Phosphorus Digestion and Absorption

Approximately 50-70% of the phosphorus in our diets is absorbed in the small intestine. The phosphorus in animal products like meat and milk is more easily absorbed than it is in foods with a high phytic acid content of nuts, seeds, grains, and legumes. Phytic acid inhibits the absorption, limiting the bioavailability of the phosphorus in these foods about to about 50%.

In addition to phytic acid, magnesium, aluminum, and calcium all inhibit the absorption of phosphorus by forming insoluble phosphates that are eliminated in the feces. All of these minerals are common components of antacids prescribed in high dosages to treat hyperphosphatemia in kidney disease patients.

Caffeine also decreases the absorption of phosphorus. Drinking a lot of coffee or tea causes more phosphorus to appear in the urine.

The digestion and absorption of phosphorus is a quick process. Studies with animals show that it passes from the intestines into the blood in less than an hour.

Between 67% and 90% of unabsorbed phosphorus is excreted in the urine, with the remaining 10-33% excreted in the feces. Thus, the kidneys serve as the main vehicle for maintaining blood serum phosphorus homeostasis. If the kidneys are functioning normally, when there is too much phosphorus in the blood, more ends up in the urine. When there is too little phosphorus in the blood, more phosphorus will be reabsorbed, and the concentration of phosphorus in the urine will be less.