We humans are not all born with the same DNA and in the same circumstances. That may seem like an open door, but in medicine too often we overlook the fact that we are all different.
Suppose you are born with a mutation in the BCO1 gene. That gene makes the enzyme beta carotene oxygenase to make vitamin A from beta carotene. If you cannot make up that enzyme, you have a high chance of a vitamin A deficiency.
This mutation is quite common, but the complaints usually develop very gradually. When you go night blind, it is clear to many doctors that you are bothered by this. But if you develop other complaints, it is usually not so clear. These people do not benefit from a daily dose of beta carotene, they will become sicker with a vegetarian diet, while this may be beneficial for others.
With a marginal vitamin A deficiency complaints of the mucous membranes, of immunity, or problems in the thyroid function, the connective tissue build-up, intestinal complaints or iron household problems arise. I bet that very few doctors will have vitamin A status checked for these types of complaints. Provided they do, they will not see a marginal shortage as the key to many complaints.
And this is just one example, due to a mutation of one gene! And of course there are many more genes, but just as many conditions that influence the functioning of those genes. You can imagine that stress combined with a vitamin A deficiency (or B, C D, E or whatever) causes many more problems. And it is so easy to blame the stress. (It’s stuck between the ears.) you may be surprised that there are masses of people sitting at the psychologist who may be bothered by a biochemical disturbance.