Dolphins are the only animals apart from humans to develop a natural form of type 2 diabetes, according to new research. The discovery offers important insights into a disease that is linked to one in 20 deaths.
American scientists have discovered that bottlenosed dolphins show a form of insulin resistance very similar to that seen in human diabetes. Unlike patients with the condition, the marine mammals can turn this state on and off when appropriate, so it is not normally harmful.
The findings indicate that dolphins could provide a valuable animal model for investigating type 2 diabetes, which promises to advance research into new therapies. If researchers can learn how the animals switch off their insulin resistance before it becomes damaging, it could be possible to develop a cure.
Stephanie Venn-Watson, a veterinary epidemiologist at the US National Marine Mammal Foundation, who led the research, said that it could have profound implications for a disease that affects an estimated 2.75 million adults in Britain.
It suggests that the bottle-nosed dolphin is “an important, natural and long-lived model for insulin resistance and diabetes, a disease that accounts for 5 per cent of human deaths globally”, she told the San Diego conference of the American Association for the Advancement of Science. “It is our hope that this discovery can lead to novel ways to prevent, treat and even cure diabetes in humans while also benefiting dolphin health.”
She emphasised that the research did not mean that dolphins should be used as laboratory animals, as their large brains and high intelligence would make this unethical. Studies of their genetic code and physiology, revealed by blood and urine samples, could nevertheless provide important clues to the biology of diabetes.
The unexpected discovery has emerged from a study of more than 1,000 blood samples collected from 52 dolphins. When the animals had fasted overnight, their blood sugar remained high and their blood chemistry changed in ways similar to diabetic patients. Unlike people with diabetes, the dolphins’ blood reverted to normal once they had been fed.
Dr Venn-Watson said that such controlled diabetes might be beneficial to dolphins. Their diet of fish is high in protein and low in sugar, and they often go long periods without eating, yet they have large brains with high energy demands.
By making their bodies resistant to insulin while fasting, they may be able to keep their brains well supplied with sugar. Once they have eaten, the insulin resistance stops to prevent damage to their health. “We propose that, while some people may eat high- protein diets to help control diabetes, dolphins appear to have developed a diabetes-like state to support a high-protein diet,” she said. “It works to their advantage to have a condition that keeps blood sugar in the body.
“If dolphins indeed have a genetic fasting switch that can turn diabetes on and off, then finding and controlling such a switch could lead to the control of insulin resistance and possibly the cure to type 2 diabetes in humans.”
Dr Venn-Watson’s team has found that dolphins with excessive iron levels, or haemochromatosis, have high insulin levels that suggest a more harmful form of diabetes similar to the human disease. High iron is associated with insulin resistance in humans.
The findings are significant because there is no ideal animal model of type 2 diabetes. While rodents, cats, pigs and some primates display some aspects of diabetes, none mimics the disease as closely as dolphins.
Mark Simmonds, international head of science at the Whale and Dolphin Conservation Society, said there were ethical objections to using dolphins to study human disease and that dolphins were too distantly related from humans to be useful. He said: “The idea that dolphins would generally be a good model for the study of human disease seems unlikely. It is a grave concern that dolphins might be used in biomedical research. Dolphins are intelligent and sophisticated animals, vulnerable to stress and suffering when confined and removed from their natural environment.”
A link with obesity
• Type 2 diabetes occurs when the body acquires resistance to insulin — a hormone that controls blood sugar
• It is often triggered by obesity, though genetic factors are also involved, and it generally occurs in people over the age of 40
• The disease causes blood-sugar levels to become elevated, resulting in progressive damage to blood vessels and nerves
• Complications include cardiovascular disease, poor circulation leading to amputation of limbs, blindness and impotence
• Type 2 diabetes has been diagnosed in about two million people in the UK
• There is no cure, though it can be controlled by diet, exercise, weight loss and drugs