Monday, November 25, 2013

Nuts! Or On What to Eat.

When  I saw the story about the new study on nuts I knew that the "miracle food" of 2013 had been found.  The miracle foods vary from soy, blueberries, cream-and-butter to the Mediterranean diet, the vegan diet etc.  Nothing wrong with the basic idea of trying to find which foods are good for us.  Plenty wrong with the idea that there is this one food which will work like a headache pill and take away all your health problems.

Have you heard about the Paleo Diet?  It's been around a while and based on the idea (familiar from evolutionary psychology) that our guts haven't evolved since some mythical age in prehistory and that we should only eat those things the prehistoric humans ate.  That the prehistoric humans ate all sorts foods, depending on where they happened to live, very much including grains, the evil in the Paleo Diet, is ignored in this theory.  Rather, the assumption is that only meat, fruit and vegetables are OK to eat.  Possibly bloody meat that you tear with your sharp prehistoric predator teeth?

That last sentence was a joke.  But the Paleo Diet, whether good or bad for someone today (and it could be good), cannot be justified by it being the paleolithic diet of all prehistoric humans.  And evolution didn't stop at some mythical point in the past.  For example, lactose tolerance after infancy, is a fairly recent introduction to some human populations.

I don't want to get into arguments about any one particular diet, however.  I want to write about nuts as the new miracle food.  The most recent study argues that those who eat more nuts on a daily or weekly basis live longer than those who do not.  And that is how the story is written up here:

People who regularly eat nuts appear to live longer, according to the largest study of its kind.
The findings, published in the New England Journal of Medicine, suggested the greatest benefit was in those munching on a daily portion.
The US team said nut eaters were likely to also have healthy lifestyles, but the nuts themselves were also contributing to their longer lifespan.
The British Heart Foundation said more research was needed to prove the link.

Why would the British Heart Foundation demand more research?  Because the study we are discussing here was observational.  It followed large groups of nurses and physicians for thirty years and then tried to correlate the death rates among the subjects with only their nut consumption.  Remember that the subjects were not watched under controlled lab circumstances for thirty years, they were not given exactly the same foods other than nuts, they were not given exactly the same amount of exercise, they were not given exactly the same living and working conditions.

This means that it is very difficult to make sure that  the found correlation between death rates and nut eating is actually a causal one from nuts to longer lives.   To give one pretty obvious example, vegetarians are likely to eat a lot more nuts than meat-eaters, on average, because the nuts are used as one substitute for meat in many vegetarian diets.

The study itself is careful about its findings and points out the problems in interpreting them.  But not all the popularizations are.  I came across a Finnish popularization (not on a rubbish website) which states (my translation):

Research:  Eat nuts so you become slim and live long

Later in the summary we learn:

The results also suggest that those who eat a lot of nuts are not fatter than anyone else.  Rather the reverse, the regular nut eaters are slimmer.

It is this that worries me:  The idea that someone not eating any nuts today should just add them to his or her daily diet and not expect a sizable weight gain but more slimness and a long life!  The summary fails to point out that those who eat nuts in the study also differed in other ways:

As compared with participants who consumed nuts less frequently, those who consumed nuts more frequently were leaner, less likely to smoke, more likely to exercise, and more likely to use multivitamin supplements; they also consumed more fruits and vegetables and drank more alcohol

If I had to make a wild guess about all that I might look into the question whether the nut-eaters were vegans or vegetarians or very health-conscious in general.  I couldn't find out if the study could control for any of that.  But note the general differences between the nut-eaters and the rest of the study subjects.  Those are many more than the consumption of nuts, and the leanness difference probably had more to do with the greater consumption of fruit and vegetables and the greater exercise levels than the fact that some munched more often on pecans and walnuts and almonds while others munched on bacon and hamburgers.

I'm not trying to disrespect nuts.  They are a great food (pesto!).  But they are high in calories and not something people should just add to their diets without dropping other food items also high in calories.  It might also be a good idea to eat more fruit and vegetables and to exercise more if you wish to live long lean and mean.