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When it comes to dietary fats, 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).
What the Study Found
Frank Hu, M.D., chair of the department of nutrition at the Harvard T.H. Chan School of Public Health, and his teamexamined 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 as they aged. They found that when people increased saturated and trans fats in their diets, they were more likely to gain weight, while upping intake of unsaturated fats had no such connection—and was even linked to weight loss.
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One explanation Hu offers is that different types of fats have different metabolic effects on the body. Saturated and trans fats seem to trigger insulin resistance, meaning 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 healthy and not-so-healthy 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.
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The Bottom Line
Be careful about what you add back when you reduce your fat intake. “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. The fat in food helps you to feel satisfied and provides the essential fatty acids your body needs. Enjoy more PUFAs and MUFAs from plant foods as part of a balanced eating plan.
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