Phosphorus
What is it?
Phosphorus is second only to calcium as the most abundant mineral in the body. It is usually combined with oxygen to make phosphate compounds and is a constituent of all plant and animal cells. The average adult contains around 500 g of phosphorus, around 85 per cent is in the bones and teeth, 14 per cent in the muscles and the rest in the fluid that surrounds the cells.
What it does in the body
Phosphate is the primary ion in extra and intracellular fluid. It aids absorption of dietary constituents, helps to maintain the blood at a slightly alkaline level, regulates enzyme activity and is involved in the transmission of nerve impulses. Phosphorus is a component of some of the major building blocks in the body, including RNA and DNA and lipids, including those in the blood and cell membranes.
Bones and teeth
Phosphorus combined with calcium, usually in the form of hydroxyapatite, is a major component of the structural part of bones and teeth.
Energy production
Phosphorus takes part in almost every metabolic reaction in the body. It is necessary for the conversion of dietary carbohydrate, fat and protein to energy. It is part of the adenosine triphosphate (ATP) molecule which acts as a reservoir of energy in cells.
Calcium-phosphorus balance
Calcium and phosphorus act together and balance each other in many body functions. An excessive intake of one mineral may cause a deficiency in the other. The intake ratio of calcium to phosphorus should be 1:1.
Absorption and metabolism
Nearly 70 per cent of dietary phosphorus is absorbed from the intestine, although absorption depends on intake levels and life cycle changes such as growth, pregnancy and lactation. In general, more phosphorus is absorbed from plant foods than from animal foods. Vitamin D is necessary for phosphorus absorption from the gut and transfer into bone from the blood. Most phosphorus excretion is via the kidney and this is mainly regulated by parathyroid hormone.
Deficiencies
Deficiency symptoms include weakness, loss of appetite, bone pain, joint stiffness, osteomalacia, irritability, numbness, pins and needles, speech disorders, tremor, and mental confusion. Red blood cell life becomes shortened, leading to anemia. White blood cells may also be affected leading to reduced resistance to infection.
Deficiencies are unlikely to occur in healthy adults as phosphorus is widespread in many types of foods. Certain medical conditions such as disorders of vitamin D metabolism, kidney or liver disorders, or alcoholism can induce low blood phosphate levels. Large amounts of aluminum-containing antacids can block phosphorus absorption and may lead to deficiency. Low levels of blood phosphorus are fairly common in hospitalized people and those on formula diets. Premature babies are also at risk of phosphorus deficiency.
Sources
Good sources of phosphorus include meat, yeast, wheatgerm, soybean flour, meat, poultry, cheese, milk, canned fish, nuts and cereals. Soft drinks also contain a lot of phosphorus and may contribute to excessive intake. Food additives may contribute 30 per cent of the dietary intake of phosphorus.
| Food | Amount | Phosphorous (mg) |
|---|---|---|
| Bran cereal | ½ cup | 770 |
| Rice bran | ½ cup | 706 |
| Salmon, cooked, dry | ½ fillet | 571 |
| Mackerel, cooked, dry | ½ fillet | 490 |
| Oats | ½ cup | 388 |
| Beef liver, fried | 85g | 392 |
| Sunflower seeds | ¼ cup | 369 |
| Raisin bran | 100g | 372 |
| Oat bran | ½ cup | 324 |
| Clam, boiled | 20 small | 304 |
| Buckwheat | ½ cup | 279 |
| All Bran | 30g | 280 |
| Tuna, grilled | 85g | 277 |
| Peanuts | ½ cup | 261 |
| Wheatgerm | ¼ cup | 232 |
| Goat's cheese | 30g | 219 |
| Bulgur, dry | ½ cup | 210 |
| Dried walnuts | ½ cup pieces | 181 |
| Swiss cheese | 30g | 182 |
| Wholewheat cereal | ½ cup | 168 |
| Cheshire cheese | 30g | 139 |
| Sardines, canned in tomato | 1 sardine | 139 |
| Sardines, canned in oil | 2 sardines | 118 |
| Shredded wheat | 30g | 106 |
Recommended dietary allowances (RDA)
The RDAs for phosphorus are based on an estimate for the preferable ratio (1:1) for calcium and phosphorus. The average American diet contains twice the RDA for phosphorus and the ratio of calcium to phosphorus is often as low as 1:2. The recommended upper intake limit is set at 4 g per day.
| Men | Women | Pregnancy | Lactation | |
|---|---|---|---|---|
| USA | 700 mg | 000 mg | - | - |
| USA (14 to 18 years) | - | - | 1250 mg | 1250 mg |
| USA (over 18 years) | - | 700 mg | 700 mg | - |
| UK | 540 mg | 540 mg | +425 mg | +425 mg |
| UK (under 18) | 770 mg | 620 mg | - | - |
| Australia | 1000 mg | 1000 mg | 1200 mg | 1200 mg |
Supplements
These are available in various forms including bonemeal, sodium phosphate and potassium phosphate. A recent study showed that sweet foods supplemented with phosphorus reduced the loss of minerals from dental enamel which these foods normally cause.1 Acute ingestion of phosphorus (phosphorus loading) may improve aerobic capacity.
Toxic effects of excess intake
High levels of phosphorus lead to calcium deficiency. (See page 191 for more information.) When the diet is high in phosphorus, calcium may be lost in the urine, increasing the risk of disorders such as kidney stones, osteoporosis and atherosclerosis.2 Soda drinks which contain large amounts of phosphoric acid may contribute to these harmful effects. Excess intakes can also prevent absorption of iron, magnesium and zinc. High levels of phosphorus also decrease vitamin D levels. This may increase the risk of bone disorders and cancer.3 A 1997 UK study involving 376 heart disease patients found a relationship between high phosphorus levels and the severity of coronary heart disease.4
Therapeutic uses of supplements
Sodium phosphate is used as a laxative as it increases the amount of water in the bowel. Phosphate supplements have been used to treat bone problems including osteomalacia, osteoporosis and rickets. They are sometimes given to premature babies. Low phosphate levels caused by disease are treated with supplements by a doctor who can monitor response.
Interactions
Interactions with other nutrients
The functions of calcium, magnesium and phosphorus are closely related and disturbances in one mineral may affect the other. Calcium aluminum, magnesium and iron reduce phosphorus absorption while vitamin D enhances it.
Cautions
Phosphorus supplements should be avoided in cases of kidney disease, liver disease, heart failure and high blood pressure. Phosphorus salts should not be given with iron salts, calcium salts, zinc salts or antacids as nonabsorbable complexes may form.