system
Given the following post, respond as if you're commenting on Reddit:
human
Why are we so far from a scientific consensus about the best diet?

I understand that human needs are complex are human bodies are adaptable, so we shouldn't expect a consensus list of The Best Things for Everyone to Eat. However, it seems to me that **some** options should be off the table.

Some eating habits are nearly diametrically opposed. When I was younger, "low-fat" was generally accepted, and now that is even subject to opinion.


I don't want to have to think about what to eat. It should be something scientifically testable! So why aren't nutrition scientists making strong claims that trim down the number of "healthy" diet options?
gpt
Testing it is actually quite hard. You have a few different options, which all have problems. 

First of you could try to look at large groups of people and their diets. This has been done by some of the high fat diet folks. They look at people from Greece, and see that they have less hearth trouble than average. So a high fat diet must be good. However, there are lots of problems with it. For one thing, there are lots of factors. Maybe it's not the high amount of fat that makes it good, maybe it's the fact that they drink more goats milk, have a better medical system or more friends. You can find groups following most diets that live a long time. So this way isn't all that effective. 

You could get a group and ask them to follow a diet, and observe them. If the group is big enough (which it often isn't), you can get some good data. However, people don't follow diets. If they break it, the data lose a lot of value. 

Note: Sometimes it can be interesting to know if people break their diets. Because maybe some diets are easier to follow than others. Should that be taken into consideration?

Those are the two big ones, but there are of course a few other ones, all of them with problems. Case studies with only one guy (or a very few), the small group makes those worth less even if you can make sure they follow the prescribed diet. Animal studies, but animals aren't the same as us.

Then you have a few systemic ones. For one thing there is the fact that you have lots of groups all earning lots of money on different diets. And since there are so many flaws in the studies, making them say what you want them to isn't all that hard. So you get sponsored tests, that show that the best diet is the one the sponsor wants. 

Independent studies often lack money, which makes them small (and that makes them inconclusive). 

And lastly you have the insane number of relevant factors. Different people want different diets (underweight people vs overweight, women vs men, old vs young), different diets are good for preventing different things (one is good for the hearth, one for the brain, one for muscle growth, etc).