If you’ve been weighing the pros and cons of the ketogenic diet, one of the most fiercely debated eating plans, a new review suggests the downsides may eclipse the benefits.
The analysis, published in the July 2021 issue of Frontiers in Nutrition, examines the potential long-term risks of the keto diet, which is a very low-carb, high-fat style of eating. People on this restrictive diet typically get 70 to 80 percent of their daily calories from fat, 5 to 10 percent from carbohydrates, and 10 to 20 percent from protein, according to the Harvard T.H. Chan School of Public Health. (The U.S. Department of Agriculture’s Dietary Guidelines for Americans say most people should get 20 to 35 percent of their daily calories from fat, 45 to 65 percent from carbs, and 10 to 35 percent from protein.)
The new review examined more than 120 studies, says one of its authors, Neal Barnard, MD, of Washington, DC, the president of the Physicians Committee for Responsible Medicine. The PCRM is a nonprofit that promotes a plant-based diet, which is essentially the opposite of the meat-heavy keto diet.
“There have been a great many hints about the risks of ketogenic diets for a long time, and this review really brings the evidence that the ketogenic diet, for some people, causes weight loss,” Dr. Barnard says. “But even when it does, the risks greatly outweigh any benefit — and the weight-loss benefits are short-lived for most people, anyway.”
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A Keto Diet May Be Harmful for Certain People
The review highlights five key findings. Among them: The keto diet is particularly unsafe for women who are pregnant or trying to get pregnant. That’s because low-carb diets are linked to a higher risk of neural tube defects in babies. Barnard says: “You’re not eating the things that you need for a healthy baby on a typical ketogenic diet. You’re not having the vegetables, fruits, and whole grains.” For unplanned pregnancies in particular, he said, the risk of neural tube defects is 89 percent higher for those on a low-carb diet.
Another finding: Keto diets raise levels of LDL cholesterol (commonly known as “bad” cholesterol) for many people — when, typically, you would expect to see those levels drop when you lose weight, Barnard says. In a six-month study that the review cites, 30 percent of participants experienced LDL cholesterol increases greater than 10 percent.
Keto diets high in protein may also cause kidney stones, plus hasten kidney disease in those with kidney disease, the review found. “That is probably because the kidneys’ job is to filter out toxins from the blood. And if you’re eating a big load of animal proteins, it’s just harder on the kidney,” Barnard says.
Speaking of animal proteins: Eating lots of red meat, processed meat, and saturated fat — which many people typically do on the keto diet — may increase the risk of cardiovascular disease, cancer, type 2 diabetes, and Alzheimer’s disease, according to the review. Food components typical of a ketogenic diet, such as red and processed meats, are linked to increased cancer risk, the study authors write. “Whole grains, fruits, and vegetables are linked to a lower risk of both cancer and all-cause mortality, yet, with the exception of nonstarchy vegetables, these foods are commonly avoided on ketogenic diets.” The study authors add that long-term data on a possible link between cancer and ketogenic diets is lacking, and that no long-term data on ketogenic diets and Alzheimer’s disease is available.
Barnard notes: “We do think that the risks go back down when a person quits that diet, which is good.”
The findings suggest that the keto diet can indeed help people achieve weight loss — but not more effectively than other dietary approaches, such as low-fat diets, when examined over the long term. The authors note that ketogenic diets can reduce seizure frequency in some people with drug-resistant epilepsy. That’s no surprise: The classic keto diet, which has adjusted macronutrient ratios, has long been used for this purpose, as the Epilepsy Foundation notes.
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The Review’s Authors May Have a Conflict of Interest
Caroline Apovian, MD, a codirector of the Center for Weight Management and Wellness at Brigham and Women’s Hospital in Boston, criticized the review’s “dramatic” findings. She pointed out that some of the study authors are proponents of a plant-based diet, which could skew their objectivity and goals.
Beyond that, she noted that many of the studies the review is based on were examining the long-term effects of, for example, high protein intake. She has recommended, and will continue to recommend, following the keto diet short term for some patients. “Ketogenic diets are not ‘diets,’” says Dr. Apovian, who’s a member of the faculty at Harvard Medical School. “They’re treatments for certain conditions,” including extreme obesity. For example, a patient with obesity might need knee surgery, but the surgeon will refuse to treat her until she loses weight, in which case a keto diet might make sense, Apovian says. In those instances, it can be safe and effective. “We would never recommend that anybody be on a ketogenic diet chronically for the rest of their life,” she says.
Her takeaway? “Don’t distort reality, and don’t use a review to denigrate a diet that we use to get weight loss,” she says.
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What to Do With This Information if You’re Considering Keto
If you’re still unsure what to make of the keto diet, Caroline Thomason, a northern Virginia–based registered dietitian nutritionist, stresses that keto is not more likely to cause sustainable weight loss than other diets. The reason it works well for some people, she says, is “that it creates high feelings of fullness and satisfaction,” because of its emphasis on protein and fat. But people on the keto diet often “overemphasize foods like dairy, eggs, and red meat,” she says. Those foods can all play a role in a healthy diet, Thomason adds, but overdoing them can harm laboratory markers of health. “It’s important to note that we still need additional research to make more conclusive statements,” she says.
Christa Brown, a registered dietitian nutritionist based in New York City, says to bear in mind that the keto diet was designed to be used in clinical settings to treat seizure disorders, not for the everyday dieter. While it will likely lead to short-term weight loss, “it’s in fact muscle loss instead of fat loss,” she says. If you’re tempted to try the diet, talk with your healthcare team first. “Overall, working with a registered dietitian can help you maintain your muscle mass” and ensure safe results, Brown says.
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