Comparative aspects of lipid digestion and absorption: physiological correlates of wax ester digestion.
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.