The Diet Killer

Your relationship may be hot, but it’s making you heavy. New research confirms moving in together often leads to weight gain.

We've all seen it: You or someone you know gets married, 10 years go by, and suddenly the communal bed is straining under an extra 40 pounds.

Needless to say, nobody deliberately sets out to gain weight when they move in with their partner. But the belief that moving in makes you fat is such conventional wisdom that researchers decided to take a closer look. The results of a new national study by a group of scientists at the School of Public Health at the University of North Carolina recently confirmed that young adults who enter a long-term live-in relationship—and especially those who tie the knot—gain weight and significantly increase their risk of a clinical diagnosis of obesity compared to those who remain uncommitted.

While men are extremely vulnerable to weight gain in the first 12 months of married life, they do better over time, whereas women just keep getting fatter and fatter.

Perhaps not surprisingly, the prognosis is worse for women. Both men and women increase their risk of becoming obese when they move in together, but women seem to be even more susceptible than men. While men, according to the new research, are extremely vulnerable to weight gain in the first 12 months of married life, they do better over time, whereas women just keep getting fatter and fatter. Women (especially short ones, and I'll write about height another time) simply don't have it easy when it comes to weight control.

Why did it happen? This kind of research doesn't explain the causes, but likely candidates include decreased care over weight control (after all, you are now hitched, not looking for someone), more dining out and relaxing over big meals, and trying to fit two different ways of eating into one kitchen.

So where does this leave us? Ultimately, the Obama administration should fully embrace weight control as a societal issue, and realize it is not something that can be left to individual responsibility. There are so many useful things the government could do that would make it easier, from subsidizing the right types of farming to reining in the food industry and funding health-maintenance programs at worksites.

Until that happens, here are some effective ways to help your own situation and stay svelte no matter your partner's habits and proclivities:

1. If you are the more health conscious one, take charge of the food. Most people at high risk of weight gain love food, but know their weakness and are happy to get help. To the extent you can take charge of shopping and cooking, you can create an environment at home that helps—rather than hinders—weight control for both of you.

2. Monitor the situation and be proactive. Nobody wants to gain weight, but it can happen slowly and subtly, and it will be much harder to get rid of 20 pounds than it would have been to shed five. So buy a good scale if you don't already have one, and weigh yourself weekly. That way, if weight starts to creep on, you can catch it early and put yourself on a diet that will get you back to your baseline. Better still, weigh in weekly with your partner, so you can stay slim together.

3. Accept that your partner's environment is now yours. It's an established fact that most of us eat just because food is there (our "food availability" instinct), and we do so in large part because food in front of us triggers real metabolic signals of hunger for things that are bad for weight control. Keeping a nutritionally healthy pantry will not be easy if your partner brings home every tempting thing, so negotiations may be needed, starting with the principle that both of your food preferences and vulnerabilities should be taken into account. And for those (hopefully few) things that he feels he has to bring in that you can't leave alone, a good hiding place can put them out of sight and out of mind.

4. Keep plenty of hunger-defying foods in the house. Casual eating never kept anyone thin, so if one of you is hunger-prone and has a tendency to gain weight, find ways to stock up on and eat more high-satiety foods. They will keep you satisfied with fewer calories, and you'll be less tempted by large portions and the fattening foods that cross your path. And since weight gain is often rooted in misunderstanding about what works and what doesn't, help your spouse understand what foods can be helpful for weight and are delicious eating in the bargain.

5. Cut down on high-calorie food variety. Every research study done on food variety shows that the more there is, the more we eat. Unfortunately, your foods plus your partner's foods can often add up to an increase in variety that encourages weight gain. The solution, of course, is horse-trading. You get to keep some (but not all) of the temptations you love, and so does your loved one.

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6. Cook in more, eat out less. This wonderful concept, promoted by celebrity chef Jamie Oliver, is not only fun and romantic, but a great way to save money and prevent weight gain. Most restaurants do a terrible job of making lower-calorie food taste good, but with the help of a good, healthy cookbook or two you can make delicious, satisfying, no-weight-gain meals and have leftovers for tomorrow. No experience cooking? No problem. Cooking isn't rocket science. Roll up your sleeves and accept that there will be mistakes along with successes, or get started with a cooking course and do it with your partner.

7. Be picky. No, you don't always have to eat the same stuff. In fact, it's good not to, because your temptations are often different from his and when you eat your favorites plus his, you eat more. And because an increasing number of research studies emphasize the importance of breakfast for weight control, your first meal of the day is a great time go your own way; you can eat the things that you enjoy and that keep you hunger-free until lunchtime, and your partner can eat something else.

8. Exercise enough. Research indicates that exercise is a low-efficacy way to actually lose weight compared to improving what you eat, but working out is great for preventing weight gain in the first place, and a fine way to enjoy time together.

Plus: Check out Hungry Beast, for more news on the latest restaurants, hot chefs, and tasty recipes.

Dr. Susan Roberts is director of the Energy Metabolism Lab at the USDA Human Nutrition Research Center and professor of Nutrition and Psychiatry at Tufts University, and author of a new approach to weight loss called The Instinct Diet, which was recently featured in The Daily Beast.