Interactions of phosphorus with other agents (micronutrients, foods):
Calcium and vitamin D
Dietary phosphorus is absorbed in the small intestine, and the excess is subsequently excreted by the kidneys. Both calcium and phosphate serum levels are regulated by parathyroid hormone (PTH) and vitamin D. Even a small drop in serum calcium levels – such as due to insufficient calcium intake – causes the parathyroid gland to respond by increasing the excretion of PTH. PTH stimulates the conversion of vitamin D into its active form (calcitriol) in the kidneys. Calcitriol, in turn, increases the absorption of calcium and phosphorus in the intestine. Both PTH and vitamin D stimulate absorption from the bones; calcium and phosphate – both of which are bone minerals – are released into the blood. PTH further provides decreased urinary excretion of calcium and increased urinary excretion of phosphorus.Increased excretion of phosphorus is beneficial for regulating serum calcium levels, because high serum phosphorus levels suppress the formation of the active form of vitamin D in the kidneys.
High-phosphorus diet and bone metabolism
Some researchers are concerned about the increasing amount of phosphorus in our diets-particularly as phosphoric acid in soft drinks and as phosphate additives in a variety of convenience foods. Serum phosphorus levels – unlike calcium – can rise somewhat as a result of ingesting too much phosphorus, especially after meals, because serum phosphate levels are not as finely regulated as serum calcium levels. High serum phosphorus levels decrease the formation of calcitriol, the active form of vitamin D, decrease calcium levels, and lead to increased PTH excretion by the parathyroid gland.Prolonged elevated serum PTH levels can have deleterious effects on bone mineral content, but this effect is detectable only in people who ate a diet high in phosphorus and low in calcium at the same time. However, similarly elevated PTH levels have also been observed in people who ate a low-calcium diet without also eating a high-phosphorus diet.A new study – double-blind randomized – found no deleterious effects of a high-phosphorus diet (3,000 mg/day) on bone-affecting hormones and biochemical markers in young women as long as the daily calcium intake of 2,000 mg/day was maintained.At present, there is no convincing evidence that increased phosphorus intake adversely affects bone density in humans. Nevertheless, the risk posed by the fortification of foods with phosphorus – soft drinks, dairy products, and other calcium-containing foods – should not be underestimated. At the same time, high serum phosphorus levels also lead to decreased urinary calcium excretion.
Fructose (fruit sugar)
Scientific studies show that a diet high in fructose-20% of total calories in the form of fructose-increased the loss of phosphorus through the urine, resulting in a negative phosphorus balance, meaning more phosphorus was excreted than was absorbed in the diet. This effect was even more pronounced with a diet low in magnesium. This may be due to the absence of a feedback mechanism that inhibits the conversion of fructose to fructose-1-phosphate in the liver. In other words, increased accumulation of fructose-1-phosphate in the cell does not inhibit the enzyme that phosphorizes fructose: On the contrary – large amounts of phosphorus continue to be consumed and fructose-1-phosphate is produced. This phenomenon is also known as “phosphate interception”. Since fructose consumption in Germany has risen sharply since the introduction of starch syrup, while magnesium intake has declined over the last century, recognition of this interaction is significant.
