I'm also a physicist and I would first say that generally (except for some specific sub-fields) we are actually not well trained on experiments (like this) that require significant statistics.
In particular the law of large numbers applies (which you should know about as a physicist). I would argue that despite the noise in the reporting the conclusions from this study are likely much more significant than a tightly controlled randomized trial with a small group (~100 people).
Self reported questionnaires are certainly a well established method in these sort of studies and the short-comings are well understood, that does not mean their results are meaningless.
In particular, I would argue that many (most) people can give meaningful quantitative answers to their eating habits over several years as their habits rarely change. Certainly most people here would be able to answer approximately with how many meals per week they eat meat, how much vegetables they eat on average, how often they drink alcohol etc.. We are only interested in rough averages.
Regarding your argument that there are too many variables that can't be controlled for, ok lets hear them name at least 10 and make a valid statistical argument why they need to be controlled for in a sample size of ~80 000.
In particular the law of large numbers applies (which you should know about as a physicist). I would argue that despite the noise in the reporting the conclusions from this study are likely much more significant than a tightly controlled randomized trial with a small group (~100 people).
Self reported questionnaires are certainly a well established method in these sort of studies and the short-comings are well understood, that does not mean their results are meaningless.
In particular, I would argue that many (most) people can give meaningful quantitative answers to their eating habits over several years as their habits rarely change. Certainly most people here would be able to answer approximately with how many meals per week they eat meat, how much vegetables they eat on average, how often they drink alcohol etc.. We are only interested in rough averages.
Regarding your argument that there are too many variables that can't be controlled for, ok lets hear them name at least 10 and make a valid statistical argument why they need to be controlled for in a sample size of ~80 000.