Acceptable foods for the keto diet include nonstarchy vegetables, dairy, protein, and healthy fats. Foods to avoid on the keto diet include processed foods, sweets, grains of all kind, high-carb fruits (some fruits, like avocados, are keto-friendly), alcohol, artificial sweeteners, and desserts.
If you search online for the term “keto diet,” you'll find a lot of health claims associated with the ketogenic diet. But before you give this approach a try, it’s important to know what the science suggests about how it may affect your health. Namely, you'll want to know about potential keto diet dangers.
Risk: You May Suffer Fatigue and Other Symptoms as a Result of the Keto Flu
One of the most common side effects of starting the ketogenic diet is “ keto flu .” This term describes the often unpleasant, fatigue-inducing symptoms that occur as the body adjusts from a high-carbohydrate to a low-carbohydrate diet . During keto flu , the body’s stored glucose begins depleting, and the body starts adapting to producing and utilizing ketones as energy.
Symptoms of the keto flu include headache, fatigue, dizziness, sleep problems, heart palpitations, cramps, and diarrhea. These side effects usually diminish and resolve in about two weeks. But to lessen the effects of any discomfort, simply consider slowly transitioning onto a ketogenic diet rather than rushing to change your eating habits. By gradually lowering your carbohydrate intake and gradually increasing your intake of dietary fat, you can transition with less negative impact and potentially prevent the keto flu altogether. But to lessen the effects of any discomfort, simply consider slowly transitioning onto a ketogenic diet rather than rushing to change your eating habits. By gradually lowering your carbohydrate intake and gradually increasing your intake of dietary fat, you can transition with less negative impact and potentially prevent the keto flu altogether.
Risk: You May Experience Constipation if You Don’t Eat Enough Fruits and Veggies
Risk: You Could Develop Dangerous Nutrient Deficiencies
Eliminating food groups can be problematic. “Ketogenic diets are often low in calcium, vitamin D, magnesium, and folic acid, which over time can lead to nutrient deficiencies if the diet is not planned carefully,” adds Marie Spano, RD, a sports performance nutritionist in Atlanta.
RELATED: What Is an Elimination or Exclusion Diet?
Risk: You May Harm Your Heart With the Diet’s Emphasis on Animal Fat and Protein
Reliance on a diet rich in animal fats and proteins may also have a negative impact on heart health, research shows. “This diet is not for anyone who is at risk of developing cardiovascular disease or who has already been diagnosed with it,” Spano cautions. “This diet is not for anyone who is at risk of developing cardiovascular disease or who has already been diagnosed with it,” Spano cautions.
This means that if you have risk factors for heart disease — such as elevated cholesterol high blood pressure (hypertension) , or a strong family history of the disease — you should use caution when following this diet. The diet's heavy reliance on fat, especially saturated fat, can raise cholesterol levels , further increasing your chances of developing heart disease in the future. A 2019 position statement from the National Lipid Association Nutrition and Lifestyle Task Force noted that research has found low-carb and very low carb plans (which keto fits into) have mixed effects on blood cholesterol levels, with some studies finding that these diets raise cholesterol. A 2019 position statement from the National Lipid Association Nutrition and Lifestyle Task Force noted that research has found low-carb and very low carb plans (which keto fits into) have mixed effects on blood cholesterol levels, with some studies finding that these diets raise cholesterol.
RELATED: Is the Paleo Diet Good for Heart Health?
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Risk: You May Experience Dangerous Low Blood Sugar if You Have Diabetes
For any individual with diabetes, discussing dietary changes — especially those as dramatic as the ones the ketogenic diet requires — with your healthcare team is essential. Because carbohydrates are broken down into glucose in the blood, cutting carbohydrates from your diet could cause levels to crash rapidly depending on your current medication regimen. Such a change may require significant adjustments to medication and insulin to prevent dangerous side effects such as low blood sugar, called hypoglycemia
Risk: You May Experience Weight Cycling and Negative Effects on Your Metabolism
Outside of physical health changes, one of the biggest concerns of the ketogenic diet may be in long-term adherence. “It’s a very difficult diet to stick to and maintain. Compliance is a challenge because it is so restrictive,” explains Dr. Mohr.
Following a strict diet for weight loss and then quickly reverting to old habits when the dietary changes are too restrictive can lead to what is known as weight cycling, or yo-yo dieting. Gaining and losing the same weight over and over is associated with poorer cardiovascular health, especially among premenopausal women, according to a study in Preventive Medicine Reports published in December 2019.
Benefit: You May See Improvements in Your Athletic Performance
For athletes, research on the keto diet highlights potential improvements in athletic performance, especially when it comes to endurance activities. An article in the British Journal of Sports Medicine found that ketogenic-type diets allowed endurance athletes to rely mostly on stored fat for energy rather than having to refuel with simple carbohydrates during endurance training and competition, and saw improved recovery times. That said, a review published in October 2020 in Sports concluded that while the keto diet may help athletes reduce their weight and body fat, there is no conclusive evidence that the method of eating improves or harms health and performance. That said, a review published in October 2020 in Sports concluded that while the keto diet may help athletes reduce their weight and body fat, there is no conclusive evidence that the method of eating improves or harms health and performance.
Benefit: You Could Lose Weight Fast — but Not Necessarily More Than You’d Lose on Other Diets
If you’re looking to lose weight, one benefit the ketogenic diet may offer is appetite suppression. A review of this form of eating suggests it may help decrease appetite, but how this actually happens needs to be studied further. Very low calorie ketogenic diets may help people who are overweight or have obesity reduce their Very low calorie ketogenic diets may help people who are overweight or have obesity reduce their BMI , decrease the circumference of their waist, and lower levels of A1C , total cholesterol, triglycerides , and blood pressure, notes a review and meta-analysis published in March 2020 in Reviews in Endocrine and Metabolic Disorders.
But when it comes to weight loss — one of the biggest keto selling points for many individuals — the benefits of the ketogenic diet may not be much different from any other diet plan. “There is no magical weight loss benefit that can be achieved from this diet,” says Spano. “The ketogenic diet may help weight loss in the same way other diets help — by restricting food choices so you eat fewer calories.”
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Mohr agrees. “Cutting so many carbohydrates is a big reduction in calories,” he says, adding that this effect will lead to a loss of water weight up front, “which is why people like the immediate response of weight loss that comes from this type of diet.” That said, the calorie reduction isn’t always the case, as it can be easy to consume more calories overall if you’re eating more calorie-dense foods (namely fat). One small study published in February 2021 in Nature Medicine found that people ate almost 700 fewer calories per day on a plant-based low-fat diet compared with a low-carb one consisting of mostly animal fats.
Benefit: You May See Better Blood Glucose Control if You Have Type 2 Diabetes
For individuals with diabetes, adapting a very low carbohydrate diet, such as the ketogenic diet, may offer some benefits when it comes to glucose management. For instance, a previous review found that dietary restriction of carbohydrates may reduce or eliminate the need for medication in individuals with type 2 diabetes.
Learn More About the Possible Benefits and Risks of the Keto Diet
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