Chemistry Reference
In-Depth Information
protein from omnivorous sources increases intestinal calcium absorption and
substitution of soy protein for meat protein causes an acute decline in dietary
calcium bioavailability (Kerstetter et al., 2006). Regardless of the source of
calcium, calcium absorption efficiency decreases with increasing intake but
total calcium absorbed continues to increase with load (Weaver and Heaney,
2006). Earlier concern that purified proteins in the diet decrease calcium
balance (Weaver et al., 1999) was muted by the finding that calcium retention
was not reduced when subjects consumed a high-protein diet from common
dietary sources such as meat (Roughead et al., 2003). Calcium absorption
efficiency is upregulated during puberty and during the third trimester of
pregnancy. On the other hand, calcium absorption efficiency declines with
age (Weaver and Heaney, 2006). Active calcium absorption is compromised
by hyperparathyroidism and diseases of the kidney (Weaver and Heaney,
2006).
In bovine milk, 99% of the calcium is in the skim milk fraction, whereas,
in human milk, 16% of calcium is present in the lipid fraction (bound to the fat
globule membrane), 47% is protein bound (6% to casein and the remainder to
whey proteins), about 38% is soluble (mainly as calcium ions) (Fransson and
Lonnerdal, 1983) and 1% is bound to
-lactalbumin (Lonnerdal and Glazier,
1985). Casein micelles are colloidal protein-calcium-transport complexes
with the colloidal calcium phosphate present in nanoclusters with a diameter
of
2.5 nm (Marchin et al., 2007). Therefore, calcium levels are higher in
milks rich in caseins (Gaucheron, 2005). Micellar calcium is not exclusively
associated with the colloidal inorganic phosphate; part is bound directly to
the phosphoserine residues of casein, which are organic phosphate. Thus, the
colloidal calcium in milk is a mixture of calcium caseinate (containing organic
phosphate) and calcium phosphate (an inorganic phosphate) (Gaucheron,
2005). In bovine milk, two-thirds and one-third of the total calcium are in the
micellar and soluble forms, respectively (Gaucheron, 2005). Micellar calcium
phosphate is exchangeable with the diffusible fraction, and the calcium bound
to phosphoserine residues is more exchangeable than that associated with
colloidal phosphate (Gaucheron, 2005). Ionized calcium in the soluble phase
accounts for about 10% of the total calcium, with the remaining soluble
portion as calcium citrate. Micellar calcium phosphate plays a key role in
the maintenance of the structure of the casein micelle, and the physico-
chemical properties are exploited for the manufacture of dairy products
(Gaucheron, 2005). Casein micelles of human and bovine milk differ in
composition such that when clotted by proteolytic enzymes under conditions
simulating those found in the infant stomach, either no clot or an almost
undetectable very fine curd forms as opposed to the large curds in bovine
milk. The small clots may be involved in the proper absorption of the milk
constituents into the body for optimal utilization (Sood et al., 1997).
