1. Refined grains

Not sure what a refined grain is? It's an ingredient found in foods like white rice, white bread, and regular white pasta. The unrefined stuff (whole wheat, brown rice and quinoa) is always healthier. Pennsylvania State University researchers found that people who ate whole grains in addition to keeping a healthy diet—of fruits, vegetables, low-fat dairy and protein—lost more weight from the abdominal area the group of people who kept the same healthy diet but ate all refined grains.

2. Potato products

A study published in the New England Journal of Medicine followed the weight changes of more than 120,000 men and women for up to 20 years. The participants were checked every four years and, on average, they gained 3.35 pounds each time—so almost 17 pounds by the time the study was finished. The foods associated with the greatest weight gain? You guessed it—potato chips and potatoes.

3. Red and processed meat

The same 20-year study found that people who ate more red and processed meat gained weight, too—about one extra pound every four years. In another study, published in the American Journal of Clinical Nutrition, researchers worked with more than 370,000 people and found that folks who ate the equivalent of a small steak a day gained about five pounds in five years.

4. Frosting

Those cupcakes your coworker makes for special occasions? Yeah, don't eat them. While the FDA has basically declared war on trans fats, store-bought frosting still contains a not-so-healthy dose of the stuff. How bad can trans fats be? Researchers at Wake Forest University gave groups of monkeys two different diets; one group ate trans fats and the other ate unsaturated fats. 

The results: The group eating trans fats upped their body weight by 7.2 percent in six years and the other only gained 1.8 percent. Not only did the trans fats add new fat, it was also responsible for moving fat from other areas to the belly. Check for trans fat in other foods like pre-made baked goods, snack foods, and frozen pizzas.

5. Diet soda

It's easy to get fooled by the zero-calorie label, yet sodas made with sugar substitutes are believed by many to play a role in weight gain. A new study published this month found that people who drank diet soda gained almost three times the amount of abdominal fat over nine years as those who didn't drink the no-cal stuff. 

Sure, that study only looked at adults ages 65 and older, but consider this: Recent research from the Weizmann Institute of Science found that mice drinking water with artificial sweeteners (saccharin, aspartame and sucralose) became vulnerable to insulin resistance and glucose intolerance—two things known to lead to weight gain.