After following about 121,000 men and women for 20 years, researchers at Harvard University published a study in the New England Journal of Medicine in 2011 that documented the foods and drinks most and least associated with gaining weight. Read on to learn which items should fill dieters' shopping carts--and which should always be avoided.
Walnuts contain omega-3 fatty acids, which protect your heart and boost your brainpower.
The monounsaturated fats and vitamin E in almonds work together to cut cholesterol.
Best: Macadamia Nuts
In addition to containing healthy fats like other nuts, macadamias also pack in an extra dose of filling fiber.
Yogurt is a good source of calcium, plus probiotics for gut health. Go for the Greek variety: It boasts all of the benefits of regular yogurt plus double the protein.
All fruits are great for diets thanks to their water content and key antioxidants and vitamins. Blueberries, however, are a true standout--studies show they may help rev metabolism and lower blood pressure.
Best: Whole Grains
Whole grains are packed with fiber, iron, and vitamin B.
Best: Leafy Greens
All vegetables are low in calories, chockfull of vitamins and nutrients, and high in fiber. Leafy greens like spinach, kale, and collards are nutritional powerhouses. They have close to zero calories per serving and give you a dose of bone-building calcium.
Worst: French Fries
Potatoes aren't great for you to begin with; adding salt and deep-frying them strips them of any value they may have had. A typical medium-size portion at a fast-food restaurant contains 410 calories and 18 grams of fat.
Worst: Red Meats
Though red meat can be healthy in moderation, the correlation between red meat consumption and weight gain in the Harvard study was undeniable. If you choose to indulge, avoid fatty cuts, such as T-bone, New York strip, and rib-eye steaks.
Worst: Processed Meats
Most contain nitrates and are very high in saturated fat.
Potatoes are often fried or covered with butter, sour cream, or sugary ketchup. People rarely eat plain potatoes.
Worst: Refined Grains
Refined grains have been stripped of nutrient-rich bran and germ.
You may be surprised to learn that in addition to waist-widening sugar and calories, soda contains flame retardants and may fatten up organs.
Worst: Fruit Juice
Something with "fruit" in the name may lead you to believe it's healthy, but most fruit juices are far from it, with just as much sugar as soda. Here are the unhealthiest juices in America.