New research has discovered a link between consuming carbohydrate and the production of a fatty acid in the gut, which protects against colorectal cancer.
The acid, called butyrate, is produced by bacteria and helps kill off cancerous cells.
But researchers from Aberdeen's Rowett Research Institute found low-carbohydrate regimes -- such as the popular Atkins Diet -- can cause a four-fold reduction in the cancer-fighting bacteria.
Professor Harry Flint, who led the research, said, "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 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.
In the study at the Rowett Institute, 19 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 24g a day -- a normal amount in low-carbohydrate diets.
The second group saw a four-fold drop in the level of the cancer-fighting bacteria.
Flint said, "The changes in butyrate production that we observed in this study are the largest ever reported in a human dietary trial."
He added it was likely the results would be the same in women.
Low-carbohydrate diets have long been controversial. Criticisms include increased risk of heart disease, increased risk of the bone disease osteoporosis in women, and high cost.
But supporters point to several studies which show the diet's effectiveness when compared to other approaches to weight loss.
The low-carb craze took off in the 1990s. Dr Robert Atkins published his book Dr Atkins New Diet Revolution and other health experts soon followed suit.
From the July 2, 2007, Prepared Foods e-Flash