Research in recent years has made it clear that losing weight and getting healthy isn't something that happens in a vacuum. One study that garnered numerous headlines several years back found that a person's chance of becoming obese increases by 57 percent if a close friend is obese, 40 percent if a sibling is obese, and 37 percent is their spouse is obese. That's some hefty (pun intended) pressure on your social circles.

But Harvard professors Walter Willett, MD, and Malissa Wood, MD, have taken the research several steps further. Their new book Thinfluence examines how friends, family, colleagues, online communities and the environment exert influence over your health behaviors -- and how you can make them work in your favor. Here, Dr. Wood talks about what it takes to stay on track.

For most people, it's whoever you spend the most time with. And that often ends up being your co-workers. You might spend more time with them than you do your family and eat more meals at work than you do at home.

The influences can be very powerful. If you work with a group of people who like to go out and eat unhealthy food every day for lunch or always order in pizza when you're working late, those decisions will shape your behavior. But, for example, I'm lucky enough to work with several women who all decided to make some efforts to get healthier by eating better and exercising more. I spend all day with these people, so that has had a very positive effect.

It can be important to tap into resources that aren't in your backyard -- whether it's joining a Meetup group to go walking, tracking and sharing workouts on a site like Fitbit or just linking up with other people who are trying to achieve the same goals. We talk in the book about 'choosing your champion,' the person who you look to most to help you succeed. If you don't have that person in your physical circle, connect with him or her online. That support will be crucial to your success.

If the people around you aren't up for making the changes you're making, you might need to spend a little less time with them during the initial phase. I liken it to a plane flight: Takeoff is like when you’re first starting to make healthy changes, and it's bound to be a bumpier ride. During that phase, you’ll probably want to avoid things like going out to eat with the people who aren't supporting your changes. But once you hit cruising altitude, you'll be better able to go out with them -- and let them order the burger and fries while you have a salad."

Sally Wadyka is a Boulder, Colorado-based journalist who writes about nutrition, health and wellness.