Modulation of digestive enzyme activities in the avian digestive tract in relation to diet composition and quality.

In nature, birds are faced with variable food sources that may differ in composition (protein vs. carbohydrates) and quality (highly digestible material vs. indigestible fiber). Studies in passerine birds and some commercial poultry demonstrate that the gastrointestinal tract can respond to varying diet composition and quality by changing morphology and/or activities of digestive enzymes. However, studies in additional avian species are warranted to understand generalities of these trends. We first fed juvenile mallards (Anas platyrhynchos), chickens (Gallus gallus), and quails (Coturnix coturnix) on either high-carbohydrate or high-protein diets. For the most part, birds fed the high-carbohydrate diet had higher small intestinal and cecal disaccharidase activities (maltase and sucrase). However, only mallards exhibited higher small intestinal aminopeptidase-N (APN) activities when fed the high-protein diet. These results differ from passerine birds, which largely modulate small intestinal proteases, but not disaccharidases. In another trial, we fed Canada geese (Branta canadensis) diets that varied in both their protein and fiber concentrations for approximately 3.5 months. Birds fed the high-fiber diets had significantly longer small intestines and caeca compared to those fed low-fiber diets. Additionally, geese fed the high-fiber diets exhibited lower mass-specific activities of small intestinal sucrase, and higher activities of APN when summed across the small intestine and ceca. Similar to the avian species above, geese fed the high-protein diets did not exhibit flexibility in their small intestinal APN activities. Overall, these experiments demonstrate that responsiveness of the avian digestive tract to diet composition may have phylogenetic or ecological constraints. Studies on other avian taxa are needed to understand these patterns.

Effect of age and diet composition on activity of pancreatic enzymes in birds.

Digestive enzymes produced by the pancreas and intestinal epithelium cooperate closely during food hydrolysis. Therefore, activities of pancreatic and intestinal enzymes processing the same substrate can be hypothesized to change together in unison, as well as to be adjusted to the concentration of their substrate in the diet. However, our knowledge of ontogenetic and diet-related changes in the digestive enzymes of birds is limited mainly to intestinal enzymes; it is largely unknown whether they are accompanied by changes in activities of enzymes produced by the pancreas. Here, we analyzed age- and diet-related changes in activities of pancreatic enzymes in five passerine and galloanserine species, and compared them with simultaneous changes in activities of intestinal enzymes. Mass-specific activity of pancreatic amylase increased with age in young house sparrows but not in zebra finches, in agreement with changes in typical dietary starch content and activity of intestinal maltase. However, we found little evidence for the presence of adaptive, diet-related modulation of pancreatic enzymes in both passerine and galloanserine species, even though in several cases the same birds adaptively modulated activities of their intestinal enzymes. In general, diet-related changes in mass-specific activities of pancreatic and intestinal enzymes were not correlated. We conclude that activity of pancreatic enzymes in birds is under strong genetic control, which enables evolutionary adjustment to typical diet composition but is less adept for short term, diet-related flexibility.