all, and that nematodes, mice and other living beings demonstrably trade off cellular resilience ("somatic maintenance") for breeding. In other words, rather than route all that energy to the gonads et al, the animal's body takes better care of itself in general. The question is why.

Have meal, don't have meal - Want mate

To start with one fact, when food is scarce, offspring are observably fewer. To continue with a second fact, the hungry mice were found to have lower incidence of deadly diseases such as cancer.

Animals exist to breed. That's how species survive. The animals can do other things between bouts of sex, such as eat and sleep and play baseball, but no breeding, no survival of the species.

Now, if an animal invests all its physiological resources (albeit unwittingly) in sex, when food runs short – it will die.

So far we are in consensus territory. From this point there are two divergent theories as to why famished mice and so on live longer.

The conventional theory is that the mouse reroutes its physiological energy from breeding to cellular repair and other body maintenance functions, thus grooming their bodies to breed better at a later date.

However, as Margo Adler and Russell Bonduriansky write in the journal BioEssays, "Why do the well-fed apear to die young?" outside the lab, animals live in peril, and not only of starving. Most can't afford that elegant strategy of waiting to breed another day – they want to breed now. Here it bears mention that the immune system is one that does suffer from starvation, which also flies in the face of the conventional theory. They postulates that with their physiological energy bypass, they're making the best of a bad situation and grooming their bodies to reproduce now, as in NOW.

And the punchline? The two scientists suspect the entire issue of dietary restriction elongating the lifespan is a laboratory artifact. That it doesn't happen in nature anyway. "Most animals living in natural environments may fail to achieve lifespan extension under DR," they write – and sum up: when nutrients are scarce, animals physiologically get better at exploiting nutrients. They are probably gearing up to maximize their reproductive output, not the contrary.

Who has it right – starved and sexy, or the opposite? Stay tuned.

Pregnant mice fed BPA gave birth to offspring that were significantly heavier upon weaning (illustration).Credit: Reuters