Chemistry Reference
In-Depth Information
10.4.
Phosphorus
Phosphorus is an essential nutrient for humans, and the adult human body
contains about 850 g of elemental phosphorus with about 85% in the skeleton;
14% in the soft tissues; and 1% in the extracellular fluids, intracellular
structures and cell membranes (Anderson et al., 2006). Food phosphorus is
a mixture of inorganic phosphate (P
i
) and various organic phosphates. Most
phosphorus absorption occurs as P
i
because intestinal phosphatases hydro-
lyze the organic forms in foods (Food and Nutrition Board: Institute of
Medicine, 1997) and the predominant form of inorganic phosphate in all
biological fluids and tissues is the hydrogen phosphate ion (HPO
4
2
), a
divalent anion. The dietary phosphorus present as phytate, the storage form
of phosphorus in food plant seeds (beans, cereals, nuts, peas), is not available
directly. Phosphorus bioavailability from these foods depends on the hydro-
lysis of phytate by phytase produced by yeasts and colonic bacteria and the
natural phytase content of all foods. Leavening breads with yeasts that
produce phytase improves phosphorus bioavailability from those foods
(Anderson et al., 2006). Because there is a multitude of factors that influence
the presence of phytase, there is considerable flux in phosphorus bioavail-
ability from food phytate in the gut over a given period of time (Food and
Nutrition Board: Institute of Medicine, 1997).
The organic phosphates are major components of phospholipids,
nucleotides and nucleic acids. Also, the hydroxyapatite-like bone crystals
contain a constant ratio of calcium-to-phosphate of approximately 2:1. On
the other hand, the whole body P
i
compartment comprises a minute fraction
of total body phosphorus and is located mainly in the blood and extracel-
lular fluid (ECF). However, the P
i
compartment is a critical pool because it
accepts phosphate absorbed from the diet and the phosphorus resorbed
from bone and is the source of most bone fluid phosphorus and most
urinary phosphorus. The normal very high fluxes of P
i
between bone and
the bone fluid compartment each day (5000 mg) occur by ionic exchange
and active bone resorption (Anderson et al., 2006). Bone turnover rates are
relatively slow, so that dynamic ionic exchange is critical for maintenance of
blood P
i
concentration.
Phosphorus is found in nearly all foods, with most food sources
having high phosphorus bioavailability (55-70%) (Food and Nutrition
Board: Institute of Medicine, 1997). The phosphorus content of the US
food supply is increasing as phosphate salts are added to processed foods
for non-nutrient functions such as moisture retention, smoothness, and
binding. As a result, near total starvation is required to produce dietary
phosphorus deficiency which manifests as hypophosphatemia (Food and
