The main criticisms include fears about fat intake and the process of ketosis.The argument over whether saturated fats are unhealthy has been settled in the research literature (they are not). If you need more info, there are several good books to read on this subject.  For more information about the type of fats to eat, see the saturated vs unsaturated fats page.  The right side column on that page has books which provide more information.

Some athletes swear by the ketogenic diet, not just for weight loss but for improved performance in their sport, as well. But Edward Weiss, PhD, associate professor of nutrition and dietetics at Saint Louis University, doesn’t buy it. “I hear cyclists say all the time that they’re faster and better now that they’re on keto, and my first question is, 'Well, how much weight did you lose?'” he says.
In 1921, Rollin Turner Woodyatt reviewed the research on diet and diabetes. He reported that three water-soluble compounds, β-hydroxybutyrate, acetoacetate and acetone (known collectively as ketone bodies), were produced by the liver in otherwise healthy people when they were starved or if they consumed a very low-carbohydrate, high-fat diet.[10] Dr. Russell Morse Wilder, at the Mayo Clinic, built on this research and coined the term ketogenic diet to describe a diet that produced a high level of ketone bodies in the blood (ketonemia) through an excess of fat and lack of carbohydrate. Wilder hoped to obtain the benefits of fasting in a dietary therapy that could be maintained indefinitely. His trial on a few epilepsy patients in 1921 was the first use of the ketogenic diet as a treatment for epilepsy.[10]
Stomach growling? Reach for a handful of macadamia nuts, which have the highest amount of fat and lowest amount of carbs in the nut family, according to previous research in the journal Nutrients. Nutrition aside, they’re snacking all-stars because they’re easy to bring with you and don’t need to be refrigerated. Keep a stash wherever you spend your day — in the car, at your desk, or in the pantry at home.
Seidelmann recently published a massive, blockbuster global study of the eating patterns of more than 447,000 people around the world. What she discovered — and what is probably not a huge surprise — is that no matter where you live or what your daily diet is like, banning entire food groups and thinking you can cheat your way into good health might work for a while, but it could also send you into an early grave.
You should check with your physician if you have any concerns about starting a ketogenic diet plan with pre-existing health conditions, especially if those conditions involve kidney or heart problems.  People with kidney disease should definitely consult with their physician about starting a ketogenic diet.  Other contraindicated conditions and health issues are listed here.  Make sure you check this list and rule these out before starting the diet.
A keto diet has shown to improve triglyceride levels and cholesterol levels most associated with arterial buildup. More specifically low-carb, high-fat diets show a dramatic increase in HDL and decrease in LDL particle concentration compared to low-fat diets.3A study in the long-term effects of a ketogenic diet shows a significant reduction in cholesterol levels, body weight, and blood glucose. Read more on keto and cholesterol >
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