At this time last year, we predicted that the high-fat, low-carb ketogenic diet was going to blow up in 2019—and in many respects, that’s exactly what happened. Health-food stores are stocked with a growing array of keto-friendly packaged foods, from pancake mix to protein powder and avocado chips. Restaurant chains such as Chipotle have introduced menu items that follow the diet’s strict high-fat, low-carb standards. And we’ve seen all sorts of keto spinoffs crop up, including lazy keto, a low-carb diet with no other restrictions; the environmentally minded eco-keto; and the Ketotarian diet, a plant-based take on the trend.

Yet at the same time, a growing number of experts have started calling out the negatives of the ketogenic diet, an eating plan designed to force your body to convert fat instead of carbs for fuel (a process called ketosis). Celebrity trainer Jillian Michaels was one of the first to speak out against keto, having publicly railed against it multiple times since 2018. Her main concern, like many other nutrition experts, is that it deprives the body of nutrient-dense foods like fruit, whole grains, and vegetables. Similarly, registered dietitian Brigitte Zeitlin told Well+Good in 2019 that she wouldn’t recommend the keto diet in most cases, since its health impacts haven’t been studied long-term.

Other health experts have come forward and questioned whether keto is truly healthy for women. One frequently-cited study, released in 2018, showed that female mice on a ketogenic diet didn’t see the same improvements in weight and blood sugar control as male mice did.

That criticism hasn’t gone away with the new year. In its annual ranking of the best diets, U.S. News and World Report put the ketogenic diet at the bottom of the list—number 34 out of 35, ahead only of the Dukan Diet. In its analysis of the diet, the publication’s panel of health experts called the eating plan hard to follow, nutritionally incomplete, and said that it could be “dangerous” for some people.

Here’s the lowdown on the keto diet, straight from a dietitian: 

On Instagram, at least, it may seem like everyone who embarks upon a ketogenic diet sees significant benefits—namely weight loss, improved energy, stable moods, and balanced blood sugar. But experts say that many of their patients who go keto are disappointed with their results. “Even from the beginning, the ketogenic-diet experts who are physicians have said there are about 20 percent of cases where [the diet] doesn’t work, and we don’t know why,” says Zandra Palma, MD, a physician at Parsley Health in Los Angeles.

In her personal experience, most patients who don’t experience positive results with a keto diet either quit the diet prematurely because it’s too hard to follow, or they aren’t reducing their carbs enough to get into ketosis. For instance, she says, they’re still putting high-carb oat milk in their smoothies while eating large amounts of fat and wondering why they’re gaining weight. This isn’t exactly surprising: With its strict macronutrient ratios, long list of off-limits foods, and multiple side-effects, it’s notoriously difficult to maintain a traditional keto diet for long. “The population I’ve seen enjoy a ketogenic diet is this group of patients I have called optimizers—they’re super-conscious of what they’re eating, they go to CrossFit, they’re pretty type-A about their health, and they’re mostly men,” Dr. Palma says.

Even following the diet perfectly doesn’t necessarily guarantee healthy results, particularly for women. “Some women seem to respond better to the keto diet than others, particularly those that are overweight or obese and deal with insulin resistance,” says Josh Axe, DNM, DC, founder of Ancient Nutrition and DrAxe.com, and author of Keto Diet and Keto Diet Cookbook. “But some women on low-carb diets might find that they struggle with problems like trouble sleeping, mood swings, or menstrual changes. There’s definitely more to learn about how gender affects results [with the keto diet].” He refers to the aforementioned mouse study, which posits that a person’s estrogen levels could play a role in whether they experience positive results from a high-fat, low-carb diet.

“We know one of the dangers of keto is that, in [some] cases, people accidentally starve the microbiome by taking out the things the microbiome likes to eat.” —Zandra Palma, MD

In severely restricting carbs (and thus most sources of fiber), a keto diet may also impact the health of your microbiome, the ecosystem of healthy bacteria living in your gut whose functioning is linked to optimal digestion, hormonal health, and even mental health. “We know one of the dangers of keto is that, in [some] cases, people accidentally starve the microbiome by taking out the things the microbiome likes to eat [such as fiber from grains, fruit, and starchy vegetables],” says Dr. Palma. Stress, calorie restriction, over-exercising, and nutrient deficiencies could also make a difference in a woman’s results on keto, add both experts. 

