Next to calcium, phosphorus is the most abundant mineral in the body, making up about 1% of total body weight. Most of it is found in bones and teeth. Phosphorus is present in the body as phosphates and, in addition to its role in bone formation, it is vital to energy production and exchange. Phosphorus helps in muscle contraction and nerve conduction. It aids kidney function and helps maintain the body's pH balance. Phospholipids are fat molecules that play a role in the maintenance of cell membranes. As a component of adenosine triphosphate (ATP), phosphorus is involved in the body's primary metabolic cycles and in protein synthesis for growth, maintenance, and repair of all body tissues and cells, as well as in the production of the nucleic acids in DNA and RNA. It is also necessary for the absorption of many vitamins and minerals, including vitamin D, calcium, iodine, magnesium, and zinc.

The parathyroid hormone (PTH) regulates the metabolism of phosphorus and calcium in the body. About two-thirds of phosphorus is absorbed from the intestine, the rate depending to some extent on levels of calcium and vitamin D, as well as the activity of PTH. While 85% of phosphorus is deposited in bones and teeth, the remainder is found in cells and other body tissues. The blood contains about 3.5 mg of phosphorus per 100 ml of plasma; total blood phosphorus is between 30 and 40 mg. Together, calcium and phosphorus assure the formation and maintenance of strong bones. The ideal dietary ratio of Ca:P is 1:1. A low Ca:P ratio can lead to bone resorption as the body draws upon existing calcium stores in the bone to pair with excess phosphorus. Not only is phosphorus absorbed more efficiently than calcium, but people are likely to get more phosphorus from their diets. The typical American diet has Ca:P ratios ranging from 1:2 to 1:4. The growing consumption of soft drinks, which are buffered with phosphates (as much as 500 mg in one serving), and high consumption of red meat and poultry, which contain 10 to 20 times as much phosphorus as calcium, are largely responsible for this imbalance. Decreased Ca:P ratios due to excess dietary phosphorus impair calcium absorption, which contributes to bone loss, osteoporosis, and periodontal disease. Low Ca:P ratios have also been associated with an increased incidence of hypertension and elevated risk for colorectal cancer.

Phosphorus deficiency, or hypophosphatemia, is rare except in people affected by certain diseases, in those receiving parenteral nutrition, or in those who have received phosphate-binding agents that contain aluminum for extended periods. It has been associated with anorexia, anxiety, apprehension, bone pain, bone fragility, stiffness in the joints, fatigue, irregular breathing, irritability, numbness, paresthesias, weakness, and weight change. In children, decreased growth, poor bone and tooth development, and symptoms of rickets may be signs of phosphorus deficiency. A greater concern for physicians is hyperphosphatemia, or an excess of phosphorus. This is most often the result of dietary imbalance. This has a negative result in terms of bone density and is of particular concern to women. Acute or chronic renal failure may also lead to hyperphosphatemia; in these cases restricting phosphorus intake to 800 to 1,000 mg is indicated.

Dietary sources of phosphorus include the following.

• Red meat and poultry
• Dried milk and milk products
• Wheat germ
• Yeast
• Grains
• Hard cheeses
• Canned fish
• Nuts
• Potatoes
• Eggs
• Soft drinks

Elemental phosphorus, a white or yellow waxy substance that burns on contact with air (thus the term phosphorescent), is used in some homeopathic remedies. However, because it is highly toxic, it is no longer used in medicine. Instead, inorganic phosphates are used to treat phosphate deficiency. The following forms are used.

• Dibasic potassium phosphate
• Monobasic potassium phosphate
• Dibasic sodium phosphate
• Monobasic sodium phosphate
• Tribasic sodium phosphate

Phosphorus is available over the counter in capsules. Because it is readily available in a variety of foods, phosphorus supplementation is usually confined to athletes who take it to reduce muscle pain and fatigue.

Phosphorus, by itself, is used in the treatment of only a few medical conditions. Along with calcium, however, it can help in healing bone fractures and in the treatment of osteomalacia, osteoporosis, and rickets. Regulating the ratio of calcium-to-phosphorus intake through dietary sources can reduce stress and alleviate problems like arthritis, which are related to calcium metabolism.

Hypophosphatemia can cause an impaired response to insulin for which supplementation with dibasic calcium phosphate (2 g tid with meals) has shown good results. Phosphate supplementation is also used in the treatment of diabetic ketoacidosis (DKA), and in constipation due to hypercalcemia. Mono- and dibasic sodium phosphates may be used as mild laxatives administered by mouth or rectally. Elemental phosphorus is used in homeopathic treatments for coughs and some types of acute gastroenteritis.

The U.S. RDA for phosphorus is 800 to 1,200 mg daily. The RDA for those up to age 24 and during pregnancy and lactation is 1,200 mg daily.

Phosphates can be toxic at levels over 1 g/day, leading to diarrhea, calcification of organs and soft tissue, and preventing the absorption of iron, calcium, magnesium, and zinc. High levels of phosphorus can promote the loss of calcium through nutritional hyperparathyroidism.

The overconsumption of foods high in phosphorus can drain calcium resources and lead to reduced bone mass. American dietary habits, particularly the high consumption of meat and soft drinks, makes low Ca:P ratios quite common. This imbalance may be the source of the high incidence of osteoporosis in the U.S. and other affluent nations where similar dietary habits prevail. Some researchers have identified this as the likely mechanism contributing to low bone mass in American women. Further retrospective studies are needed to investigate this hypothesis. Low ratios of dietary Ca:P also reduce the efficacy of treatments for osteoporosis. A 1986 experimental study of 158 females, aged 20 to 75, found that treatment of osteoporosis may in fact be fruitless when dietary Ca:P ratios exceed 1:1.25. Patients need to be aware of the dietary sources of both calcium and phosphorus so that they can take a more active role in balancing these two elements in their diet.

No clinically significant interactions between phosphorus and conventional medications are known to have been reported in the literature to date.

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