Food fads and superstitions unhappily still delude too many people--and spawn too many cure-everything best sellers--even in these enlightened times. In fact, it is perhaps because these times are enlightened that food faddism is possible. Through most of man's history, his food consisted of whatever could be produced in the vicinity, of which there usually was no very much. It was eat what there was to eat or starve; no one could afford to be choosy, and without choosiness there can be no food faddism.
Modern transportation has made it possible to ship food form any part of the earth to any other, particularly since the use of large-scale refrigeration has arisen. This reduced the threat of famine, which, before modern times, was invariably local, with neighbouring provinces loaded with food that could not be transported to the famine area.
Home storage of a variety of foods became possible as early man learned to preserve foods by drying, salting, increasing the sugar content, fermenting, and so on. It became possible to preserve food in states closer to the original when methods of storing cooked food in vacuum were developed. (The cooking kills micro-organisms and the vacuum prevents others from growing and reproducing.) Vacuum storage was first made practical by a French chef, Francois Appert, who developed the technique in response to a prize offered by Napoleon I for a way of preserving food for his armies. Appert made use of glass jars, but nowadays tin-lined steel cans (inappropriately call 'tin cans' or just 'tins') are used for the purpose. Since the Second World War, fresh-frozen food has become popular and the growing number of home freezers has further increased the general availability and variety of fresh foods. Each broadening of food availability has increased the practicality of food faddism.
All this is not to say that a shrewd choice of food may not be useful. There are certain cases in which specific food will definitely cure a particular disease. In every instance, these are 'deficiency diseases', diseases produced by the lack in the diet of some substance essential to the body's chemical machinery. These arise almost invariably when a person is deprived of a normal, balanced diet--one containing a wide variety of foods.
We are wonderfully insulated from cold snaps and droughts when it comes to agriculture. I think it is important to consider the impact of reverting to non-modern techniques in our food supply.