What is Magnesium? Magnesium is an essential dietary mineral; although the human body contains a relatively small amount (approximately 25g, 60% of which is in the skeleton), magnesium plays a vital role in the normal functioning of the body (see below). The best dietary sources of magnesium are green leafy vegetables and cereals. Magnesium levels in the body may be depleted following intense physical activity, or by drinking alcohol. Many individuals may not have an adequate intake of magnesium, because of their reliance on processed foods which contain little magnesium.