Which is irrelevant, as it only speaks of total risk. If butter was bad, it would be bad for them too, in a controlled study against not butter eating Tibetans, regardless if they all run 10k miles per day and eat only vegetables.

If the sample size was large enough and you could factor our everything else, then yes. I don't think anything like this was done, though.

What exactly is your point, then?

That the lifestyle difference is irrelevant since any study that measures the impact of a food in health should control for it.

It can't be controlled for if there is no variation in the population. At that point it becomes a fixed effect--a population-specific intercept, as it were--and the only way to net that out would be through a difference-in-difference type study.

>It can't be controlled for if there is no variation in the population.

You think there are no Tibetans who don't exercise much and don't eat much vegetables either?

Depends. The external validity may be questionable due to the height of the Tibetan plateau, epigenetic expression that is potentially common in the nation, or any number of other factors. In other words, conditional randomization is not true randomization required.

However, the grandparent comment I understood to be relevant to dietary factors.

+1 Additionally, even if you could and would factor out everything else, "obvious" would hardly be the right word to use.