Abstract
Bottlenose dolphins have a metabolism that mimics diabetes, including a sustained hyperglycemia following ingestion of either dextrose or large quantities of fish.1-3 While a diabetes-like metabolism is believed to be natural in dolphins, they are susceptible to diseases that are associated with insulin resistance in humans, including iron overload and urate nephrolithiasis.4-8 A series of studies was performed to assess the role of insulin resistance, hyperglycemia, and pathologic diabetes in dolphin metabolic diseases. The first study compared fasting and 2-hour postprandial insulin levels in dolphins with and without iron overload; dolphins with iron overload were more likely to have higher 2-hour postprandial insulin compared to controls (least squares means, 25 versus 7 μIU/ml, respectively; P < 0.01).3 A follow-on investigation found that 67% of dolphins in a study population had excessive hepatic iron deposition at time of death; assessment of clinical pathologies and location of iron deposition support that iron overload is more likely due to metabolic versus hereditary factors.9 The final retrospective study looked for the presence of fatty liver disease, a hallmark of metabolic syndrome with insulin resistance in humans, among 18 dolphins; 37% of dolphins had fatty liver disease, and dolphins with fatty liver disease were more likely to have postprandial hyperglycemia (> 140 mg/dl) compared to controls (31% versus 13%, P < 0.01).9 There is growing evidence that insulin resistance may be a common underlying cause of chronic metabolic diseases in dolphins, including iron overload and under-detected fatty liver disease. Improved biomarkers to better characterize metabolic syndrome and insulin resistance are being investigated, as well as the ability to treat insulin resistance with changes in diet, decreased overnight fasting duration, and increased activity levels.
Acknowledgements
The authors wish to thank the animal care staff and management at the Navy Marine Mammal Program for their support of this work.
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