In addition, there are medical conditions that should make you think twice about starting keto — or at least talk to your doctor before trying it out. Those include people on insulin, as well as those on oral and noninsulin injectable medications for high blood sugar or high blood pressure, says Hultin. Even struggling with GI issues may be a barrier to starting. “One of the side effects of a ketogenic diet is constipation, so if that’s a struggle, there’s serious reason not to go on this relatively low-fiber diet,” says Hultin. Last consideration: If existing personal dietary restrictions require you to avoid foods like soy, eggs, nuts, dairy, or seafood, a ketogenic diet may be too limiting for you. Coming from a place of elimination in an already restrictive diet can make it incredibly tough to follow, she says.

But beyond that, experts aren't convinced that the keto diet has any other scientifically proven health benefits. In fact, it may have some distinct downsides. If you follow the keto diet incorrectly, for example (like by eating lots of saturated fats, versus healthy unsaturated fats), you're at risk of raising your cholesterol levels. “The best strategy to keep your heart healthy is to get as much fat as possible from unsaturated sources such as olive, avocado and canola oils, nuts, seeds, avocados, and olives," says Ansel. For some people, it's possible to actually improve cholesterol if the fats in their diet are from varied and healthy sources, says Harvey.

Around this time, Bernarr Macfadden, an American exponent of physical culture, popularised the use of fasting to restore health. His disciple, the osteopathic physician Dr. Hugh William Conklin of Battle Creek, Michigan, began to treat his epilepsy patients by recommending fasting. Conklin conjectured that epileptic seizures were caused when a toxin, secreted from the Peyer's patches in the intestines, was discharged into the bloodstream. He recommended a fast lasting 18 to 25 days to allow this toxin to dissipate. Conklin probably treated hundreds of epilepsy patients with his "water diet" and boasted of a 90% cure rate in children, falling to 50% in adults. Later analysis of Conklin's case records showed 20% of his patients achieved freedom from seizures and 50% had some improvement.[10]

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