Magnesium is a mineral that is vital for energy production, muscle contraction, nerve function, and the maintenance of strong bones. It comes into the body through the diet and is absorbed by the small intestine and colon. Magnesium is stored in the bones, cells, and tissues. Normally, only about 1% of total body magnesium is present in the blood and this makes it difficult to get an accurate measurement of total magnesium content from blood tests alone. However, this test is still useful for evaluating a person's magnesium status.