Eat fat to lose fat: That’s the thinking at the heart of the ketogenic diet. This popular weight-loss regimen requires followers to fill their plates with high-fat foods—think bacon, avocado, cheese, butter, and nuts. Meanwhile, carbohydrates are strictly limited, even when they come from nutritious sources such as fruits, veggies, and whole grains. Since the keto diet started trending again in recent years (it’s been around in some form for generations), many health and nutrition experts have voiced concerns about its safety and long-term efficacy.
Before we get to those concerns, here’s a quick primer: The standard keto diet is a super-low-carb, high-fat plan with moderate protein. It typically contains 70% to 80% of calories from fat, about 10% to 20% protein, and only 5% to 10% carbohydrates. The goal of a ketogenic diet is to induce a metabolic state called ketosis, a phenomenon in which the liver converts stored fat into substances called ketones to serve as an energy source for cells.
If you’re like most U.S. adults, your body normally gets much of its cellular energy by breaking down dietary carbs into glucose. Sharply curb your carb intake, however, and your system is forced to find fuel elsewhere, explains Angela Cusimano, RD, a clinical dietician at the University of Kansas Hospital in Kansas City. First, the body taps stored glucose (glycogen) in muscles and other tissue. Soon after glycogen runs out, ketosis kicks in and the body switches to ketones for cellular energy. Fans of the keto diet say this use of stored fat is what accounts for the weight loss many followers experience.
General drawbacks of the keto diet
With its high-fat, moderate-protein, ultra-low-carb rules, the keto diet comes with some big caveats. “Keto does not align with public health guidance, which emphasizes eating plenty of fruits, vegetables, whole grains, and plant-based proteins,” says registered dietitian Jaclyn London, MS, CDN, head of nutrition & wellness at WeightWatchers®. As a result, keto dieters may miss out on important nutrients, she notes, including fiber, minerals, B vitamins, antioxidants, potassium, and magnesium. Another concern: The keto diet fails to differentiate between saturated and unsaturated fats. Foods high in saturated fat are associated with an increased risk of developing heart disease and other health issues, London says.
The first week or two of a ketogenic diet can cause a number of temporary but unpleasant side effects, including headaches, constipation, fatigue, dizziness, muscle cramps, halitosis, irritability, and difficulty focusing. Beyond that, the safety of a keto diet may depend on your personal health picture. If you have diabetes or a condition affecting your heart, liver, or kidneys, seek the advice of a physician before undertaking any significant changes to your diet, London advises.
Read on for an in-depth, science-backed look at some considerations to keep in mind before trying a ketogenic diet.
Is the keto diet safe for people living with obesity?
There’s no evidence to suggest that the ketogenic diet poses special safety concerns to people living with obesity, Cusimano says. At the same time, however, it’s worth examining whether keto offers a meaningful weight-loss edge.
Many people who try a keto diet report rapid weight loss early on—this is largely the result of the body burning through its glycogen stores, a process that results in water shedding, London says. The calories lost from cutting out nearly all carbs may contribute to dramatic changes on the scale, as well.
Beyond the first few months, research doesn’t offer much support for the popular theory that keto imparts a major metabolic advantage. A review of 13 randomized controlled trials, published in 2013, compared adults on very-low-carb keto diets with those on low-fat diets and found that after 12 to 24 months, the keto group had lost only about 2 pounds more on average.
Still, on certain health-related measures, the keto diet may beat old-school calorie cutting, research suggests. One small study of adults living with obesity, published in the Annals of Internal Medicine, found that subjects on a super-low-carb diet had more favorable levels of triglycerides and HDL (“good”) cholesterol after a year than those who were simply told to slash 500 calories a day. Long-term effects on health, however, were beyond the scope of this analysis. (More on that matter below.) Also worth noting: Both diet groups reported poor adherence and high dropout rates.
In general, any benefits of weight loss are sustained only to the extent that the weight itself is sustained, says Jen Bruning, MS, RDN, LDN, a spokesperson for the Academy of Nutrition & Dietetics. The restrictive nature of the keto diet can make sticking with it difficult, she says. When people stop following a keto plan and return to their prior eating habits, the weight they lost tends to come back.
Is the keto diet safe for people living with type 2 diabetes?
It’s possible—under close medical supervision. The keto diet is designed to reduce levels of glucose, the carbohydrate-derived sugar that normally serves as the body’s main fuel source. Because type 2 diabetes is a condition that affects how the body metabolizes glucose, Cusimano strongly advises patients to partner with their doctors in creating any carb-restricted eating plan. “If you change your diet and not your medication, it could drop your blood sugars to dangerous levels,” she cautions.
Some evidence suggests that in the short term, a carefully managed ketogenic diet may be helpful for improving blood sugar control and managing weight among people living with type 2 diabetes. Within that group, a low-carb diet may also reduce the need for oral medication.
Right now there’s not enough evidence to recommend the keto diet as a therapeutic approach for people living with type 1 diabetes, a 2019 research review in Nutrients concluded. Type 1 diabetes is a condition that occurs when the pancreas produces too little (or none) of the insulin needed to move glucose into cells. If you have type 1 diabetes and are considering a keto diet for weight-loss purposes, chat with your doctor to ensure the diet aligns with your overall health plan.
Is the keto diet safe for people with fatty liver disease?
