By Rachel Ehrenberg, Science News
SAN DIEGO—Fish might be brain food, but it doesn’t supply the high levels of fuel needed to keep a dolphin brain functioning. New research adds to evidence suggesting that bottlenose dolphins go into a harmless diabetic state during overnight fasting, thereby maintaining high levels of glucose in the blood. The research, presented at a news briefing February 18 at the annual meeting of the American Association for the Advancement of Science, suggests that dolphins may be a good model for studying diabetes and could offer insights into treating the disease in people.
Carbohydrates typically provide animals a glucose fix. But dolphin diets are high in protein and very low in glucose-rich carbs. Dolphins may have a “diabetic switch” that “helps keep the brain well-fed” even when they haven’t eaten for a while, said veterinary epidemiologist Stephanie Venn-Watson of the National Marine Mammal Foundation in San Diego. “Brains need sugar to function, but a diet of fish has no sugar,” she said.
The proposed fasting “switch” may allow dolphins to turn this diabetic state on and off. In people with type 2 diabetes, high levels of blood glucose result from insulin resistance. These individuals don’t respond to signals from their own insulin, which tell body tissues to absorb glucose from the blood. But in dolphins, what in people look like dangerously high levels of circulating glucose may provide fuel for dolphins’ big brains during the fasting period between dinner and breakfast, Venn-Watson explained.
Preliminary findings suggest that this insulin resistance in dolphins isn’t always good and that dolphins may get a pathological form of diabetes. For 21 weeks, Venn-Watson and her colleagues measured insulin levels in six dolphins two hours after the animals ate. One dolphin that had especially high insulin levels compared to others, also had a 10-year history of iron overload, or hemochromatosis. Iron overload is associated with type 2 diabetes in people, Venn-Watson noted. This new work adds to previously published research showing that fasting dolphins show changes in blood chemistry, including in glucose levels, that mirror changes in the blood of people with diabetes.
While humans and dolphins aren’t closely related, both have big brains and blood cells that can carry large amounts of glucose. A similar diabetic “switch” may lurk in the physiology of humans, Venn-Watson said. Several years ago scientists J. Brand Miller and Stephen Colagiuri proposed that before the ice age, the human diet was rich in carbs, but when the freeze came, humans may have switched to a diet high in protein. “Basically everything with lots of carbs froze,” said Venn-Watson. In this environment, evolution may have favored insulin resistance to keep glucose available for the brain. The same thing might have happened when dolphin ancestors colonized the seas, she said. “Maybe we can find the switch in humans.”