Would eating a low-fat diet save you from depression and obesity? Not so fast—not all fats belong on the naughty list

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You've heard a lot of hype about how great high-fat diets are for you-they help a lot of your favorite celebs lose fat and stay full longer. But several recent studies have found that a high-fat diet not only causes you to overeat and gain weight, but it can also harm your arteries and even dampen your mood. So what gives?

"When you take a closer look at the studies, it becomes clear that the type of fats you eat matters," says Rebecca Blake, R.D., director of Clinical Nutrition at Mount Sinai Beth Israel Hospital in New York City. In most instances, researchers found the bummer consequences in diets packed with saturated fats-like greasy bacon, pizza, and ice cream. (Clean up your favorite recipes with the Top Substitutions for Fatty Ingredients.)

Let's start at the beginning: In the most recent study, published in Neuropsychopharmacology, rats that ate a diet packed with saturated fats for eight weeks became less sensitive to the neurotransmitter dopamine. "Dopamine is the brain's feel-good chemical and when production or uptake is low, it can contribute to depression," Blake says. "Many antidepressants are designed to help regulate levels of dopamine in the brain."

What's more, low levels of dopamine could lead to overeating. Researchers theorize that when levels are low, you don't reap as much pleasure or reward from eating as you're used to, so you may down even more high-fat foods to feel the level of pleasure that you'd expect.

However, these findings weren't true of all types of fat. Although all diets contained the same amount of sugar, protein, fat, and calories, the rats that consumed a diet high in monounsaturated fats (the kind found in fatty fish like salmon and mackerel, plant-based oils, walnuts, and avocado) didn't experience the same repercussions on their dopamine system as those that scarfed the saturated kinds.

Another recent study, presented at the annual meeting of the Society for the Study of Ingestive Behavior, found that feeding rats a high-fat diet affected the makeup of naturally-occurring bacteria in their gut. These changes lead to inflammation that damaged nerve cells that carry signals from the gut to the brain. As a result, the fuzzy signals dampened how the brain sensed fullness, which could lead to overeating and weight gain, researchers say. Once again, not all fats were to blame though-saturated fat appeared to be the inflammation-causing culprit.

Based on these findings, definitely don't nix fats entirely-even the main culprit in these studies, saturated fats, shouldn't be blacklisted, Blake says. "Healthy foods that contain saturated fats often contain other nutrients that your body needs, such as iron in steak or calcium in dairy," she says. Instead, Blake suggests focusing on upping your intake of healthy monounsaturated fats. After all, diets high in healthy fats like salmon, olive oil, and nuts have been shown to help trim body fat and may help improve athletic performance (find out the whole story in The Truth About the Low-Carb High-Fat Diet). Plus, a Low-Fat Diet Sabotages Weight Loss, and consuming some high-fat foods could even boost your mood-Ohio State researchers study found that people who upped their intake of fish oil, which is rich in omega-3 fatty acids, experienced a decrease in inflammation and anxiety.

Consuming more monounsaturated fats can alter the ratio of good to bad fats that you get in a beneficial way too. "Unfortunately, the proportion of healthy fats to unhealthy fats in the Western diet is very bad," says Krzysztof Czaja, Ph.D., associate professor of neuroanatomy at the University of Georgia and lead author of the first study mentioned. "We consume too much pro-inflammatory fats." Achieving a healthier balance, by eating more monounsaturated fats and fewer saturated fats could tip the scale in the opposite way.

"This doesn't mean you can never have pizza or steak again," Blake says. "But knowing which foods are on the 'good' fat list and what's on the 'bad' fat list can help you make decisions at each meal to eat more of the good fats so you can experience all of the benefits of having more of them in your diet."