Calcium is one of the most tightly regulated substances in the body. In addition to the role of calcium as a structural element in bones and teeth (99% of the body’s calcium is in the bones), calcium is critically needed for nerve function. When calcium in the plasma drops about 30%, the person may develop tetany, a condition that is often fatal due to overstimulation of the nerves in both the central nervous system and peripheral nervous system, leading to tetanic contraction of the skeletal muscles.
The concentration of calcium in the plasma is one of the most constant laboratory values ever measured. In the great majority of normal people, calcium only varies from 9-11 mg per dL, regardless of the diet. An average adult ingests about 750 mg per day of calcium and secretes about 625 mg of calcium in the intestinal juices. If all the ingested calcium is absorbed, there would be a net absorption of 125 mg per day of calcium. Since the average person excretes about 125 mg calcium per day in the urine, the average person has a zero net calcium balance except when bone is being deposited. If bone is being deposited due to the stress of exercise or following a fracture, the regulation of the amount of urinary calcium excretion is the major factor to allow for bone growth. Since urine is the major controlling element for maintaining calcium balance that is under tight hormonal control, it appears that urine calcium is the best indicator of adequate dietary calcium.
The most common reasons for low urine calcium are inadequate dietary calcium and/or a high oxalate diet. Other reasons for calcium deficiency include hypoparathyroidism, pseudohypoparathyroidism, vitamin D deficiency, nephrosis, nephritis, bone cancer, hypothyroidism, celiac disease, and malabsorption disorders.
Magnesium is an essential element like calcium and is also in the bones (66% of the body’s magnesium is in the bones). It is a cofactor with many enzymatic reactions especially those requiring vitamin B6. Like extremely low calcium, extremely low magnesium can also cause tetany of the muscles. Low magnesium in the diet may also increase the incidence of oxalate crystal formation in the tissues and kidney stones. Early signs of magnesium deficiency include loss of appetite, nausea, vomiting, migraine headaches, fatigue, and weakness.
The most common reason for high urine magnesium is high magnesium in the diet. Less common causes of high urine magnesium include insulin resistance, alcoholism, diuretic use, primary aldosteronism, hyperthyroidism, vitamin D excess, gentamicin toxicity, and cis-platinum toxicity. Symptoms of marked magnesium excess can include diarrhea, hypotension, nausea, vomiting, facial flushing, retention of urine, ileus, depression, and lethargy.
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