BBC Radio 4 - The Food Programme, Keto: Diet fad or food fix?
Why would you eat this way?
In the show on Sunday, I asked a nutritionist, Frances Quinn, about the long-term health implications.
She said: "These are very disruptive foods. They're high on the glycemic index, so they have the potential to spike your blood sugar and send your insulin levels high.
"We know very clearly that insulin is a critical factor in leading to cancer, heart disease, diabetes and other serious illnesses.
"That means long-term use can actually be quite dangerous. For people who are exercising regularly, this is not something to use as a weight loss diet."

The 30-Day Ultra-Fast keto challenge
What is the advantage of Keto?
There are many ways of trying to control your weight. Some people like to limit their calories in other ways, such as eating smaller portions of fat and protein.
What you should eat on a Keto diet is very, very low in carbohydrates
Ketosis is the term for when your body produces more ketone bodies - molecules containing fewer than four carbons, rather than carbs - in your blood.
This means that you consume very little glucose (sugar) and this helps you to burn fat.
"What you should eat on a Keto diet is very, very low in carbohydrates," explained Frances.
"But it's possible to consume any type of food - that is what's called 'ketogenic' eating."
The main way that this occurs is through a very low-carbohydrate diet. A typical carbohydrate allowance would be 10% of a person's total daily calories, but this will be much higher for people with diabetes.