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Dietary fat is one of the most confusing — and controversial — food topics around. And no wonder: Americans have been hearing bizarrely mixed messages about whether it's okay to eat fat for more than half a century.
In the 1950s and '60s, saturated fat — the stuff found in red meat and butter —began acquiring a bad reputation. Back then, researchers were finding that people with diets lower in saturated fat appeared to be healthier. Public health officials worried that eating too much saturated fat could lead to heart disease, a major killer in the United States.
This specific concern about saturated fats eventually mutated into a generalized panic about all types of fat. In the 1980s, the official US dietary guidelines began warning Americans to cut their total fat intake. This recommendation wasn't very scientifically grounded, since it didn't distinguish among types of fat (at the time, researchers were also finding that unsaturated fats, such as those found in vegetable oils and fish, had health benefits). But as Marion Nestle describes in Food Politics, the meat industry didn't want the government telling people to eat less red meat, a huge source of saturated fat. So the message became the vague "eat less fat, period."
It's not how much fat you eat that's important — it's what kinds. As best scientists can tell, trans fats (found in foods like margarine) contribute to cardiovascular diseases, whereas unsaturated fats (found in vegetable oils and fish) actually have the opposite effect — lowering the risk of cardiovascular disease. Saturated fats (found in butter and red meat) fall somewhere in between.
Some of the biggest controversies surround saturated fats. Scientists disagree about the extent to which saturated fats contribute to important health outcomes like heart disease, stroke, and cancer. The available research does suggest, however, that there are health benefits from replacing saturated fats with unsaturated fats in the diet, and that eating lots of nutrient-poor carbs (like sugary cereals, soda, and white bread) instead of fat is a bad idea.
Stay away from foods that are high in trans fats. And you're likely better off eating foods rich in unsaturated fat instead of saturated fat. But there doesn't seem to be any need to worry about your total fat intake. So as long as you're eating a variety of real foods and not too many calories, you should be okay.
Those warnings helped spur the "low-fat" diet craze of the past couple of decades. They also had a number of harmful unintended consequences. Food manufacturers began replacing the fat in their products with sugar — think Snackwell's cookies — and marketed them as healthy alternatives. This turned out to be a bad idea: Those sugars and refined carbohydrates were often just as bad for health. Worse still, food manufacturers and consumers started moving away from saturated fats and toward artificial trans fats — as seen in the switch from butter to margarine. This, too, was a disaster, since trans fats turned out to be very bad for the body.
Today the conventional wisdom is shifting yet again. Some critics now argue that saturated fat isn't actually that bad for you, and that we all made a terrible mistake switching to low-fat diets that were higher in sugar. In 2014, the former New York Times food writer Mark Bittman declared, "Butter is back," going through the research on how saturated fat wasn't nearly as harmful as we thought and arguing that we should ditch artificial foods (like margarine) in favor of natural foods (like, well, butter).
I decided to sift through the available evidence, interviewing eight researchers and reading more than 60 journal articles on the subject. What I learned is that there's still a ton of controversy about fat — although there is also clarifying consensus in important areas.
For starters, just about everyone agrees that the 1980s-era recommendations about switching to a low-fat diet were not supported by science. There isn't any high-quality evidence to back up that advice. In fact, researchers today generally don't think the total amount of fat you eat has much effect on obesity and heart health (so long as you're eating healthy foods and not consuming too many calories).
Instead, they focus on what types of fat we should eat. Not all fats are created equal. (More on that in the next section.) Artificial trans fats appear to be extremely harmful, which is why they're now being banned from foods. Unsaturated fats, like those found in vegetable oils and fish, appear to have some health benefits. Saturated fats fall somewhere in between. We've also learned that other types of ingredients, such as the highly refined carbohydrates that make up cookies and soda, can actually be just as unhealthy as "bad fats."
Now, this doesn't mean it's okay to eat a cheeseburger every single day. What it does mean, however, is that not all fats are bad and that fat can be a part of a healthy diet. These revelations have also triggered a debate among researchers about whether it's still useful to give dietary advice about macronutrients like fat and carbohydrates — or whether we should focus on foods instead.
Broadly speaking, there are three main types of dietary fats: saturated, unsaturated, and trans fats. Foods with fat contain some mixture of these three, and chemically they're all pretty similar (chains of carbon atoms bonded together by hydrogen). But they seem to do different things to the body.
Saturated fats are usually solid at room temperature. They're found at high levels in animal-based foods such as red meat (beef, bacon), poultry, and full-fat dairy milks, butter, and cheese. Some plant-based foods, like coconuts and palm oil, are also high in saturated fat.
By contrast, unsaturated fats typically remain soft or liquid at room temperature. These are more likely to appear at high levels in fish and certain vegetables. There are two types: monounsaturated fats (found in olive, peanut, and canola oils, avocados, almonds, pecans, pumpkin, sesame seeds, etc.) and polyunsaturated fats (found in fish and seafood, sunflower, safflower, corn, soybean, and flaxseed oils, walnuts, and flaxseeds).
