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Dunnart Studies

Researchers at the Royal Adelaide Hospital have been experimenting on the tiny native fat-tailed dunnart for a number of years. One of the researchers suggested in the Sydney Morning Herald that these animals were sufficiently like humans to provide useful information about human obesity. However, dunnarts are nocturnal, insectivorous animals that weigh only 10-20 grams. They can eat 90% of their body weight in insects in a night, and can go into torpor (slowed metabolic rate) when temperatures are low and food is scarce. The tail is a specialised fat storage area, and contains 25% of the body's fat reserves. Some of the experiments that have been done on these dunnarts are described below.

1. Examine food intake, activity and body weight/fat distribution in relation to:

   • gender

   • 24 hour or 36 hour food deprivation

   • long or short night cycle

   • surgical tail removal

   Not surprisingly, most feeding occurred at night, food intake increased after fasting, body weight decreased after fasting, and body weight decreased after tail removal but recovered within 3 weeks. The mutilated animals did not increase their food intake, but recovered their weight as a result of decreased activity. The importance of exercise in human weight control is already known.

2. Examine the effects of naturally occurring opioids on food intake and choice.

   There are receptors for naturally occurring opioids in the body - opioids produce a feeling of well-being by binding to these sites. There are other chemicals - opioid antagonists - that bind to these same sites and thereby prevent the opioid effect. In these experiments, dunnarts were injected with the opioid antagonist naloxone.

   Under normal circumstances dunnarts preferred high fat mealworms to standard laboratory diet. However, after naloxone injection food intake generally decreased, especially the intake of the high fat mealworms. So, naturally occurring opioids (and the rewarding feeling they produce) made eating, especially high fat food, less pleasurable. However, the effect of the opioid antagonists naloxone and naltrexone have already been investigated in humans so there is no need for these dunnart studies.

3. Examine metabolic changes in dunnarts as a result of temperature change.

   The animals were kept in perspex tubes at either 28degC or 15DegC. At the lower temperature, body temperature, oxygen consumption (activity) and tail width all decreased. The dunnarts did not increase food consumption or use body brown fat stores to maintain a stable body temperature, a point on which they are obviously different to humans.

4. Examine the effects of a high fat diet and the protein leptin (involved in regulating food intake and energy expenditure).

   Dunnarts were fed a high fat (mealworm) or normal laboratory diet, and not surprisingly those on the high fat diet gained weight. Half the animals were then given only laboratory diet, while the other half had both lab diet and mealworms to choose from. Half in each group were injected with leptin twice daily. Leptin reduced body weight and food intake only with the normal lab diet, not when dunnarts could eat high fat mealworms.

   The researchers concluded: "... from a therapeutic standpoint leptin is most likely to be effective for treating human obesity when combined with a low-fat, low-calorie diet (or modified protein diet) ". The importance of a low-fat, low-calorie diet for human weight reduction is already known. In addition, many studies of leptin in humans have already be done, and it is only in such studies that the effect of leptin in our species can be validly assessed.

It is very difficult to see what new information these studies have contributed to the understanding and treatment of human obesity, but they will no doubt continue to be approved by the Ethics Committee for many years to come.

References