Tannins are brown chemicals that provide the colour in many foods and drinks. They occur naturally in many plants, such as nuts, bruised fruit, red grapes and any woody parts of plants. They also occur in many processed foods, such as red wine, brandy, apple juice, tea.
Tannins seem to be created to discourage animals from eating the plants. They seem to be designed to disturb the digestive system of whatever is eating the plant, and to do other things.
Through evolution, our bodies have learned over the years to be able to cope with such substances in our food. But we do not cope very well. They can still hurt us.
Tannin can also be beneficial. And it can provide a mild "buzz" in our brain. That is why we can crave food that contains tannin. And it is why some alcoholic drinks are matured in the barrel - so that tannins can be absorbed from the wood.
Unfortunately, some people are more sensitive to tannins than others. They get far too much of a "buzz", in the form of a migraine.
Some people have found that it is better to control the tannins, rather than to consume powerful drugs.
Now we should explain that this is not scientifically proven fact at this stage. But we are beginning to have good anecdotal evidence for this theory. And it does seem to work beautifully for some people.