When I was in college, every time I got a new roommate, my weight fluctuated. One roommate was a runner who was addicted to healthy eating. Naturally, I followed her habits and improved my own eating. But the next year, my roommate was a midnight-pizza-eating junk-food junkie. Although I knew her eating habits weren’t good, she obviously had an effect on me: I gained 10 pounds that year.

A Harvard study, conducted in 2007 by James Fowler and Nicholas Christakis, showed that our friends’ habits have a huge influence on our health. In fact, this study showed that friends have even more of an impact on our health than our spouses. That fact surprised me a little. In most cases, we don’t eat with or even see friends every day, but we usually cook or eat with our spouses daily. In order to understand why our friends have such an impact on our health, let’s take a closer look at this study.

The study found that if your friends are overweight, you are 57 percent more likely to be overweight yourself; but if your spouse is overweight, your chances of being overweight increase by only 37 percent. Here’s why: Friends of the same gender have more influence on us than a spouse of the opposite gender. This may be because we are more likely to compare our body image with that of our friends. In other words, if your friends weigh more than you, you’re less likely to feel pressure to watch what you eat. Why should you? You’re the “skinny one,” after all.

But, it’s not only our friends’ eating habits that influence us. The same Harvard study found that you are 36 percent more likely to smoke cigarettes if your friends also smoke. Similar results were found with drinking alcohol and being happy.

So, now that you know how much of an influence your friends can have on you, maybe it’s time to do something about it. Of course, I would never suggest that you drop a friend because she’s overweight or unhappy. But, maybe it’s time you start taking more of an interest in your friends’ health and happiness. While you’re working to build up the friends that you already have, think about making friends who put more of a priority on health than you currently do. This way, their influence will build you up, and it can have a domino effect on your current circle of friends.

With all of this in mind, it might be a good time to look at who your children are hanging out with. If you notice there is bad influence swaying the group, you may want to get involved.

This study reminds me of a great line I once heard about how other people influence our behavior: “People are like elevators, they either bring you up or down.” Try to be the one who brings other people up, and find others who can do the same for you.

Healthy Bites appears on MyHealthNewsDaily on Wednesdays. Deborah Herlax Enos is a certified nutritionist and a health coach and weight loss expert in the Seattle area with more than 20 years of experience. Read more tips on her blog, Health in a Hurry!