Be it a hyper-critical parent or that constantly complaining friend who sucks you dry, we all know how exhausting a negative person can be. They somehow find a way to be a fun-sponge even in the happiest and most pleasant situations, and their sky seems to be a perpetually gloomy gray.What you may not know, however, is how much that Negative Nancy may be sabotaging your own health and wellbeing—and even making it harder to button your favorite pair of skinny jeans. According to a new study published in the American Journal of Public Health, exposure to negative relationships can lead to more than a 10 percent increase in your body mass index (BMI) and waist circumference—even when the researchers adjusted for age, smoking, diet and physical activity levels.QUIZ: What’s Your BMI?So what gives? It’s all about the feelings and behaviors a negative person triggers in you.”Negative relationships can affect our health for so many reasons, not the least of which is they can increase our stress levels and affect how we feel about ourselves,” says YouBeauty Wellness Advisor Beth Ricanati, M.D. “This can have a spiral effect: If we don’t feel as good about ourselves, we may make choices that affect our health negatively. You may reach for an unhealthy snack or blow off a workout.”MORE: How to Identify and Ditch Your “Frenemies”The best course of action, according to Dr. Ricanati, is to spend time with people who make you feel good instead. That may sound like a no-brainer, but it’s easy to get stuck in bad relationships, especially when it’s someone you are close to or have been friends with for years. In cases where you can’t realistically escape the person—say, a critical parent whom you still love—the way you respond to the person is important.”Stress management techniques, such as remembering to breathe, meditate and do yoga, can be extremely helpful in the moment and they have a long-term benefit,” she says.MORE: Meditate for Stress Relief“In addition, if a negative relationship is really causing havoc that you can’t get a handle on, sometimes talking to someone objective can help.”Whatever you do, just remember: Someone else can’t define how you feel about yourself and your world. And misery loves company, so try not to engage. Debbie Downer will soon move on to sulkier pastures.Another reason to be mindful of the company you keep? Bad habits can be contagious.