Food Structure for Nutrition - Food Colloids: Self-Assembly and Material Science - page 9

selective pressure have been guiding its evolution in many ways. Therefore, it is
reasonable to interrogate the basic structures of milk for lessons on how to
design molecular food structures for optimal nutrition.
1.6.1 Milk and Glucose Bioavailability
The rate of delivery of glucose has been under considerable scrutiny for several
years because of the implications for adverse health of rapidly absorbed glucose
from foods of high glycemic index. 8 It is thus potentially instructive to examine
how glucose is, in effect, delivered in milk.
The vast majority of glucose is present in milk not as glucose, but as the
disaccharide lactose. This is formed from glucose and galactose as a final step in
the assembly of milk in the lactating epithelial cell. Lactose is a disaccharide that
is strikingly non-digestible by humans. Only a single enzyme - lactase - has
emerged through mammalian evolution to digest lactose, and this enzyme
exhibits an equally remarkable regulation. This enzyme is produced by mam-
malian infants during the suckling period, and then, in virtually all mammals,
this activity is down-regulated on weaning. Thus, intriguingly, mammalian
mothers are unable to digest the lactose that they produce. Although various
functions have been suggested for lactose's unique structure, there is one
indisputable consequence of this structure to digestion. By delivering glucose
in the form of lactose, mammalian evolution has placed the delivery of glucose
exclusively under control of the infant and its lactase activity. Glucose will be
digested and absorbed only as rapidly as the activity of its endogenous intestinal
lactase enzyme allows. So, through the evolution of lactation, glucose is deliv-
ered as lactose, and this lactose invariably causes slow delivery of glucose.
1.6.2 Milk and Protein Bioavailability
The rate of delivery of protein has not been as well studied as that of other
macronutrients, although recent research 38,39 has begun to examine variations
in proteins as delivery agents for amino acids. Casein is the most abundant
single protein source in most mammalian milks, and it is the protein component
that is uniquely associated with milk. 29 Thus, it is interesting to examine casein
structure for its nutritional value in terms of structure, digestion, and rate of
absorption.
Proteins in general are digested by proteases as a consequence of their
structure. In fact, all levels of protein structure - primary, secondary, tertiary,
and quarternary - are known to affect proteolysis. The greater the degree of
native structure, the less digestible is the protein. In general, denaturation of a
protein increases its digestion by proteases. Thus, if evolutionary pressure were
to be actively providing a selective advantage to highly digestible proteins, then
the less the structure, the more digestible a given protein would be expected to
be. Consistent with this interpretation, casein is perhaps the least structured
protein known. 40 There is remarkably less secondary structure within any of the