system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is nutrition such a difficult topic to research?

There is a massive amount of conflicting research/information on nutrition out there. Eggs are great for us, eggs are clogging our arteries, eating carbs is good and gives us energy, carbs make us lethargic and fat. As someone who, after years of treating their body like crap, wants to make an effort and eat things that are good for me, it seems impossible because at this point I feel like whatever arbitrary statement about food you take (like, eating 1/2 green apple increases your metabolism but only on Tuesdays and Fridays), you will find some type of research "confirming" it. Why is it so hard to have concrete research/evidence of what is good for our bodies and what isn't, at least generally? Isn't it science? How are we supposed to know what to eat??
gpt
Ethics - Researchers can't lock people up and force them to eat certain foods.  So researchers have to rely on willful compliance, which is difficult for most people.

Money - Human research is expensive.

Conflicts of interest - Food is a big business, and businesses do not like threats to their profits.  So they are more than happy to finance research of questionable quality in the name of profits.

Time Scale - Some issues relating to diet take years to manifest.  And on such a long time scale, there are many other potential contributing factors which must be rule out.

Difficulty - Science is hard and new tools & techniques emerge which help researchers do and explore more than they ever could.  This drives a lot of the findings that "turns the field on its head."   Communicating scientific findings to layman is also very difficult.