Note: This post doesn't have much to do with running or exercise, but it does talk about staying healthy into old age, which I think is something of interest to most active people.
A few years ago I was watching a segment on the Discovery Channel about diets and aging. It was discussing how starvation conditions (a man was trapped in a cave) tell our bodies to shut down the aging process. (When the man emerged from the cave weeks later, he hadn't aged a day.) Fascinating stuff. It went on to suggest that if we reduced our calorie intake we would live longer. Still not too much of a stretch. But then it used an example of someone slashing their normal diet in half, from 4000 calories/day to 2000 calories/day, and (surprise!) that would lead to a longer life.
That's when I turned the TV off. First of all, 2000 calories/day is not near starvation, it's the recommended amount for the typical adult. (Whether or not people are actually eating closer to 4000 calories is besides the point.) Obviously cutting back from overeating will lead to a longer life. But what about reducing a 2000 calorie/day diet? Is there a benefit to that? Ever since the Discovery Channel completely misinterpreted the point, I've cringed a little when I read calorie restriction studies.
Granted, it's difficult to do these studies in humans. There aren't many volunteers ready and willing to be nearly starving subjects, let alone graduate students who want to wait a lifetime to get the results. The next best thing is primate studies. In the 80s, two groups started putting monkeys on calorie restricted diets (30% fewer calories than normal diets) and then let them age (for 20+ years). One study was done at the Wisconsin National Primate Research Center (WNPRC) and published in 2009. It showed some promise for calorie restriction: the monkeys fed less outlived the monkeys on a normal diet. However, another study, done at the National Institute of Aging (NIA), came out this month and found much the opposite.
The NIA study looked at two cohorts, one that was started on the restricted diet at an old age, and one that started at a young age. The monkeys put on restricted diets in old age had no difference in survival compared to monkeys eating the normal amount of calories. However, there were some health benefits; compared to monkeys on a normal diet, they weighed less and had lower levels of triglycerides, cholesterol, fasting glucose, and oxidative stress. The animals put on a restricted diet at a young age also had no benefit in survival. Furthermore, no other health benefits were observed. Although they weighed less, there were no striking differences between triglycerides or glucose levels. They did have fewer cases of cancer, but diabetes and cardiovascular disease were not prevented.
The results from this study suggest that diet restriction doesn't increase longevity. The major difference between the WNPRC and NIA studies (besides the genetics of the animals, which is a factor, but I won't get into it) is the diets given the monkeys. The diets from the NIA study were healthier: they had less sugar, more antioxidants, and fish oil. (Give the WNPRC some credit; they started these studies in the 80’s when most people were concerned only with total calories.) Additionally and importantly, the control animals in the NIA study weren't allowed to eat as much as they wanted, like the WNPRC animals. They were slightly restricted in order to maintain a healthy calorie intake. The control animals in the WNPRC study may have represented more of an overweight population. So perhaps the WNPRC study was doing more of what I feared these studies would do: just slash the calories of a diet that's too caloric and not very healthy to begin with. Of course you'll be healthier after eating less of a poor diet.
There's a lot more research to be done here. There may be some benefit to smaller, nutritionally complete (an important point!) diets, but we aren't sure yet. The diets themselves matter, as well as the genetics, the current age, etc. Another study looked at people and found those within a normal weight range (BMI 20.0-24.9) had the lowest mortality rates. So we shouldn't be gorging ourselves, but maybe not starving ourselves either. That sounds good to me.