Do fad diets cause bowel cancer?

Do low-carb dieters face a higher risk of bowel cancer? Jacky Law investigates.

What is the story?
Low-carbohydrate diets such as the Atkins diet could increase the risk of colon cancer, according to media reports.

Scottish researchers have discovered a link between eating less carbohydrate and lower levels of a cancer-fighting chemical found in the gut called butyrate.

The researchers found that drastic cuts in carbohydrate intake prompted a four-fold reduction in butyrate production. This fatty acid has been shown to reduce the risk of colon cancer in rats.

The low-carbohydrate craze first took off in the late 1990s following the publication of the Atkins diet. While nutritionists have raised concerns in the past about the health consequences of increased protein or fat, few studies have been carried out to show how a low-carbohydrate diet can affect microbial activity in the gut.

What is the research?
The findings come from research conducted at Aberdeen's Rowett Research Institute.

For the study, the researchers gave 19 healthy but obese men, with a BMI of between 30 and 42, three diets containing different levels of carbohydrate.

The men started the study with three days on a high carbohydrate diet containing around 400g a day, which is the amount needed to maintain their weight.

This was reduced to a medium carbohydrate diet of 160g a day for four weeks, before the final stage when they were required to eat just 24g of carbohydrates a day for a further four weeks.

The lowest regime contains around the same level of carbohydrates as that of someone just starting the Atkins diet.

At each stage of the nine-week study, the stools of the participants were analysed for the amount and type of bacteria present and for butyrate.

After four weeks on the lowest 'Atkins' level of carbohydrates, the researchers registered a four-fold fall in the level of butyrate.

What do the researchers say?
Lead researcher Professor Harry Flint said the changes in butyrate production were 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,'he said.

'But this doesn't automatically lead to the conclusion that reduced butyrate production causes colon cancer.'

However, Professor Flint said that previous research had shown a link between butyrate production from dietary fibre and large bowel cancer in rats.

'Studies in cell culture have also suggested a link between butyrate and colon cancer,' he said.

'This study is part of a general enquiry into how to prevent obesity in humans.

'Obesity is one of the top health issues in society and the Atkins diet has been shown to be highly effective in helping people lose weight,' he said.

The study was designed to investigate why substituting protein for carbohydrate makes people feel full and whether such an extreme low-carbohydrate intake is actually needed to achieve the weight loss, said Professor Flint.

'We would like people to get the best of both worlds. That means knowing in greater detail what goes on in the gut when on a low-carb diet.'

Professor Flint added that the study would produce more results, which would be published to help give a fuller picture.

Short-term bursts on Atkins-type diets were not bad. But longer term, people could be storing up health problems, he warned.

'It should be possible to lose weight by taking out sugar and starch and maintaining some of the fibre that supports bacterial activity in the intestine.'

What do other experts say?
Professor Annie Anderson, from the department of epidemiology and public health of Ninewells Medical School, University of Dundee, said: 'Compared to other low-fat diets, there is little merit in low-carbohydrate diets, apart from the fact that that they can help people to lose weight.

'There are no long-term benefits of cutting down on fruits and fibre and, as this report shows, doing so is likely to have a negative impact on your bowel health and may increase your risk of bowel cancer.'

Informing patients

  • Low-carbohydrate diets such as Atkins can lead to a four-fold reduction in a fatty acid thought to have a role in preventing colon cancer.
  • Short two-week bursts on Atkins-type diets are fine. It is long-term deprivation of carbohydrate that causes the damage to the gut.
  • Further research is required to show that butyrate can cause colon cancer in humans.
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