Excess sugar is toxic to rock iguanas on remote islands

We all know the potential consequences of including too much sugar in our diets – higher blood pressure, inflammation, weight gain, diabetes and fatty liver disease. In addition, all these conditions are linked to an increased risk for heart attack and stroke. Eating a high-sugar diet, in spite of these potential hazards, is a personal choice but it seems that people cannot resist feeding wildlife with snacks from their lunch packs and this may well expose vulnerable species to the same sort of health risks.

“A prolonged high-sugar diet can ‘exhaust’ the ability to regulate blood glucose,” explained Susannah French from Utah State University. “Catalina Monzon-Arguello and colleagues discovered in 2018 that juvenile green sea turtles in the Canary Islands have high fat and cholesterol levels in their blood as a result of being fed by tourists.”

Endangered rock iguanas on the Exuma Islands in the northern Bahamas are also attractions for visiting ecotourists, who feed them with items such as grapes in order to interact more closely with them. French, along with Dale Denardo (Arizona State University), and northern Bahamian iguana expert Charles Knapp (John G. Shedd Aquarium), wondered whether these iguana populations would also show physiological evidence of being fed sugar-charged items not usually present in their diet.

In the initial stage of their research, the scientists tested the impact of added dietary sugar on the blood glucose levels of captive iguanas in the laboratory. 

“We chose to study the more common green iguana in the lab, because rock iguanas are critically endangered,” said French, who supplemented the iguanas’ usual meals of “Tortoise Diet” and greens with either a high (5g/kg) or low (2.5g/kg) glucose drink.

“The low dose provides glucose in an amount similar to that found in consumed grapes,” said Denardo. 

Then, after 17 days, each iguana was given an intermediate strength glucose drink (3.75g/kg) and its blood glucose levels were monitored over the following two days. As expected, the reptiles’ glucose levels peaked three hours after consuming the sugar, but those iguanas that had previously been fed the high glucose diet had the most difficulty bringing their blood sugar levels under control. Glucose in their blood reached a concentration of ~520mg/dl, in contrast to iguanas that had been fed Tortoise Diet and greens alone, whose levels peaked at only ~420mg/dl. 

The researchers interpreted the iguanas’ hyperglycaemia as evidence that a high glucose diet impacts the captive iguanas’ ability to regulate their blood glucose levels after a meal. This is a symptom typical of people who suffer from type 2 diabetes.

In the second part of the study, French, Denardo, Knapp and colleagues from various US institutions traveled to the remote Exuma Islands to measure the blood glucose levels of the rock iguanas in situ. 

“When we pull up onto picturesque sandy beaches on the tourist islands, the sound of the boat motors draws the iguanas down, but landing on the islands where tourists do not travel requires carefully timing your jump from the boat,” said French. 

The researchers carefully captured 24 iguanas from islands frequently visited by tourists and another 24 from islands that are too rugged for tourists to access. Each iguana was fed a glucose drink and its blood sugar levels were monitored for the best part of a day. 

The results, published today in the Journal of Experimental Biology, are cause for great concern, say the scientists. The iguanas from one island where they are fed frequently by tourists experienced the highest glucose peak ~570mg/dl after five hours, and the levels were still high eight hours later. Iguanas from another island on the tourist route had a less extreme response, showing glucose levels of ~505mg/dl five hours after receiving the glucose drink. 

By contrast, the blood sugar levels of the iguanas from the islands that rarely see tourists were much lower, peaking at ~450mg/dl. Blood glucose concentrations in these iguanas also increased at a slower rate and returned to normal more quickly.

It is clear to the researchers that feeding the wild iguanas on the Exuma Islands is impacting their ability to regulate their blood glucose levels and keep them under control. Whether this will lead to a deterioration in the health of the animals is not yet known. 

“If these were humans, we would be talking about diabetes, however, it is not yet clear what the health implications are here. That is something we are continuing to work on,” said French.

“The opportunity to develop extraordinary connections with wildlife through tourism can lead to compassion and conservation, but we need to work towards a future where animals and the livelihoods of people who rely on them are safeguarded,” said Knapp. “While we continue to expand our understanding of how tourism affects these endangered iguanas, we hope that our research will guide science-based recommendations for sustainable wildlife activities.”

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By Alison Bosman, Earth.com Staff Writer