Mark Hoofnagle has a MD and PhD in physiology from the University of Virginia, and is now a general surgery resident. His interest in denialism concerns the use of denialist tactics to confuse public understanding of scientific knowledge.
This article in PLoS caught my eye today. It's entitled, "Calories Do Not Explain Extension of Life Span by Dietary Restriction in Drosophila", and is an extension of the body of science showing that caloric restriction in a variety of animals, from fruit flies to non-human primates, may dramatically extend life-span.
Currently the mechanism is not well understood, but this surprising new result suggests that rather than absolute calorie restriction, decreased protein intake may be more critical for this beneficial effect.
In this study, fruit flies were fed a mixture of carbohydrate and protein, and their life span was measured with restriction of the carbohydrate, the protein, or both. Their results were surprising:
Flies fed food media with very similar caloric content showed marked differences in their life spans (see Figure 3). This finding is in direct contrast to what would be predicted if ingested calories were the key mediator of life span in D. melanogaster and demonstrates that the nutritional composition of the diet affects life-span extension by DR in this species. Reduction in the concentration of either sugar or yeast levels increased life span (see Figures 2 and 3). However, the magnitude of the effects on life span when the caloric content of the food was changed via altering yeast concentration far exceeded that seen when calories were changed to the same extent via manipulation of sugar levels, suggesting that protein/lipid levels have a greater effect on Drosophila survival than does carbohydrate. The differing effect of sugar and yeast on mortality in Drosophila could occur if different pathways sense nutrients during DR, possibly with different outputs affecting life span.
Here's a table summarizing some of the data
DR SY corresponds to restriction of both carbohydrate and protein (sugar and yeast), DR Y is dietary restriction of protein, DR S is dietary restriction of carbohydrate, and the control SY is normal diet. As you can see, both the restriction of both protein and carbohydrate together, as well as restriction of just protein, had a dramatic effect on the median and maximum life span of the fruit flies.
Now, this study has only been completed in fruit flies, an animal very distant from humans, but I still think it's interesting for a couple reasons. One, the extension of lifespan through restriction of caloric ntake has been consistent across a variety of animals, and appears to be a very real effect. It seems likely, to me at least, that this effect will be replicated in mammals especially considering a brief statement from the introduction suggesting studies in mice in rats are already underway showing a similar effect, "However, in other experiments, rats fed isocaloric diets with altered nutritional composition [17,18] or reduced protein [19] showed life-span extension. Furthermore, reducing just one amino acid (methionine) increases life span in both mice (R. Miller, personal communication) and rats [20]."
This doesn't mean anyone should make drastic changes in the composition of their diets, but this sure will be interesting to follow. In the future it may become apparent that protein-rich diets are not as beneficial as some would currently like us to believe. I look forward to the extension of these studies to more relevant animal models.
Comments
In the future it may become apparent that protein-rich diets are not as beneficial as some would currently like us to believe.
Can you expand on this a tad? Who, and why -- to get the backstory on the impending rumble.
Posted by: Ted | July 18, 2007 9:13 AM
Aha! Stanley Green is vindicated at last!
http://www.flaneur.org.uk/html/green/green.html
Posted by: Anonymous | July 18, 2007 9:33 AM
So... We should expect to see a lot of very longlived ruminants and grazers of multiple species, no?
Posted by: Janne | July 18, 2007 10:37 AM
We cannot interpret the meaning of the median without knowing the distribution. Why have they left out the arithmetic mean? Something smells off here.
Posted by: Grackle | July 18, 2007 10:40 AM
Ted, I was referring obliquely to Atkins diets which are mostly fat and protein, and very little carbohydrate.
It's too early to make any generalizations from a study in Drosophila, but since this effect is really pervasive across multiple species I think there is a fair probability this result will hold up.
As far as the median and mean etc., the paper is at Plos so you can see the rest of their data. It looks like a pretty robust effect.
Posted by: MarkH | July 18, 2007 11:35 AM
I thought you were talking about the meat protein lobby on one side and the "well organized coalition forces" representing vegan/peta/dreadlocks/tat/pierced constituency mobilizing to confront. I'm thinking that the SciBloggers would be akin to the women's auxiliary corps in that epic struggle.
Live longer? Is that the end all goal? What of the goal of trading a few years of crappy old-age quality of life for the daily pleasure of delicious meat proteins? What will slaughterhouse workers do in this new, healthier age? Probably live longer on unemployment and other public dole venues.
Posted by: Ted | July 18, 2007 12:40 PM
That's your choice. Plenty of protein in a vegetarian diet though. It also might get tied to a specific amino acid composition, namely methionine, strangely enough.
Posted by: MarkH | July 18, 2007 1:47 PM
My great grandmother died just short of 102, and I am sure she never followed *any* of these goofy diets, so.. But more to the point, my mother is on a diet right now that allows proteins, but not carbohydrates, because she has some medical issues, and medications, while carbohydrates create problems with. Mainly, they tend to make some organs work harder, and those are the same ones that may be slightly weakened by some of the meds she takes.
At some point I am sure we will have a clear picture of what is going on, but right now this stuff is about 40% fact, 40% crank and 20% who the hell knows. And its damn hard to tell if you are going to die faster because the crank+who_knows ratio is *against* what you are doing, or the fact+who_knows ratio is *in favor* of what you are doing. Best bet is to just not do anything too obviously stupid and hope. lol
Posted by: Kagehi | July 18, 2007 6:04 PM
In 2006 a study was published in which mice were fed a normal diet, a calorie restricted diet, a CR diet in which only protein was restricted, and a CR diet in which only carbs were restricted. The mice who were given unrestricted protein and fat but CR in carbs faired the best and had the best muscle and skin. Don't forget that carbs cause glycation, but protein doesn't. The best diet is a Rosedale diet, of high protein and fats from FISH, leafy greens and low-glycemic veggies, a limited amount of berries, and NO breads, pasta, etc.
See: Zangarelli et al. Synergistic effects of caloric restriction with maintained protein intake on skeletal muscle performance in 21 month old rats: a mitochondria-mediated pathway. FASEB J 20:2439-50 (2006).
Posted by: James | July 18, 2007 7:21 PM
Grackle,
Yes the distribution would be nice. Survival data is often summarized by medians rather than means. Survival data is often right-skewed, which pulls the mean away from the center (median) of the data. Also, the median has a much lower breakdown.
Posted by: other bill | July 19, 2007 9:15 AM
It's worth pointing out that these life extension studies, while interesting, all involve animals kept environments with regular supplies of food and protected from predators, disease and most injuries. I haven't yet seen such a study that even allowed the animals to breed.
It could well be that the life extending benefits of restricted diets are only of benefit if you somehow manage to avoid contracting any diseases or suffering injury for your whole life!
Posted by: Paul | July 19, 2007 1:03 PM
The key piece of data missing here is: Were these diets protein rich, balanced, or protein deficient? If there is an optimum level of protein, and the control diet had a surplus, the results make perfect sense.
We should not assume that the relationship between protein consumption and life is linear. It may well be a curve(or complex shape), with both of these diets on one side of the peak. It takes many more data points to define the shape of the curve.
Posted by: Brian | July 23, 2007 7:14 PM
I think these studies in rats have overstated the increased lifespans of the low protein group of rats as opposed to the meat-eaters. While this has little effect on rat health, it affects the validity of the studies when translated to humans.
The studies fail to correct for unrealized pretermination.
Rats are not as smart as humans. The numbers of veggie rats that oulived the meat-eaters was skewed simply because some unknown, but undoubtedly significant, number of them couldn't figure out how to kill themselves early on.
Posted by: pete | July 24, 2007 10:19 PM