Earlier this year, Shape Magazine reported that more than half a million Instagram users have shared photos of their progress on the diet. But, in another Shape essay criticizing the diet, dietitian Toby Amidor raised several issues with the high-fat, low-carb diet. “With all the side effects and complications this diet has, I'm truly surprised by the popularity it has gained – it's just so darn unhealthy and unappetizing in so many ways,” Amidor wrote. Some of those side-effects include constipation and lack of sustainability. According to Amidor, if you stray from your diet, it can take weeks to get back into ketosis. There’s also a nasty little side-effect called the keto flu, which can cause dizziness, nausea and even cramping.