Just wondered how we got to know what each vitamin does in our bodies, like vitamin A for example which is supposed to be good for eye health.
Wouldn’t it be pretty hard to conclude this by studies? How can one control for other variables?
A lot is just extensive research like anything else in the science world. You get people who are deficient in say vitamin A, and compare that to people who are sufficient in vitamin A, what are the changes if any? If someone deficient in vitamin A suddenly takes a supplement for it, what changes for them? Through observation, blood work, and people who donate themselves for science, you can start understanding how the human body is impacted by various nutrients.
Find people who are deficient in vitamin A, control for possible confounding factors, observe their health outcomes, repeat.
Basically you’re trying to identify a person with a deficiency and compare them to someone else who is alike in all other ways except the lack of a deficiency.
There’s also longitudinal studies where you take people with deficiencies and treat it while observing the changes post intervention.