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The VMH Rat

A classic illustration in many physiological psychology textbooks shows an enormously fat rat sitting on a postage scale. The rat (from the laboratory of Neal Miller) had a portion of its brain removed: the ventromedial (front-middle) part of the hypothalamus, an important brain structure. The rat with a ventromedial hypothalamus lesion is called a VMH rat.

The VMH rat

After surgery, the VMH rat with constant access to tasty food will eat until it is twice its normal weight. For years, scientists assumed the VMH rat had a defect in fat cell regulation: a high set-point.

What causes VMH obesity according to Duggan & Booth?

However, later research showed that VMH rats do not show the active defense of high fat levels implied by the concept of a set-point. For example, the VMH rat does not conserve energy when its weight drops. Moreover, the VMH rat loses weight if given unpalatable food. By contrast, a rat with an altered set-point defends its new weight even if this means eating unpleasant-tasting food. So apparently the VMH rat does not suffer from a high set-point. It suffers from overeating caused by uninhibited consumption of tasty food.

The real underlying problem of the VMH rat turned out to be rapid daytime gastric emptying according to Duggan & Booth (1986). Food does not linger in the stomach of the VMH rat long enough to trigger feelings of fullness. Therefore the rat continues to eat as long as it is provided with palatable food. If this research is correct, the rat should lose weight if provided with a rat-sized gastric bubble (or stomach-reduction surgery). It has a satiety defect.

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