[Sebb767]

> There is no way to keep the other variables like, sport, social interaction, stress and such out of the study.

Sure, but with sufficiently many people, these effects will usually average out. I'm not saying the situation is great, but it's still an important field of study and we need to make progress in some way.

[jcranmer]

> Sure, but with sufficiently many people, these effects will usually average out.

In the case of dietary studies, not really. There are a few factors which are known to have a big effect on your health--being wealthy, active, and moderate in particular--and a lot of the big studies are really just uncovering yet another proxy for those factors.

Of course, you can turn that around and make the realization that your diet doesn't really matter: there's no diet that will magically make up for being a couch potato. And outside the main well-known interventions (e.g., eating less calories), the solution is generally to just be more active and things like that rather than trying to tweak your diet.

[echoangle]

It only averages out if the factors are unrelated though. If a lot of asians eat rice and don’t have a high alcohol tolerance, your study would still show a correlation between eating rice and alcohol tolerance when looking at every single person on earth.

[skirge]

Compare people with vegetarian diet from India (over 1 billion, a good sample!) with European meat eaters, what will be the conclusions? Do effects "average out"? Or people drinking alcohol with millions of muslims? There are some obvious criteria which should be used for example divide people in age, income and cultural groups (my grandfather used to eat and did different things I did, including avoiding doctors, despite living in same country and even same home).

[constantcrying]

>Sure, but with sufficiently many people, these effects will usually average out.

No they won't. If you have two correlated factors and only measure one of them you can easily get to totally wrong conclusions.

If you have a food that is more often eaten by people doing a lot of sports, you will measure that eating that food is correlated with being more healthy. But it would obviously be fallacious to conclude that this food is more beneficial to health than other foods.

[f1shy]

No if they correlate strongly: people eating more vegetables are more likely to do sport, and care about sleeping. Not to mention visiting a doctor much often. That is just one example.

[amanaplanacanal]

Yeah this is a major issue. The first study that reports a link between some specific thing and health pollutes the data for all follow-up studies, because the folks that care the most about their health are going to change their behavior based on it. So after that you will always see a correlation with all the other things that have been reported to be healthy.