Caloric Restriction: Comment and Response

by Nina and Brad

Brad’s post on caloric restriction (see Caloric Restriction and Longevity) prompted an interesting comment from Kathleen Summers MD PhD that I thought it worthwhile to share with you. Rather than having Brad respond in the comments section (I'm not sure how many of you actually read the comments), I asked him to reply to her comment at the end of this post. So have a look! This debate is a very good example of how little scientists currently understand about the aging process (a theme we return to periodically) and provides a hint of some of the many issues surrounding the controversy regarding the best diet for healthy aging.

Posted by Kathleen Summers MD PhD to YOGA FOR HEALTHY AGING

The biggest lesson here is that an excess of energy intake brings disease, disability, and early death. Restricting intake protects against cancer - and also diabetes, arthritis, and heart disease to some degree, although the numbers in the latest study didn't reach significance for the latter (potentially due to limited sample size). Teasing out just what the optimal amount of intake is takes time and research. And it's complicated - there's nutritional, environmental, mental/emotional health, and genetics among other factors playing a role.

The monkeys in both groups of the Wisconsin study ate more and weighed more than the NIH monkeys. The researchers used different sources for their proteins, fats, and carbs as well as a different approach to vitamin and mineral supplementation.

We have one primate study showing extended life span with calorie restriction and one not showing the same but yet other positive results. Let's not jump to absolute conclusions about what the latter study means.

Response from Brad Gibson PhD

Kathleen,

You make some good points, and I agree that this most recent NIA caloric restriction study needs to be considered within the context of other published work. There are undoubtedly many nuances in experimental design and interpretation that future experiments will need to address. That said, my major point is that the data for CR in primates is weak at best.  Many of my colleagues were very critical of the earlier Wisconsin study on two counts: the fact that they fed the control group a fairly high caloric diet and that they removed animals from their final analysis on the basis that they died from non age-related reasons. The removal of animals in the final statistical analysis was a very dubious call. And, as it was pointed out in the NYT article—and by many critics of the Wisconsin study—if those animals were included there was no difference in longevity between the two primate groups. Combined with other studies on more diverse genetic backgrounds in mice that show very mixed effects of CR, one really has to wonder how much traction the CR models has left in it, at least in mammals. Oddly, the data on other model organisms (flies, worms etc.) remain strong.  But one can only push these conserved evolutionary arguments so far.

While there may be benefits in a CR diet as you indicate (e.g., cancer and heart disease), one needs to make a distinction between a low calorie diet and caloric restriction. Many years ago a very prominent scientist in the field of aging who practiced the CR diet stated at the end of his seminar that "we scientists" needed to make a case to the public about the benefits of CR. I challenged him on this assertion, saying that American's relationship to food is so screwed up as it is, that to send a message that food is your enemy is not good advice. There is no evidence that CR in humans is beneficial. In contrast, there's plenty of evidence that a sound, balanced, healthy, low-to-moderate caloric diet (especially one that limits or avoid meat and dairy) is good for you. I suspect we are in agreement on that point.

I also agree that we are still far from drawing a final conclusion on the benefits of CR on human longevity. I was a bit flippant on this point in my last blog post. Guilty as charged. And there is little doubt that there will be more NIH studies examining CR and longevity in various mammalian and primate models as there is still compelling and interesting evidence that needs to be sorted when all animal models of CR and longevity are considered. However, I for one, will be placing my bets elsewhere.