People need fat to live. The same is true of protein and some essential vitamins and minerals.
But we can live without carbs, according to Harvard nutrition professor and world-renown diet expert Dr. Walter Willett.
This truth undergirds the trendy low-carb keto diets that celebrities like Halle Berry and Kim Kardashian say they adore. Keto diets can be as high as 90% fat, and typically limit a person’s carbohydrate intake to no more than 5% of daily calories (meaning no bread and no sugary drinks).
The goal is for the body to enter an atypical fat-burning metabolic state called ketosis, where instead of transforming carbohydrates into glucose first, fat is a person’s primary energy source. In practice, this can mean that keto dieters munch on a fair amount of high-fat animal products, including butter, cream, and bacon.
But researchers still have a lot to learn about the merits (and potential pitfalls) of going keto for life. This dearth of evidence means that there’s no way to know if a consistently high-fat plan, especially one that’s rooted in saturated forms of fat, might prompt more cancer cases, strokes, or chronic illnesses down the road.
Willett’s nutrition advice: don’t shun entire nutrient groups, whether it’s fat, protein, or carbs. While it’s possible to adhere to a healthy high-fat eating plan — drenching food in liquid oil, for example, can be a boon for the heart and provide essential fatty acids the body can’t make on its own — Willett also has a hunch there may be some dangerous fats lurking in some keto diets, just as there are dangerous carbs in other kinds of diets.
“The reality is that we don’t have long term studies of people on keto diets,” Willett said. “Given that big gap, what we see in general for good health, is that the type of fat is really important. It’s not all bad, it’s not all good, and emphasizing mostly unsaturated fats in a diet has positive health benefits.”
Sugar is a dangerous carb, but not all others are like it
“I eat full-fat cream in everything,” kinesiologist and cancer researcher David Harper, who’s been following a keto diet for more than six years, recently told Business Insider.
But while he pays virtually no attention to how much fat he consumes, sugar is a major no-no in his house, and he thinks it should be banned in everyone else’s too, keto or not.
“Try and get as much sugar out of your diet as possible,” he said.
While Willett agrees with Harper that sugar (which is the simplest kind of carbohydrate around, chemically speaking) is not great for our bodies, he says the same isn’t necessarily true of other more complex carbohydrates.
“There are positive benefits of modest amounts of whole grains, for example,” Willett said. “And it’s not clear that in the long run, a keto diet is going to be better than a diet with some modest amount of whole grains in it.”
Willett has been studying what people eat for 30 years, and is one of the most cited scientists in the world, in part because his studies have overturned conventional wisdom about eating well.
He has collected evidence that low-fat diets often backfire, and do not help prevent breast cancer in any measurable way. He was also one of the first nutrition scientists to suggest that fat doesn’t necessarily make us fat, but that the types of fats we choose to eat are paramount to overall health.
And despite the fact that Willett comes from a family of dairy farmers, the doctor’s body of research also overwhelmingly suggests that saturated fats— the kind that are plentiful in meat, coconut oil, cheese, and (yes) cow’s milk — are especially bad for human hearts in large doses.
The healthiest fats in the world are not from animals, they’re from plants
Greeks, Romans, and other populations around the Mediterranean sea have known this intuitively for centuries. The traditional Mediterranean Diet, which is a swimming-in-healthy-fat eating plan of as much as 30% olive oil, has been linked with better health outcomes and less cognitive decline in older adults.
“It’s not too hard to do,” Willett said. “If you have a big salad every day that’s sort of swimming in olive oil, lots of vegetables that are swimming in olive oil, you put some on your bread, you cook with it. You can get up to 30% pretty easily.”
One of the central problems with studying keto diets long term is that unlike traditional Mediterranean plans, they’re notoriously tough to follow. Cardiologist Ethan Weiss, who is keto himself and suggests patients who want to try the diet stick to a “heart-healthy” version, says high-fat eating is not for the faint of heart.
“That’s a big problem with keto, right?” Weiss previously told Business Insider. “It’s not simple to do it.”
That difficulty staying in ketosis means it is extremely challenging to get the same kind of long-term data about the health effects of keto that researchers have about Mediterranean diets, which have been studied in hundreds of thousands of eaters around the world.
It’s possible there’s something different about a body running on fat that makes saturated fats from animal sources less harmful to ketoers versus people who tend to eat more carbs. The scant evidence we do have, though, suggests to the contrary that keto dieters can suffer kidney and heart problems, and some may struggle with high cholesterol.
“Dirty” keto, as some call it, is a fast food version of the keto diet, loaded with high-fat processed foods that are low-carb. Dirty keto staples include beef jerky, sliced full-fat cheese, bacon cheeseburgers, and Egg McMuffins (hold the buns and bread).
But Weiss suggests that his patients who want to try keto instead prioritize “the fats that you find in things like nuts, or avocado, or olive oil, and fish like salmon or trout,” (which carry essential fats like omega-3 and omega-6) instead of “the sort of traditional bacon, butter, and eggs keto,” which he said “may not be optimal for everybody.”
People can have a “healthy keto” diet, but the best diet for people — and for the planet— is always one that does not prioritize animal fats, according to Willett.
“If someone has glucose intolerance problems, they can get better, they will get better on a keto diet,” Willett said. “But pay attention to the kind of low-carbohydrate foods that you are eating, because we have lots of evidence from other kinds of diets, across the range of carbohydrate intake, that a high consumption of [saturated fat] — especially red meat and dairy fats — is not going to be an optimally healthy diet.”