A lot of studies are done on high-level, competitive NCAA / college athletes. Both cuz they're, like, already at a university, but also NCAA has mandatory physicals, and high level athletics lends itself to controlled lifestyles.
Both tend to rule out most health issues as well; in theory you can be diabetic and play NCAA Div 1 ball, but selection bias tends to weed them out, while the Army explicitly does so.
You can only control what people eat if you lock them up in a prison or similar institution.
These days locking up thousands of people and experimenting with their diet is not cool.
The best nutritional / food studies come from prisons and similar institutions.