It’s not only the foods you eat that determine whether you gain or lose weight, but how you combine them, new research from one of the nation’s top nutritional institutions shows.

"Counting calories is not the most effective strategy for long-term weight management and prevention,” says Dariush Mozaffarian, M.D., head of the Friedman School of Nutrition Science & Policy at Tufts University.

“Some foods help prevent weight gain; others make it worse. Most interestingly, the combination of foods seems to make a big difference.”

Dr. Mozaffarian is the lead author of a major new study in which his team discovered that it isn’t only the foods you eat, but what you pair them with that can make the difference between long-term weight loss or gain.

The study looked at food choices as ranked by glycemic index, a number correlated to how much a food raises blood sugar levels.

Foods with a high glycemic index are generally breads with refined flours, potatoes, and sugary foods. Foods with a low glycemic index include vegetables, some types of beans, and some fruits with low sugar content, such as grapefruit.

“Our findings suggest we should not only emphasize specific protein-rich foods like fish, nuts, and yogurt to prevent weight gain, but also focus on avoiding refined grains, starches, and sugars in order to maximize the benefits of these healthful protein-rich foods, create new benefits for other foods like eggs and cheese, and reduce the weight gain associated with meats,” said Dr. Mozaffarian.

The study was based on more than 16 years of follow-up among 120,000 men and women from three long-term studies of U.S. health professionals.

Here are three weight-loss lessons that emerged from the study:

1. Pair protein with low-glycemic side dishes.
When eating a protein-rich food like meat, eggs, or cheese, choose a low-glycemic food to go with them – like vegetables, instead of white bread or potatoes. Protein, when paired with high-glycemic foods, leads to weight gain over time.

2. Add these foods to your diet: yogurt, seafood, skinless chicken, and nuts.
They are strongly associated with weight loss. However, eating them in combination with high-glycemic foods cancels their beneficial weight loss effects.

3. Don’t shy away from high-fat dairy.
It seems counterintuitive, but when choosing dairy foods, like cheese or milk, opt for higher-fat versions. Study participants who ate low-fat dairy actually tended to consume more carbohydrates, which lead to weight gain over time. This may have been because the lower-fat products left them less satisfied, the researchers theorized.