It’s that time of year again: The gym is packed with New Year’s resolutioners sweating off all their winter weight gain. But what if you were an angel all through the holidays and let barely a cookie pass your lips — yet still gained a few?

New research shows even the most stalwart dieters could blame their weight regain on something other than their willpower, or lack thereof: hormones. A study published in The New England Journal of Medicine found that hormones involved in appetite regulation could be the culprit.

Scientists put 50 overweight or obese people on a low-calorie diet for ten weeks, then measured key hormone levels before the diet, right after it and a little more than a year later. The results showed that a year after losing weight, the hormone ghrelin, which regulates hunger, actually increases. Meanwhile, leptin, the hormone that tells us to stop eating, decreases.

It all goes back to our ancient ancestors, say scientists. Changes in hormones are supposed to keep us from starving when food is scarce. But that’s hardly an issue in today’s society, where high-calorie food is just a microwave away and the most exercise many people get is the daily walk to and from their cars. Far gone are the days of hunting and gathering food by physical means, so it’s no wonder we gain — and regain — weight. Studies have shown that most people who lose weight gain it back within five years.

The key to keeping it off, though, say experts, is suppressing appetite safely. Drinking lots of water, eating fiber- and protein-rich foods like nuts and yogurt and filling up on nutrient-packed foods like eggs and avocados can keep the snack attacks at bay.

Experts also advise eating breakfast, stepping on the scale regularly, exercising at least one hour a day and following a low-fat diet. Remember what you worked for and keep the weight off!