When it comes to energy metabolism, hummingbirds are the heavyweight champions of vertebrates. Pound for pound, the thumb-sized birds have higher energy demands than elephants.
For this reason, hummingbirds have been widely used as a model to study energy balance. For the past 20 years, this model has been predicated on active absorption of sugars-that hummingbirds expend energy moving sugar molecules from the gut to the bloodstream. However, reporting online this month in Biology Letters, a University of Wisconsin-Madison researcher has shown sugars can enter the bloodstream passively, without energy expenditure. The article will also be published in the December print version of the journal.
“It”s important to understand digestion, how you get the energy from your food into your body. If there is an incorrect explanation for the physiology of how digestion occurs, that could impact other interpretations of energy balance,” says Todd McWhorter, a wildlife ecologist in the College of Agricultural and Life Sciences. “Currently, passive absorption is an under-appreciated means of getting nutrients and other substances into the body.”
McWhorter”s findings may necessitate a revision of current thinking about how nutrients are absorbed in vertebrates. For instance, the results indicate passive absorption may play a significant role in human medicine and health.
“There are implications for membrane function, human nutrition, and absorption of orally delivered drugs in humans. There are also implications for toxicology in humans and animals. If we understand how water-soluble substances are absorbed in the gut, then we can know more about potential exposure to natural and man-made chemicals in the environment,” says McWhorter. “For instance, if you have a more permeable gut, you are more vulnerable to water-soluble toxins.”
Absorption of nutrients occurs in the small intestine, after food has been digested in the stomach. The small intestine is lined with a layer of cells, called the epithelium, which actively moves nutrients from the small intestine to the bloodstream. During active transport, proteins on the surface of the epithelial cells selectively grab desirable nutrients from the gut and bring them inside the cell. Then, the cells deposit the nutrients into the bloodstream. The body expends energy running cellular machinery to accomplish these tasks.
While McWhorter”s results do not contest the important role of active absorption in digestion, he found, in hummingbirds, that more than half of sugar molecules simply slip between the cells of the epithelium straight into the bloodstream, without the help of cellular machinery. This process, called passive paracellular absorption, requires no energy expenditure by the body.
“Imagine that the epithelial cells of the small intestine are organized like a six-pack of Coke. Each can represents one cell. In the space between the cans, where they are connected by plastic, we have found that small molecules like sugars can pass right through to the other side, straight into the bloodstream,” explains McWhorter.
To study sugar absorption in hummingbirds, the birds were fed water containing indigestible sugars-sugars the protein transporters on epithelial cells can not grab and move to the bloodstream. If active absorption were the only absorption mechanism, these sugars would be unable to enter the body system, and so would pass in the feces. However, McWhorter detected the indigestible sugars in the bloodstream, indicating the sugars entered the bloodstream via passive absorption.
After plugging the data into a mathematical model, akin to the models used to assess the absorption of orally delivered medications in humans, hummingbirds were found to have high passive sugar absorption, similar to findings in other birds.
McWhorter and his collaborators are systematically gathering passive absorption data for all kinds of vertebrates. “We are involved in a broad survey of passive absorption in vertebrates. We are working in birds, mammals and reptiles to find out, using consistent methods, what the patterns are across vertebrates,” says McWhorter. “Understanding patterns of gut permeability in vertebrates in general helps us understand what is going on in humans.”
In the future, McWhorter plans to explore how passive absorption occurs and how the body regulates the process, if at all. Advances in this area stand to play a vital role in understanding and ultimately controlling nutrient and toxin absorption in humans and animals.
“The research we are doing helps us understand epithelial function, so it absolutely has biomedical implications for humans,” says McWhorter.
McWhorter”s collaborators include William Karasov at the UW-Madison, and Bradley Bakken and Carlos Martinez del Rio at the University of Wyoming. The project was funded by the National Science Foundation.
Citation for online article: McWhorter, T.J., B. Hartman Bakken, W.H. Karasov and C. Martinez del Rio. 2005. Hummingbirds rely on both paracellular and carrier-mediated intestinal glucose absorption to fuel high metabolism. Biology Letters (doi:10.1098/rsbl.2005.0388).
Male broad-tailed hummingbird (Selasphorus platycercus). Photo by Bradley Hartman Bakken.
hummingbird sugars 10/05
writer: Nicole Miller