Finally, many health experts are concerned about the types of food that people typically eat on a ketogenic diet. “A traditional ketogenic diet is made up of a lot of conventional meat and dairy products, like bacon and butter, [to which] many people can have sensitivities,” says Ketotarian author Will Cole, IFMCP, DC. Not to mention the genuine health concerns associated with eating a ton of red meat and saturated fat from animal products. “This can continue to exacerbate symptoms and impair the health benefits of going keto in the first place.” Plus, he adds, many keto dieters limit vegetables to stay within their carbohydrate range and end up missing out on essential nutrients, like the above-mentioned fiber, antioxidants, vitamins, and minerals.

Packaged keto foods, which are supposedly designed to make following the restrictive diet easier, may not be much better for health. “Pre-packaged foods that are labeled ”keto-friendly’ are still highly processed and filled with unnecessary ingredients that aren’t nutrient-dense,” says Dr. Cole. They also sometimes inadvertently stray from the diet’s guidelines, adds Dr. Palma. “It’s hard to get the fat composition right on the ketogenic diet, and [some] packaged foods are putting together the wrong fats,” she says. “Those ‘zombie foods’ are a really bad idea.”

So what do these downsides mean for the future of the keto diet?

It’s worth remembering that the ketogenic diet was originally developed to help people with seizure disorders, so from a medical perspective, it will likely continue to endure. “There are still situations where we use the keto diet clinically,” says Dr. Palma. “I think medically it will always be a tool.”

Some research indicates that the diet may also be helpful when used therapeutically for people with diabetes or uncontrolled blood sugar. “The only benefit I’ve been able to suss out of the research is it can have blood-glucose lowering effects,” says Dana Hunnes, PhD, MPH, RD, a dietician the Ronald Regan-UCLA Medical Center and professor at the UCLA Fielding School of Public Health. That said, she says she’d never recommend a traditional keto diet to this population or any other, as she feels that its benefits don’t outweigh its downsides—instead, she’s an advocate of “a plant-based diet rich in plant-based fats and proteins.”(It’s important to emphasize that anyone with diabetes should work closely with their doctor or a registered dietitian before trying keto or any other kind of diet.)

Indeed, outside of the clinical setting, the tides seem to be turning towards a more plant-heavy take on ketogenic eating. Dr. Axe advocates for what he calls a “clean keto diet,” which includes plenty of non-starchy vegetables, along with healthy fats and moderate amounts of high-quality protein. Dr. Cole’s “ketotarian” approach also revolves around plants, with an emphasis on high-fat vegetarian staples like coconuts, nuts, seeds, and avocado. “Many of my female patients also do quite well with a cyclical ketotarian approach, when they increase their healthy carb intake around their period and/or ovulation—once or twice a month. This [can help] with hormonal balance and energy levels during these times of the month,” he says.

“If you could truly replicate a very high-fat, super low-carb diet in a plant-based format, I might not be so opposed to it.” —Dana Hunnes, PhD, MPH, RD

Even Dr. Hunnes, who is highly skeptical of the ketogenic diet, says that a plant-based keto eating style could be an evolution in the right direction. “If you could truly replicate a very high-fat, super low-carb diet in a plant-based format, I might not be so opposed to it. Plant-based fats and proteins are not nearly as harmful to our health as their animal-based counterparts,” she says. “With that said, however,  it would not really be replicable to do a ketogenic diet that is primarily plant-based without adding in a higher carbohydrate component. Even if you’re doing primarily non-starchy vegetables and nuts and seeds, eventually you’d still be adding in more carbohydrates than you’d be getting from a traditional keto diet,” she adds, since plant-based proteins like legumes are inherently higher in carbs than meat. And with a greater carbohydrate intake, it’s unclear whether the desired keto diet results would be the same.

Despite the keto diet falling out of favor in some circles, Dr. Axe believes the eating plan is “here to stay” in some capacity, thanks to people continuing to try it and seeing short-term results. But as popular interest in the keto diet will likely persist into 2020 and beyond (and marketing of food products will likely continue to rise to meet it), it seems the more we learn about the keto, the less likely mainstream medical and nutrition experts are to recommend it.

Speaking of the Mediterranean diet, here’s what a healthy plate looks like on the eating plan. And if you’re more interested in sustainable food patterns, here’s what you should know about the planetary health diet.