New research from Harvard University found some surprising results about fat and weight gain.

Gone are the days when fat was verboten (thank the avocado gods!) but we also know that not all varieties are created equal. Some, namely polyunsaturated and monounsaturated fats, are more heart-healthy than others (that would be you, saturated and trans fats). But how does this apply to the age-old question: Does eating fat make you fat? In a first-of-its-kind study, researchers found that certain types of fat are more likely to encourage weight gain-even though all fats have the same calorie count (9 calories per gram).

Frank Hu, M.D., chair of the department of nutrition at the Harvard T.H. Chan School of Public Health, and his team examined 20-plus years of data from more than 120,000 women and men, looking specifically at how changes in the types of fat that people ate affected their weight. They found that when people increased saturated and trans fats in their diets, they were more likely to put on pounds, while upping intake of unsaturated fats had no such connection-and was even linked to weight loss.

More: 8 Best Foods to Eat for Weight Loss

One explanation Hu offers is that different types of fats have different metabolic effects in the body. Saturated and trans fats seem to trigger insulin resistance, meaning that your cells don't absorb glucose like they're supposed to. As a result, your body increases its output of insulin, a hormone that promotes fat accumulation.

As for unsaturated fats, that's where it gets interesting. In general, adding more monounsaturated fatty acids (MUFAs) caused a slight increase in weight. However, when the researchers teased out animal sources of MUFAs (like red meat and dairy) versus those from plants (nuts and olive oil), only the animal sources increased weight, while plant sources helped keep the scale steady. That's likely because MUFAs and saturated fats are usually a package deal in animal foods, so you get both good and not-so-good fats.

Meanwhile, foods- like tuna, salmon, walnuts and sunflower seeds­-that are rich in polyunsaturated fatty acids (PUFAs) may prevent weight gain, or even help it go down. How? PUFAs appear to promote insulin sensitivity-a good thing for helping the body use food for energy instead of storing it as fat.

Related: Top Sources of Omega-3s

Bottom Line

Weight gain from eating more trans and saturated fats averaged about 3/4 pound a year in the study. This scale creep may not sound like a big deal, but it is enough to increase the risk of diabetes, heart disease and cancer. Hu recommends replacing 5 percent of your saturated fat intake with equal amounts of MUFAs and PUFAs. On a 2,000-calorie diet, if you're eating 22 grams of saturated fat, you'd just need to cut back by about 1 grams. That's like swapping a slice of crumbled bacon on your salad with a generous teaspoon of sliced almonds.

And don't confuse shifting fat with slashing fat. "When people reduce the overall fat in their diets they tend to replace it with refined carbohydrates and added sugar, which may be worse for your health." says Hu. As long as you focus on quality, there's no need to fear fat.

Get started: How to Cut 31 Grams of Saturated Fat from Your Diet

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Watch: How to Eat to Reduce Saturated Fat in Your Diet

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