Introduction to Human Nutrition

198 Introduction to Human Nutrition

phosphorus and hydroxyapatite mineral phosphorus
are derived (Figure 9.4). This compartment is also the
primary source from which the cells of all tissues
derive both structural and high-energy phosphate.
Food phosphorus is a mixture of inorganic and
organic forms. Intestinal phosphatases hydrolyze the
organic forms contained in ingested protoplasm and,
thus, most phosphorus absorption occurs as inor-
ganic phosphate. On a mixed diet, absorption of total
phosphorus ranges from 55% to 70% in adults. There
is no evidence that this absorption varies with dietary
intake. Furthermore, there appears to be no apparent
adaptive mechanism that improves phosphorus
absorption at low intakes. This situation is in sharp
contrast to calcium, for which absorption effi ciency
increases as dietary intake decreases and for which
adaptive mechanisms exist that improve absorption
still further at habitual low intakes. While a portion
of phosphorus absorption is by way of a saturable,
active transport facilitated by 1,25(OH) 2 D 3 the fact
that fractional phosphorus absorption is virtually
constant across a broad range of intakes suggests that
the bulk of phosphorus absorption occurs by passive,
concentration-dependent processes. Phosphorus

absorption is reduced by ingestion of aluminum-

containing antacids and by pharmacological doses of

calcium carbonate. There is, however, no signifi cant

interference with phosphorus absorption by calcium

at intakes within the typical adult range. Excretion of

endogenous phosphorus is mainly through the

kidneys. Inorganic serum phosphate is fi ltered at the

glomerulus and reabsorbed in the proximal tubule. In

the healthy adult, urine phosphorus is essentially

equal to absorbed dietary phosphorus, minus small

amounts of phosphorus lost in shed cells of skin and

intestinal mucosa.

Metabolic function and essentiality

Structurally, phosphorus occurs as hydroxyapatite in

calcifi ed tissues and as phospholipids, which are a

major component of most biological membranes,

and as nucleotides and nucleic acid. Other functional

roles of phosphorus include:

● buffering of acid or alkali excesses, hence helping to

maintain normal pH

● the temporary storage and transfer of the energy

derived from metabolic fuels

GIT

Plasma phosphorus

(3.5 mg/100 ml)

Feces

500 mg

1400 mg

Diet

1100 mg

5000 mg

+

5000 mg

Bone

6100 mg

Kidney

900 mg

200 mg

Ca2+ Parathyroids

7000 mg

PTH

Figure 9.4 Homeostatic regulation of serum phosphorus. PTH, parathyroid hormone.