Friends and couples often gain weight together, but the opposite is also true: When one member of a couple embarks on a weight loss program, the other partner can lose significant weight as well. A recent study looked at this “ripple effect” in weight loss.

The finding is good news, and it suggests another way to address the epidemic of overweight and obesity.

The most impressive findings showed up in the members of a couple who did not take part in either the Weight Watchers or self-guided diet plan. They lost weight as well.

Just as kids who have active friends are more likely to become active, couples often show parallel courses in their weight loss and weight gain. One study found that when one partner becomes obese there is a 37 percent increase in the likelihood that the other member of the couple will develop obesity as well.

Researchers recruited couples living near Hartford, Connecticut. Participants were over 25 years old, and 93 percent were married. All were overweight: Each person had a Body Mass Index, or BMI, of between 27 and 40. A BMI is 18.5 to 24.9 is considered normal weight; a BMI of 25 to 29.9 is overweight; and a BMI of 30 or higher is considered obese.

The 130 couples were divided into two groups. One group of 65 couples was enrolled in a six-month Weight Watchers program. The other group of dyads was assigned to a self-guided program and received a four-page handout on weight loss tips. The self-guided group did not receive any ongoing interventions or counseling. Partners' weights were measured at the start of the study, at three months and at six months.

One person in each dyad — whether they were enrolled in the Weight Watchers or the self-guided group — was not enrolled in any weight loss management program. Sixty-eight percent of those not enrolled in a program were male; 66 percent were obese.

People enrolled in the Weight Watchers program lost more weight at three months than those in the self-guided group, but by six months there was no difference in the amount of weight lost between the two groups.

By far the most impressive finding was that members of a couple who did not take part in either the Weight Watchers or self-guided diet plan lost weight too. This group of untreated spouses or partners lost an average of three pounds at three months and 4.5 pounds at six months, regardless of whether their partner was enrolled in the Weight Watchers or self-guided diet plan. By six months, almost a third of the non-dieting people in both groups had lost at least three percent of their initial body weight, the amount that, according to healthcare providers, must be lost in order to achieve measurable health benefits.

The study highlights the community nature of weight control. As the authors write, “Weight and weight change within married couples is highly interdependent.” With the ripple effect bringing a positive result for the non-enrolled member, having one partner in an overweight couple in a weight loss program may be cost effective, essentially treating two people for the price of one. The findings also suggest that couples may be able to successfully lose weight even without a highly structured program such as Weight Watchers.

It is not clear how the ripple effect is achieved, and more research is needed to understand and maximize its impact. Changes in the amount and kind of food available in the home, the kind of relationship a couple has and dietary changes initiated voluntarily by untreated family members are all possibilities.

The study is published in Obesity.