Biologisch Medisch Centrum

Hydratatie, Kalium en Magnesium

Arts Paul van Meerendonk
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Phosphorus is a major structural component of bone in the form of a calcium phosphate salt called hydroxyapatite. Phospholipids (e.g., phosphatidylcholine) are major structural components of cell membranes. All energy production and storage are dependent on phosphorylated compounds, such as adenosine triphosphate (ATP) and creatine phosphate. Nucleic acids (DNA and RNA), which are responsible for the storage and transmission of genetic information, are long chains of phosphate-containing molecules. A number of enzymes, hormones, and cell-signaling molecules depend on phosphorylation for their activation. Phosphorus also helps to maintain normal acid-base balance (pH) by acting as one of the body's most important buffers. Additionally, the phosphorus-containing molecule 2,3-diphosphoglycerate (2,3-DPG) binds to hemoglobin in red blood cells and affects oxygen delivery to the tissues of the body (1).

Nutrient interactions:

Fructose

A study of 11 adult men found that a diet high in fructose (20% of total calories) resulted in increased urinary loss of phosphorus and a negative phosphorus balance (i.e., daily loss of phosphorus was higher than daily intake). This effect was more pronounced when the diet was also low in magnesium (3). A potential mechanism for this effect is the lack of feedback inhibition of the conversion of fructose to fructose-1-phosphate in the liver. In other words, fructose-1-phosphate accumulates in the cell but this compound does not inhibit the enzyme that phosphorylates fructose, which consumes large amounts of phosphate. This phenomenon is known as phosphate trapping (1). This study's finding is relevant because fructose consumption in the U.S. has been increasing rapidly since the introduction of high fructose corn syrup in 1970, while magnesium intake has decreased over the past century (3).

Calcium and vitamin D

Dietary phosphorus is readily absorbed in the small intestine, and any excess phosphorus absorbed is excreted by the kidneys. The regulation of blood calcium and phosphorus levels is interrelated through the actions of parathyroid hormone (PTH) and vitamin D (diagram). A slight drop in blood calcium levels (e.g., in the case of inadequate calcium intake) is sensed by the parathyroid glands, resulting in their increased secretion of PTH. PTH stimulates conversion of vitamin D to its active form (calcitriol) in the kidneys. Increased calcitriol levels in turn result in increased intestinal absorption of both calcium and phosphorus. Both PTH and vitamin D stimulate bone resorption, resulting in the release of bone mineral (calcium and phosphate) into the blood. Although PTH stimulation results in decreased urinary excretion of calcium, it results in increased urinary excretion of phosphorus. The increased urinary excretion of phosphorus is advantageous in bringing blood calcium levels up to normal because high blood levels of phosphate suppress the conversion of vitamin D to its active form in the kidneys (4).

Is high phosphorus intake detrimental to bone health?

Some investigators are concerned about the increasing amounts of phosphates in the diet which can be attributed to phosphoric acid in soft drinks and phosphate additives in a number of commercially prepared foods (5, 6). Because phosphorus is not as tightly regulated by the body as calcium, serum phosphate levels can rise slightly with a high phosphorus diet, especially after meals. High phosphate levels in the blood reduce the formation of the active form of vitamin D (calcitriol) in the kidneys, reduce blood calcium, and lead to increased PTH release by the parathyroid glands. However, high serum phosphorus levels also lead to decreased urinary calcium excretion (2). If sustained, elevated PTH levels could have an adverse effect on bone mineral content, but this effect has only been observed in humans on diets that were high in phosphorus and low in calcium. Moreover, similarly elevated PTH levels have been reported in diets that were low in calcium without being high in phosphorus (7). Recently, a controlled trial in young women found no adverse effects of a phosphorus-rich diet (3,000 mg/day) on bone-related hormones and biochemical markers of bone resorption when dietary calcium intakes were maintained at almost 2,000 mg/day (8). At present, there is no convincing evidence that the dietary phosphorus levels experienced in the U.S. adversely affect bone mineral density. However, the substitution of phosphate-containing soft drinks and snack foods for milk and other calcium rich foods does represent a serious risk to bone health (see Calcium).

Deficiency

Inadequate phosphorus intake results in abnormally low serum phosphate levels (hypophosphatemia). The effects of hypophosphatemia may include loss of appetite, anemia, muscle weakness, bone pain, rickets (in children), osteomalacia (in adults), increased susceptibility to infection, numbness and tingling of the extremities, and difficulty walking. Severe hypophosphatemia may result in death. Because phosphorus is so widespread in food, dietary phosphorus deficiency is usually seen only in cases of near-total starvation. Other individuals at risk of hypophosphatemia include alcoholics, diabetics recovering from an episode of diabetic ketoacidosis, and starving or anorexic patients on refeeding regimens that are high in calories but too low in phosphorus (1, 2).

The Recommended Dietary Allowance (RDA)

The recommended dietary allowance (RDA) for phosphorus was based on the maintenance of normal serum phosphate levels in adults, which was believed to represent adequate phosphorus intake to meet cellular and bone formation needs (2).

Sources

Food sources

Phosphorus is found in most foods because it is a critical component of all living organisms. Dairy products, meat, and fish are particularly rich sources of phosphorus. Phosphorus is also a component of many polyphosphate food additives and is present in most soft drinks as phosphoric acid. Dietary phosphorus derived from food additives is not calculated in most food databases, so the total amount of phosphorus consumed by the average person in the U.S. is not entirely clear. A large survey of nutrient consumption in the U.S. found that the average phosphorus intake was 1,495 mg/day in men and 1,024 mg/day in women. The Food and Nutrition Board estimates phosphorus consumption in the U.S. has increased 10% to 15% over the past 20 years (2).

The phosphorus in all plant seeds (beans, peas, cereals, and nuts) is present in a storage form of phosphate called phytic acid or phytate. Only about 50% of the phosphorus from phytate is available to humans because we lack enzymes (phytases) that liberate phosphorus from phytate (9). Yeasts possess phytases, so whole grains incorporated into leavened breads have more bioavailable phosphorus than whole grains incorporated into breakfast cereals or flat breads (2). The table below lists a number of phosphorus rich foods along with their phosphorus content in milligrams (mg). For more information on the nutrient content of foods, search the USDA food composition database.