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A team from the Rowett Research Institute in Aberdeen followed up 19 obese men while they tried a series of different weight loss diets varying in their carbohydrate content (399 g carbohydrate/day; 164 g/day or 24 g/day). While the volunteers lost similar amounts of weight on the high protein versions regardless of carbohydrate content, differences in the health of their gut bacteria were apparent. When the men followed the very low carbohydrate diet, their feces contained 4 times fewer 'friendly' bacteria than when they followed the higher carbohydrate diet. 'Friendly', or probiotic, bacteria such as Lactobacillus and Bifidobacteria, are believed to release beneficial chemicals into the gut that may help prevent bowel cancer. Examples of such chemicals are butyrate and propionate.

Lead researcher, Dr Harry Flint, commented: "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".

For more information, see
Duncan SH et al (2007) Reduced dietary intake of carbohydrates by obese subjects results in decreased concentrations of butyrate and butyrate-producing bacteria in feces. Applied and Environmental Microbiology, Vol 73, pg 1073-8.

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