The holiday period spanning Thanksgiving, Christmas, and New Year's Eve is a special time in the United States when we gather our loved ones and celebrate abundance, community, and renewal. Yet a growing body of evidence suggests that this six-week period has a concerning side effect: It accounts for most of our country's weight problem. Understanding this fact, and why it happens, gives us powerful insights into why we gain weight—and how to prevent it.

It's a fact: We eat too much during the holidays.

In a recent study, scientists set out to answer a simple question: How does a person's body weight change over the course of the year? To find out, they used internet-connected scales to collect daily body weight data from nearly 3,000 volunteers in the United States, Germany, and Japan. After crunching the data, a striking pattern emerged: No matter where you live, or what you celebrate, holidays are likely your period of greatest weight gain.

In the United States, our ascent begins at Thanksgiving when we enjoy our iconic feast and well-meaning relatives inundate us with delectable treats. Yet we don't reach our peak weight until we've danced with the sugar-plum fairies of Christmas and the New Year. After the New Year, many of us dutifully step into our jogging shoes and try to work off the extra flab, but on average we lose less than half of it. Each holiday season, our weight ratchets up a little bit, gradually bringing us closer to obesity. More than half of our annual weight gain comes from just 12 percent of the year.

In Germany, the situation is similar, minus Thanksgiving. Most annual weight gain occurs around Christmas and the New Year, with a smaller bump around Easter. Could it be that our bodies simply want to gain weight in winter, perhaps as a way of guarding against the winter famines our ancestors faced? Maybe our weight gain has nothing to do with the holidays after all? The data from Japan put this idea to rest. In Japan, the largest weight gain doesn't occur in winter but in spring. This gain coincides with the Golden Week, a holiday period in April and May during which Japanese people celebrate nature, children, and their national constitution. Our conclusion is inescapable: Holidays themselves are making us fat.

The brain's role in cravings

Superficially, the explanation for this is no great mystery: During the holidays, we simply eat and drink too much, and some of those excess calories get shunted to our love handles. But to really understand why this happens and empower ourselves to fight back, we need to dig deeper. We need to take a detour to the brain, the organ that generates all behaviors, including eating.

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Craving trigger no. 1: food composition

The human brain is the product of millions of years of survival in the face of scarcity, and it has a number of hard-wired tricks up its sleeve that helped us stay alive in the world of our distant ancestors. One of these is an important function called reward, which mostly whirrs away below our conscious awareness. Here's how it works in a nutshell. Every human brain is wired with specific motivations that drive us to seek the things that are good for us, including physical comfort, sex, social interaction, water, and of course, food. But not just any kind of food: The brain has no instinctive love for celery sticks or Brussels sprouts; it wants rich sources of fat, starch, sugar, and protein that would have met the rigorous demands of ancient life, and the more concentrated, the better. In everyday experience, we feel cravings as we smell a glazed ham emerging from the oven or see a slice of pecan pie obscured by a generous dollop of whipped cream. This craving, along with the enjoyment we feel as we eat delicious foods, is the conscious manifestation of reward.

Craving trigger no. 2: food proximity

But reward isn't solely triggered by a food's composition. The human brain is fundamentally opportunistic and always on the lookout for a good deal. In other words, it wants to get you the most benefit for the least sacrifice (for more on this effect, see Brian Wansink's book Mindless Eating). Food within arm's reach fires up your motivation to eat, even if you've already eaten more calories than you need. By the time our brains throw the emergency brake to limit the metabolic damage of a feast like Christmas dinner, we've already eaten far too much.

Craving trigger no. 3: sensory-specific satiety

The story doesn't end there. Another textbook brain function, called habituation, also chips in to drive up our food intake. Habituation is one of the oldest and most basic functions of the nervous system, and it simply makes us respond less and less to a specific stimulus the more we encounter it. For example, if you see a new billboard on your way to work, you might glance at it, but after a week it becomes part of the background and you stop paying attention. Food works the same way, thanks to a property called sensory-specific satiety. What this means is that your feeling of fullness (satiety) is specific to the particular foods you ate. You may be completely sated by what you've already eaten but still be ready and willing to eat something that tastes different. So while your "first stomach" may be full of ham and sweet potatoes, your "second stomach" still has room for apple pie, vanilla ice cream, and maybe a Christmas cookie or three. That's because your second stomach isn't in your abdomen—it's in your brain. If your only option were more ham and sweet potatoes rather than pie, you probably wouldn't eat them.

In practice, what this means is that we eat more total food as the variety in a meal increases. Some people call this the "buffet effect," and chances are, your personal experience confirms that this name is apt.

Craving trigger no. 4: the satiety system

As if that weren't enough, another brain system jumps into the holiday fray to help us pack on the pounds: the satiety system. Located in the brain stem where your brain meets your spinal cord, the satiety system receives information from your digestive tract about the food you've eaten at a meal and uses it to determine your feeling of fullness. Although fullness may feel like a simple physical sensation in your stomach, it's actually your brain's interpretation of complex food-related signals coming from several parts of your gut.

As sophisticated as this system is, it's surprisingly inaccurate in one key way: the amount of fullness you feel doesn't correspond very closely to the number of calories you've eaten. In particular, the more calorie-dense the food—in other words, the less water and fiber it contains—the less fullness you get out of it, per calorie. So a bowl of oatmeal that's bulky with water and fiber will deliver much more fullness than the same number of calories of white flour crackers, meaning that you may need to eat twice as many calories of crackers to feel full. Since Christmas dinner and Aunt Sally's famous egg nog are especially calorie-dense, we tend to eat more calories than usual before we reach that feeling of fullness that tells us to stop eating.

Together, the instinctive functions of your brain conspire to parlay the abundance of the holidays into an extra layer of body fat, as insurance against a future scarcity that never arrives.

This might seem grim, but understanding the problem empowers us to eat better this year. If more than half of our annual weight gain comes from only 12 percent of the year in the United States, then just a little vigilance will go a long way toward keeping us feeling good. And if we know why we eat too much during that period, we can do something about it. Here are four tips that will help you work with your brain's instincts to make this your healthiest holiday ever.

1. Be mindful of your food environment.

If the reward trigger—such as the sight or smell of cookies—isn't present in your immediate surroundings, it won't make you crave, and you won't have to fight yourself to avoid eating it. The most effective way to do this is to purge your home and work environments of tempting foods that don't support a healthy weight. If a friend brings over a pie, say thanks, eat a small piece, and find another home for the rest.

2. Create small effort barriers to eating.

Small effort barriers, like having to open a cupboard, unscrew a jar, peel an orange, or shell nuts, make food a less attractive "deal" to the brain and help reduce your intake of unnecessary calories. If you have to work a little bit to snack, chances are you'll only do it when you're genuinely hungry.

3. Simplify.

The fewer types of food that are available at a meal, the fewer calories you'll eat overall. Consider cutting out appetizers, or only offering one option and keeping it light. Rather than providing the riot of dishes that normally defines holiday feasts, pick a few traditional items and stick with them. Instead of serving three different desserts, choose only one.

4. Lighten up.

Serve lighter versions of your favorite dishes, snacks, desserts, and drinks. For foods that you prepare, the easiest way to do this is to use less fat, sugar, and white flour in your cooking and stick with ingredients that are minimally processed. For foods that you buy already prepared, look for lighter options that you still like. When it comes to alcoholic drinks, which are inherently high in calories, wine is the best option because it delivers fewer calories per serving than beer or cocktails. And you might want to lay off the egg nog.

If you follow these four simple tips, chances are you and your family will enjoy the holidays as much as ever, without the long-term consequences.


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