Low carb diets lower levels of good bacteria in gut

Scientists at Aberdeen's Rowett Research Institute have shown that a very low carbohydrate weight-loss diet results in a four-fold reduction in the numbers of certain types of bacteria in the gut of obese men.

This is a significant finding because these gut bacteria produce a substance called butyrate, which has been shown to be important for keeping the gut healthy including helping to prevent colorectal cancer. The study raises questions about the impact of the prolonged use of very low-carbohydrate weight-loss diets on gut health.

Very low-carbohydrate weight-loss diets (such as the so-called ‘Atkins-type diets) are popular with people struggling to lose weight and are used in some weight-loss clinics. Nutritionists have raised concerns about the low fruit and vegetable content of such diets as these contain nutrients that help protect against a number of diseases and cancers within the body. Less attention has been paid to the consequences of the low carbohydrate intake on the bacteria within the gut and how this might alter the release of either beneficial or harmful compounds from the food.

In this study, 19 healthy, obese men were given three diets containing different levels of carbohydrate (high, medium and low). Two of the diets also contained a high proportion of protein, as this is known to help reduce appetite and is used in a number of diets that help produce weight loss. Indeed, the volunteers lost similar amounts of weight and body fat on these two diets. Stool samples were analysed for the amount and type of bacteria, and for butyrate.

“The changes in butyrate production that we observed in this study are the largest ever reported in a human dietary trial. The results provide strong evidence that butyrate production is largely determined by the content of a particular type of carbohydrate in the diet that the bacteria in our guts can utilise,” said Professor Harry Flint who led the research at the Rowett Institute.

“We can’t be sure from this study about the impact of butyrate production on gut health, but there has been quite a lot of work done which shows that butyrate stops cancer cells from growing, and so helps prevent colorectal cancer.

“If low carbohydrate diets are to be consumed for long periods of time, it may be important to ensure that there is enough of the right sort of carbohydrate in the diet which can be used by the bacteria to produce compounds such as butyrate, which are beneficial for human health. This means making sure you continue to eat plenty of sources of fibre – such as fruit and vegetables,” said Professor Harry Flint.

The work is published in Applied and Environmental Microbiology, 73:1073-8.