Anxiety following increased hind-gut fermentation

Background - Previous investigations into the effects of carbohydrate on behaviour have focussed on behavioural changes 2-4 hrs after consumption of the diet and have not considered the effect of site of digestion. Fermentation and lactic acid production in the caecum and colon can lead to detrimental effects in several animal models, including adverse behaviour in horses. Objective - To determine changes in anxiety promoted by the consumption of fermentable carbohydrate and increased fermentation in the hind-gut of rats. Design - Randomised control trial with 3 iso-energetic dietary treatment groups, a soluble carbohydrate diet containing wheat (n=12), a fermentable carbohydrate diet based on cooked and cooled rice (n=12) and a basal control rat and mouse Chow diet (n=12). Behaviour was assessed 3 and 21 hrs after dietary consumption by the light dark emergence test. Outcomes - The 3 diets promoted different fermentation patterns in terms of pH and lactic acid concentrations in the caecum of rats 3 or 21 hrs after consumption. The length of time spent in the dark compartment of the light dark emergence test, indicating increased anxiety, was associated with increased concentrations of D- and L-lactic acid in the caecum (r2 = 0.97 and 0.96 respectively; P<0.01) irrespective of dietary group. Conclusions - Fermentation of carbohydrate leading to increased concentrations of lactic acid in the caecum of rats was associated with increased anxiety in rats. This has important implications in terms of those diets promoting increased fermentation (eg. with a high intake of resistant starch) without considering any possible detrimental effects.