AJP - Regulatory, Integrative and Comparative Physiology, Vol 263, Issue 3 464-R471, Copyright © 1992 by American Physiological Society
Comparative aspects of lipid digestion and absorption: physiological correlates of wax ester digestion
A. R. Place
Center of Marine Biotechnology, University of Maryland System, Baltimore 21202.
Our understanding of lipid digestion and absorption is largely derived from
studies with glyceride-based dietary lipids. Yet for a great majority of
marine species, wax esters (long-chain fatty alcohols esterified to
long-chain fatty acids) are the dominant dietary neutral lipid. Many birds,
especially seabirds (9 species) and some passerines (2 species), have a
unique capacity for assimilating wax esters with higher efficiencies
(greater than 90%) than that attainable by mammals (less than 50%). This
unique capacity is correlated with several factors. One factor is an
elevated intestinal bile salt concentration approaching 50 mM and a
gallbladder concentration exceeding 600 mM. A second factor involves
regular retrograde movement of duodenal contents to the gizzard. Thus not
only is gastric emptying closely tied to the receptiveness of the duodenum
for further handling of digesta as in mammals, but in birds the reflux
returns the digesta (both gastric and duodenal) for further processing to
the gizzard. A third key factor in wax ester utilization is a nearly
equivalent hydrolysis of wax esters and triglycerides. Although
similarities in fat digestion exist between birds and mammals, there do
exist differences that make nonpolar lipid assimilation (i.e., wax esters)
more efficient in birds, especially for seabirds.
