Fears that Atkins-type diets could increase the risk of bowel cancer have been highlighted by a scientific study.

In the past nutritionists have raised concerns about low carbohydrate diets because they have low contents of fruit and vegetables which help protect against a number of diseases and cancers.

Now a team at the Rowett Research Institute in Aberdeen has discovered that the diets lead to a four-fold reduction in bacteria which keep the gut healthy and fight colorectal cancer. These bacteria produce a substance called butyrate, which has been shown to be important for keeping the gut healthy including helping to prevent, colorectal cancer.

Very low-carbohydrate weight-loss diets - the so-called Atkins-type diets - are popular with people struggling to lose weight and are used in some weight-loss clinics. Although the fears about their low fruit and vegetable content have been well-aired, less attention has been paid to the consequences on the bacteria and how this might alter the release of beneficial or harmful compounds from the food.

For the study, 19 healthy, obese men were given three diets containing different levels of carbohydrate (high, medium and low). Those on the high diet consumed 400g of carbohydrate daily - the level in an average diet. On the low diet, the men consumed only 24 grammes a day - a normal amount in low-carbohydrate diets. They saw a four-fold drop in the level of the cancer-fighting bacteria.

"The changes in butyrate production that we observed in this study are the largest ever reported in a human dietary trial," said Professor Harry Flint, who led the research at the Rowett Institute.

"In the long run, it is possible that such diets could contribute to colorectal cancer. It is a preventable disease, and there is evidence that poor diet can increase your risk."

Typically, the diets begin with a drastic cut in carbohydrate intake, including foods such as potatoes, fresh dairy products and alcohol. This lasts around two weeks, and most report a huge drop in weight at this time.

Carbohydrate levels are then slowly increased over the following weeks. This, according to Professor Flint, is when the situation can become dangerous. Dieters may be tempted to keep the level low and risk a greater likelihood of bowel cancer.

He said: "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.

"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."

He said that although men were used in the research the results were likely to be the same for women.