The keto diet has become very popular in recent years. It involves changing the diet so that almost all carbs are eliminated. In consequence it has very high levels of fat and protein, mostly derived from eating meat. Despite the diet having lots of fat in it, some people on this diet lose amazing amounts of body weight. Although the ‘keto diet’ fad is quite recent the idea of cutting out carbs to lose weight is in fact very old. The first person who wrote about it was a man called William Banting who published an article on it in 1863. In fact, some versions of the keto diet are still called the ‘banting diet’. Over the years, various other people have advocated the same approach probably the most famous of which was Atkins, who launched the ‘Atkins diet revolution’ in the 1970s. Although the diet produces good overall results it isn’t very clear why it works. One popular idea to explain its actions is called the ‘carbohydrate-insulin model’ or CIM. 

The CIM suggests that carbohydrates cause obesity because they cause a post meal surge in insulin which promotes the deposition of ingested energy into body fat. That creates a sort of internal starvation signal which is suggested to cause a suppression of metabolic rate and stimulates further intake. By cutting out carbs the keto diet breaks this cycle. The result is no longer a craving for food (hence reduced intake) and an increase in metabolic rate, which causes the spectacular weight loss that some on the keto diet enjoy. The CIM model is consistent with the keto diet effects – but is it correct? 

To answer that question John Speakman of the Shenzhen Institutes of Advanced Technology (SIAT) of the Chinese Academy of Sciences, who has worked extensively on energy balance and diet effects in mice teamed up with Dr. Kevin Hall of the USA NIH who has also done extensive work on energy regulation in humans. They summarize a series of recent experiments which refute most of the predictions of the CIM. They then build on this work to propose an alternative model that preserves some of the aspects of the CIM that seem correct – like the role of insulin in fat deposition.  

Speakman said 'The carbohydrate insulin model was a good attempt to try and explain why we get fat, and why keto diets help reverse it. However, our data in both mice and humans clearly show it is wrong.'

 Speakman J.R. and Hall, K.D. (2021) Carbohydrates, Insulin and Obesity. Science  (May 7^th 2021)