It's not just a money issue. It would be impractical and unethical to confine study subjects in controlled environments for long periods of time. That's what we would have to do in order to gather reliable data and eliminate confounding factors.
Most human nutrition studies are only observational and barely a step above junk science. Don't take them too seriously.
I'm not so confident RCTs of nutrition are unethical. The data are too damn valuable for society to ignore. Perhaps participants could be informed and willing to partake in the challenge trials, which although these participants might not be reflective of the overall population, we can get a better sense than correlational studies.
Humans are incredible explorers, self-sacrificing for the greater (perceived) good. We should consider continuing our legacy by understanding ourselves better through rigorous science. (And hopefully end the historical legacy of making thousands of other species extinct via our exploration.)
Randomized control trials are done all the time. They can and are done ethically. I don't know where you people are getting the idea that RCTs are unethical.
Proper large scale, long term RCTs have almost never been done in the human nutrition field. In order to really get the "controlled" part of an RCT the researchers have to confine people somewhere that all of their food and beverage intake can be accurately counted. Obviously this is very expensive and researchers can't ethically confine people in controlled environments for years. At most those studies have been done with very small subject counts for a few months at a time: hardly sufficient to obtain reliable results.
So as a practical matter most studies tend to rely on subject reported data, which is notoriously inaccurate. People forget to record little snacks, or don't weigh their portions, or lie about their behavior because they're ashamed.
You're totally missing the point. If the study relies on subject reported data then there's no proper control regardless of number of subjects. More funding won't fix that quality problem.
What other method is there to gather nutritional data? The researchers either have to confine and monitor the subjects in a controlled environment, or leave them loose and rely on subject reported data. Maybe someday we'll have better technical solutions but for now those are the only practical options.
If you have a better approach for study design then I'm sure nutrition researchers would love to hear it.
Yes, to do a proper RCT you need to confine and monitor subjects in a controlled environment. YOU used the word "years" and nobody ever suggested a study that long. If you're implying that you need a study that long to gather meaningful results then that's just simply not true.
Let me put this plainly, you can design RCTs that are:
A) Safe for participants.
B) Aren't too long.
C) Don't rely on subject reported data.
D) Still give meaningful results.
These are not mutually exclusive and they are done all the time. These are gold-standard experiments in the nutrition field.
Like you said, they are expensive to do and usually only have a small number of subjects. That's why I'm suggesting that wealthy governments should collaborate to fund massive-scale trials at maximum quality.
In the general case "B" and "D" are mutually exclusive. For the important questions in human nutrition like whether a diet high in saturated fat increases the risk of all cause mortality it takes many years for significant effects to appear.
That has been done in a few studies. But delivering food doesn't mean that study subjects actually ate it, or didn't eat other food. So it doesn't solve the data quality problem.
Most human nutrition studies are only observational and barely a step above junk science. Don't take them too seriously.