The human species started out of Africa and spread over the whole planet. At those times the human world population was small compared to today. The roaming and scattering was done in small groups. As with all species, once separated through distance, independent breeding and environmental differences in selection our development forked. But we all did quite well. The populations grew and some groups became peoples, the first civilizations burgeoned. This was facilitated by domesticating animals and farming the land. From there on we talk about history. But the time before, the prehistory, was a lot longer. It is there that genetic differences developed.
In the long prehistory time humans adapted genetically to their environment, especially in what we ate. The tribes following the herds to the north fed on meat mostly, especially during the long winters. Those in the warm regions ate more plants and fruits because they were available either throughout the year or at least during summer.
When the populations grew, the peoples spread and met again: They waged war, fought for resources, mixed genetically again. These are time periods we learned to survey in our history education, periods of 2, 3 thousand years. That is no time compared to the hundreds of thousand of years before that. That is no time for genetic adaption, which counts in generations. The history time accounts for a hundred generations, the prehistory time for 10000 generations.
The history time brought us in today's small world for the 7 billions of us. We now can reach every spot of the world within a day. We mixed up and we mix up more than ever today, genetically, culturally and, in regard to the topic, especially in food culture.
We may be born into a "homogeneous" culture, but actually might not be so much in a genetic sense, because the mixing went on for some time already. So we start to live in a food culture that is good for some of us, but is bad for others. Some of us can drink milk and eat sweets throughout their life, others get intolerant to the lactose in the milk and can't eat fruits when they become adults. Others report issues when eating meat.
Health issues were not very much linked to food throughout history, because there was not the awareness brought by through the scientific method. Surely some individuals found out by observing themselves and couldn't explain why everybody else seemed to have no problems at all. Still today diets are advertised as the good diet for all. But it is not the case. We are different in what we can eat and in what we can't, even if we live side by side.
Were I was born people have a saying: "You eat what is on the table". I was born on a little mountain farm and I know of grandfathers who had stomach issues and died early. These people lived before the modern world had reached the village. They lived of the farming, and a lot of it was grain. The land was quite split already and rather small. So one had to be parsimonious to get along though the winter. One could not slaughter too often and so one ate grain products mostly. Their health issues were not associated to what they ate. They themselves were not aware of it and the people around them would also not have accepted special food for one of them.
All people have a long history of natural genetic selection behind them and are or were well adapted to their environment. This means that they also felt well, felt lively and full of energy. The fast change, compared to evolutionary times, in the food culture though cultural mixing and systematic creation of new food products, made some of us feel tired most of the times.
If you have health issues, mentally scan your diet first and do trial and error with food until you feel better. Observe your digestion.
A doctor will not see how you feel day in, day out. In general one cannot give away responsibility to others, also not to a doctor. Doctors often don't search carefully, because they don't have time, because they are in a routine to walk people through their practice. Don't become a congestion of the health system and don't waste money for doctors or health gurus.
But what is a health issue in the first place? If you feel tired and can't concentrate although you have slept enough, you might think, that's the way you are. But think again, think of what you ate, Tiredness is the first symptom and can last for decades, possibly accompanied by red eyes or light skin issues. The next symptoms show a lot later, maybe when you are 40. You get swollen eyes, more visible skin issues. But then your inner organs and all the rest of your cells have a long time of suffering behind them and have spent their very resourceful ability to cope with the poisonous milieu you have kept them for too a long time.
That is a big problem about eating related issues. It takes a long time until the symptoms become serious. So people tend to neglect them.
Take the first signs serious. I've heard that from an old professor who was shaking like hell while lecturing. Ultimately the nerves are the most precious part that will cease functioning: shaking, dementia an the like. Of course everything will cease functioning finally at one time. It is just that eating related issues tend to be a slow decline.