Introduction to Human Nutrition

190 Introduction to Human Nutrition

a tightly regulated range. This tight regulation of
plasma calcium concentration is achieved through a
complex physiological system comprising the interac-
tion of the calcitropic hormones, such as parathy-
roid hormone (PTH), 1,25-dihydroxycholecalciferol
[1,25(OH) 2 D 3 ] and calcitonin, with specifi c target
tissues (kidney, bone, and intestine) that serve to
increase or to decrease the entry of calcium into
the extracellular space (plasma) (Figure 9.2). Only in
extreme circumstances, such as severe malnutrition
or hyperparathyroidism, is the serum ionized calcium
concentration below or above the normal range. The
secretion of these hormones is governed wholly, or in
part, by the plasma concentration of ionized calcium,
thus forming a negative feedback system. PTH and
1,25(OH) 2 D 3 are secreted when plasma calcium is
low, whereas calcitonin is secreted when plasma
calcium is high.
Calcium in food occurs as salts or associated with
other dietary constituents in the form of complexes
of calcium ions. Calcium must be released in a soluble,
and probably ionized, form before it can be absorbed.

Calcium is absorbed in the intestine by two routes,

transcellular and paracellular (Figure 9.3). The trans-

cellular route involves active transport of calcium by

the mucosal calcium transport protein, calbindin, and

is saturable and subject to physiological and nutri-

tional regulation via vitamin D. The paracellular route

involves passive calcium transport through the tight

junctions between mucosal cells; it is nonsaturable,

essentially independent of nutritional and physiologi-

cal regulation, and concentration dependent. Most

calcium absorption in humans occurs in the small

intestine, but there is some evidence for a small

colonic component. Transcellular calcium absorption

responds to calcium needs, as refl ected by changes

in plasma calcium concentration, by hormone-

mediated up- or down-regulation of calbindin in

mucosal cells; for example, reduced plasma calcium

evokes a PTH-mediated increase in plasma

1,25(OH) 2 D 3 , which stimulates increased calbindin

synthesis in intestinal mucosal cells.

On average, between 10% and 30% of the calcium

is absorbed from a mixed diet by healthy adults.

GIT

Plasma calcium

(9–10.4 mg/100 ml)

Feces

800 mg

1000 mg

Diet

300 mg

400 mg

+

+

+

+

+

+





400 mg

Bone

Thyroid

CT

9 800 mg

Kidney

1,25(OH) 2 D 3

200 mg

100 mg

Ca2+

Ca2+

Parathyroids

10 000 mg

25(OH)D

PTH

Figure 9.2 Homeostatic regulation of serum calcium, showing the integration of hormone action at the tissue level. CT, calcitonin; PTH, para-
thyroid hormone; 1,25(OH) 2 D 3 , 1,25-dihydroxycholecalciferol (to convert from mg/day to mmol/day multiply by 40).