A ketogenic diet may be helpful to some people living with nonalcoholic fatty liver disease (NAFLD). That’s the finding of a 2020 analysis of 21 studies, published in the journal Obesity Reviews. NAFLD occurs when fat accumulates in the cells of the liver, a condition that can sometimes lead to complications if left unaddressed. While people with NAFLD are generally advised to lose weight, the research cited above suggests that ketosis may have a beneficial effect on liver composition beyond weight loss. (Fatty liver disease can also occur as a result of excessive alcohol consumption; the main recommendation in those cases is to stop drinking alcohol.)
NAFLD often has no symptoms but may occur along with type 2 diabetes, metabolic syndrome, or obesity. Cusimano recommends speaking with your doctor if you suspect something’s going on with your liver and are considering a keto diet.
Is the keto diet safe for people with epilepsy?
You’d expect so—the keto diet was first developed as a treatment for epilepsy in the 1920s. Epilepsy is a neurological disorder in which aberrations in brain activity result in symptoms such as seizures, confusion, and loss of consciousness. Though the exact modes of action are still being investigated, a 2013 research review in the European Journal of Clinical Medicine determined that the keto diet indeed shows “strong evidence” for reducing epileptic seizures, noting that a keto diet may reduce the need for anticonvulsant medications. One possible explanation is that the metabolic changes resulting from a keto diet affect certain neurotransmitters in the brain to normalize activity. Nevertheless, says Bruning, people with epilepsy “should only follow a keto diet under close medical supervision.”
Is the keto diet safe for people with kidney disease?
A keto diet might not be ideal if you’ve been diagnosed with a kidney issue. People with stage 1, 2, or 3 kidney disease are commonly advised to cap their protein intake at 12% to 15% of daily calories to minimize the kidneys’ processing load, says Cusimano. This would preclude most keto plans, which set protein intake at roughly 20% of calories.
The keto diet’s specific impact on kidney function is still being investigated. A small observational study of men and women with obesity and mild chronic kidney disease found in 2020 that a very-low-calorie version of the keto diet resulted in weight loss without compromising kidney function. Close medical supervision is key, the researchers write, echoing advice from WW’s experts.
Is the keto diet safe for people with a history of eating disorders?
As of early 2020, the safety of a ketogenic diet for people living with or recovering from eating disorders was still in the early stages of investigation. To date, no large studies have outlined the risks or benefits of a ketogenic diet for people with anorexia nervosa, bulimia, or binge-eating behavior. Until we know more, starting a keto diet might not be advisable if disordered eating is or was a concern for you at any point. In Bruning’s opinion, “the highly restrictive nature of the diet might be triggering for individuals with an eating disorder or a history of one.” Your doctor or nutritional care provider can better advise you based on your individual health picture.
Is the keto diet safe for athletes?
Don’t bank on bacon to amp up your soccer skills: A 2019 review of 13 studies looking at the impacts of the keto diet in both elite and recreational athletes turned up “no clear performance benefit.” Nor was keto linked to any performance drawbacks.
A 2020 analysis of seven small studies, published in the Journal of the International Society of Sports Nutrition, turned up mixed results for endurance athletes on keto vs. high-carb diets. Some findings seemed to point to increased endurance (as measured by VO2 max) for keto dieters, while the majority of research found no meaningful difference between the eating approaches.
Even if you’re not looking for a performance boost, take note: Because the early phases of a keto diet are often marked by increased water losses from the body, it’s especially important to stay hydrated during physical activity.
Is the keto diet safe for kids?
Children and teens should not follow a keto diet unless directed to by a doctor as a medical treatment for a condition such as epilepsy, Bruning says. Children need ample carbohydrates from nutritious sources such as whole grains to support healthy development and learning. And young kids might mistake the rules of the keto diet for value judgments—and come to think of certain foods as “good” or “bad.” Rather than restricting a major food category, Bruning encourages parents to model a positive relationship with food and support children in making healthy choices at the table.
Is the keto diet safe long-term?
Recent research suggests that strict, long-term carb deprivation could affect lifespan. In 2018, a team publishing in The Lancet looked at 25 years of data on carbohydrate intake for more 432,000 adults in diverse U.S. communities and found that those who reported eating low-carb diets—with carbs making up less than 40% of their overall intake—were at the highest risk of dying of any cause during the study period. High-carb diets, composed of 70% or more carbs, also were associated with increased risk of mortality, though to a lesser extent. Low-carb diets high in animal-based protein were more strongly associated with death than low-carb diets that emphasized plant sources of protein. The group with the lowest observable risk? Adults whose diets were 50% to 55% carbs.
The upshot: Is the keto diet worth trying?
As a short-term eating approach, a very-low-carb, high-fat, moderate-protein keto diet is generally considered safe for healthy adults. Anyone with an underlying health condition is advised to speak with their doctors before making major changes to their diet.
Note that the keto diet does not adhere to the latest Dietary Guidelines for Americans, and long-term compliance with the keto diet is low. Says London, “The best eating plans are ones that you can stick with for the long term, include a wide variety of foods from all the main food groups, and don’t require you to eat from a list of ‘approved’ foods.”
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Liz Krieger is a freelance writer and editor in Brooklyn, New York.
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This article was reviewed for accuracy in July 2021 by Tiffany Bullard, PhD, manager for clinical research at WW. The WW Science Team is a dedicated group of experts who ensure all our solutions are rooted in the best possible research.
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