Unfortunately the soil in which crops are grown can become depleted of their mineral content meaning our food supply is not always a dependable source.
Although minerals are not destroyed by cooking, they can leach out of the food stuffs during this process. Peeling of foods (including polishing of rice and cereals) however, causes a major loss of minerals from our foods.
Taking a multimineral is by far the most cost effective way of supplementing your diet with minerals.