ARI SHAPIRO, host:

If you overindulged during the holiday season and you're having a hard time cutting back now, turns out there's a biological reason for that. NPR's Patti Neighmond has the story.

PATTI NEIGHMOND: Dr. Rita Redberg says she sees it all the time, those January patients lugging around an extra five pounds or even more. Holidays can do that to you, says the University of California at San Francisco cardiologist.

Dr. RITA REDBERG (Cardiologist, University of California, San Francisco): I see it on the scales, because we'll have a very earnest talk in October about weight loss and then the visit after the holidays is generally a weight gain.

NEIGHMOND: And year after year...

Dr. REDBERG: People get discouraged when they realize they're now up 50 pounds. You know, it started as five pounds, and then, you know, 10, 20, and those pounds just start piling up quickly. And I see that commonly in my patients.

NEIGHMOND: When you overeat, biological changes in your body can set you up for a lifetime of overeating. Take the biological clock, an innate mechanism that tells you when to sleep and when to eat. Dr. Joe Bass, an endocrinologist and molecular biologist at Northwestern University, studies the body clock in mice. And he says when mice are overfed, their body clock changes, and not for the better.

Dr. JOE BASS (Endocrinologist & Molecular Biologist, Northwestern University): So mice, for instance, if they eat a high-fat diet they actually wake up during what is nighttime for them and eat. It would be as if you were waking up every night and eating all the sweets in your refrigerator.

NEIGHMOND: All this suggests that people who eat less fat will sleep better and not engage in nighttime bingeing. And the body clock's just one of a number of changes that occurs when we overindulge.

Dr. SASHA STILES (Obesity Specialist, Tufts Medical Center): It sets your body chemistry sort of into red alert.

NEIGHMOND: Dr. Sasha Stiles is a family physician who specializes in obesity at Tufts Medical Center.

Dr. STILES: An inordinate amount of what you eat will dump into fat storage, where it will remain as fat in your body.

NEIGHMOND: And all that food can trigger an unfortunate cycle. The pancreas produces extra insulin to process the sugar load and remove it from the bloodstream. It doesn't stop producing insulin until the brain senses that blood sugar levels are safe. But often too much sugar is removed.

Dr. STILES: So the most common thing that people will chuckle about is that, well, I'll just get up and I'll eat some more chocolate or some, you know, pancakes with syrup, or something, or some more doughnuts and carbohydrates to bring their blood sugar back up to normal.

NEIGHMOND: And if you drink lots of icy beverages with your food, it only gets worse.

Dr. STILES: When you drink cold liquids, your stomach will start contracting. And what that means is that it will then contract and it will massage the food that will again quickly leave your stomach to the rest of your GI tract. Your stomach will be empty.

NEIGHMOND: And you'll be hungrier sooner. And if you consistently overeat, your stomach changes too. The neurological tissue at the top of the stomach, which signals the brain when the stomach is full, starts to malfunction.

Dr. STILES: When you overeat time and time again, this electrical conduit pathway gets tired and it doesn't tell your brain that you're full anymore.

NEIGHMOND: So here's some food for thought. The next time you're tempted, just think about all those biological body cycles, about your sleep cycle, your metabolism, and how easily they can get disrupted by that extra piece of cake or bunch of cookies or juicy cheeseburger. Patti Neighmond, NPR News.

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SHAPIRO: For more on the biology of eating, check out what experts have to say about fasting at npr.org.

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