Celebrities linked: Kim Kardashian, LeBron James, Halle Berry, Mick Jagger, Rihanna, Alec Baldwin, Matthew McConaughey, Tom Jones.
What is it? Dieters consume little to no carbs (around 20-50g per day or 5% total calories), filling up on fats and proteins instead.
BDA's verdict: "A carefully dietitian-planned ketogenic diet can be a very effective treatment for people with epilepsy. For weight loss, there's no magic, the diet works like any other by cutting total calories and removing foods people tend to overeat.
"Initial side effects may include low energy levels, brain fog, increased hunger, sleep problems, nausea, digestive discomfort, bad breath, and poor exercise performance.
"It can be an effective method of weight loss in the short term with careful planning but it is hard to sustain for many in the long term and most of the initial weight loss seen is often associated with water/fluid losses."
Is it worth it? No. The BDA said over-restricting any food group is a bad idea and leads to uncurbable cravings, often causing the weight to pile back on.