You can do meaningful science with people. There are plenty of solid drug studies out there, and even good education research. What I mean is there are experiments you don't get to do with people and when that happens you need to move on and not try and get by half assing it.
So, you don't get to see what happens when people eat the exact diet you want them to for 20 years. Having people fill out servery on what they ate is not a reasonable substitute. In this case there are options like sending people actual food. You don't know if they eat it, but you do know you sent it and you can measure outcomes based on what food was sent.
PS: Also, most of what your talking about is almost pure politics. When politics and science really clash it's rare for science to make much headway. See: Climate Change, Evolution, etc.
So, you don't get to see what happens when people eat the exact diet you want them to for 20 years. Having people fill out servery on what they ate is not a reasonable substitute. In this case there are options like sending people actual food. You don't know if they eat it, but you do know you sent it and you can measure outcomes based on what food was sent.
PS: Also, most of what your talking about is almost pure politics. When politics and science really clash it's rare for science to make much headway. See: Climate Change, Evolution, etc.