Medicine isn't hard science, it's barely a social science

Econ gets a lot of grief for its science pretensions. It proports to be able to describe cause and effect relationships with precise math, but only if one assumes the truth of several obviously false premises. Physics, meanwhile, relies on assumptions that make sense. Force equals mass times acceleration, assuming we are talking a normal human scale, not quantum mechanics.

Econ seems contrived and wrong not because it makes assumptions, but because the assumptions are substantively crazy, e.g., all humans are rational profit maximizers all the time.

We like to pretend that doctors are scientists of the human body, but the unstated assumptions behind most medical advise turn out to be more far fetched than rational market theory.

For example: "Try to eat healthy snacks like carrots or celery." The assumption is that when humans are hungry, our bodies are looking for a certain quantity of calories. If we get that number, we will feel better till our next meal. Does hunger work like that? Of course not! If it did, you'd be snacking on carrots already (well, maybe not).

As you already know from paying attention to your own body: No matter how many carrots you eat, you will still feel hungry until you get some protein or fat. You could eat 1000 calories of carrots, but you'll keep going until you eat a slice of pizza. Sans the bad advice, you would have skipped the carrots and gone straight for the pizza. Thus, the net result of the medical advice: you gain weight because your "healthy" 1000 carrot calories are simply added on top of your current diet.