More than a third of adults in the U.S. are obese. And many of those already overweight continue to put on even more pounds. Now researchers have a clue why.

Two new studies in the Journal of Clinical Investigation suggest how obesity and high-fat diets might actually alter the way the brain works, changing in particular the hypothalamus. This almond-sized area of the brain helps regulate hunger and thirst, as well as sleep and body temperature. So if it’s out of whack, people can feel hungry even when they've consumed plenty.

One study found that in the brains of both obese humans and obese rats, neurons around the hypothalamus were damaged by inflammation. High-fat diets have been known to promote inflammation throughout the body, but that usually takes weeks or months to appear. Changes in the brain, however, can happen fast—even within 24 hours. [Joshua Thaler et al, Obesity is associated with hypothalamus injury in rodents and humans]

The second study found that mice on a fatty diet were slow to replace old neurons in the hypothalamus, which could also hamper its function. [David McNay et al., "Remodeling of the arcuate nucleus energy-balance circuit is inhibited in obese mice"]

So you might consider starting 2012 by watching the ball drop—and dropping the cheese ball.

—Katherine Harmon

[The above text is a transcript of this podcast.]

