The keto diet isn’t new, and it’s been around for nearly a century. It was originally developed to treat people with epilepsy. In the 1920s, researchers found that raised levels of ketones in the blood led to fewer epileptic seizures in patients. The keto diet is still used today to treat children with epilepsy who don’t respond well to anti-epileptic drugs.
If you're healthy and eating a balanced diet, your body controls how much fat it burns, and you don't normally make or use ketones. But when you cut way back on your calories or carbs, your body will switch to ketosis for energy. It can also happen after exercising for a long time and during pregnancy. For people with uncontrolled diabetes, ketosis is a sign of not using enough insulin.
Some research suggests that ketogenic diets might help lower your risk of heart disease. Other studies show specific very-low-carb diets help people with metabolic syndrome, insulin resistance, and type 2 diabetes. Researchers are also studying the effects of these diets on acne, cancer, polycystic ovary syndrome (PCOS), and nervous system diseases like Alzheimer's, Parkinson's, and Lou Gehrig's disease.
Cyclical ketogenic diet: The Bulletproof Diet falls into this category. You eat high fat, low carb (less than 50 grams of net carbs a day) five to six days of the week. On day seven, you up your carb intake to roughly 150 grams, during what’s called a carb refeed day. Carb cycling this way helps you avoid the negative effects some people experience when they restrict carbs long term, like thyroid issues, fatigue and dry eyes.  Full ketosis isn’t for everyone, and adding carbs such as sweet potatoes, squash, and white rice one day a week keeps your body systems that need some amount of carbs functioning properly.
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Without peer-reviewed clinical trials, many of the benefits remain anecdotal. For instance, Weiss himself has been on a low-carb high-fat (though not strictly ketogenic) diet for more than six months, and claims he does feel much better. But he’s clear about what he knows and what he doesn’t. He’s lost weight and his borderline pre-diabetes is gone.
Staying in Ketosis long-term: Chronic Ketosis can cause fatigue, muscle soreness, insomnia and nausea. "Unless you have a medical condition that requires you to stay in Ketosis for long-term, you shouldn't stay in that state for a prolonged period without any carb ups," Mavridis suggests. And if you're a beginner, "it’s recommended that you go through the fat-adaptation phase so that your body becomes accustomed to burning both glucose and fat for fuel," says the nutritionist.
On a ketogenic diet, your entire body switches its fuel supply to run mostly on fat, burning fat 24-7. When insulin levels become very low, fat burning can increase dramatically. It becomes easier to access your fat stores to burn them off. This is great if you’re trying to lose weight, but there are also other less obvious benefits, such as less hunger and a steady supply of energy. This may help keep you alert and focused.