The definitive answer as to why you ate that second slice of yule log.

Welcome back. If you're reading this, it means you probably just emerged from your holiday meal-induced sugar coma and you're ready to leap back into the world of fitness and nutrition.

I don't want to kick a dog while he's down, but today I've got a cool new study that explains why we (yes, even the Fitness Nerd porks out on occasion) over-indulged during the month of December. According to researchers at the UT Southwestern Medical Center, the culprit is a "hunger hormone" called ghrelin.

"What we show is that there may be situations where we are driven to seek out and eat very rewarding foods, even if we're full, for no other reason than our brain tells us to," said Dr. Jeffrey Zigman, assistant professor of internal medicine and psychiatry at UT Southwestern and co-senior author of the study appearing online and in a future edition of Biological Psychiatry.

Scientists previously have linked increased levels of ghrelin to intensifying the rewarding or pleasurable feelings one gets from cocaine or alcohol. Dr. Zigman said his team speculated that ghrelin might also increase specific rewarding aspects of eating.

"(Rewards) give us sensory pleasure, and they motivate us to work to obtain them," he said. "They also help us reorganize our memory so that we remember how to get them."

They came to this conclusion after a weird series of tests using mice and high-fat food rooms that you can read about through the link. Frankly, I'm not even vaguely surprised. We haven't evolved that much from our caveman days -- we're still the same old homo sapiens. Back then, body fat was considered a good thing, given you never knew when your next meal would come from. With this in mind, it stands to reason that when Ugg had access to food, especially highly caloric food, his body would do everything it could to coax him into eating as much as possible, as to store more fat.

But here's the deal. Back then, if Ugg felt threatened by another person, he'd generally bash their brains in with a rock. With the exception of UFC, divorce court and road rage, we've gotten past that bit o' instinct, so it shouldn't be that tough battling our little ghrelin gremlins as well.

Of course, when I started espousing this theory over Christmas dinner, my family didn't take to it, especially when I symbolically bashed the pumpkin pie to bits with a rock, shouting "Ugg no like ghrelin!"

Some people have no appreciation of the high road.