Honey is a good way to boost the carbohydrate level of your diet, and is therefore a standard item in many an endurance athlete's store-cupboard. But it can cause gut problems for some.

Honey mainly consists of a mixture of sugars, together with various trace elements. The sugars are glucose, fructose and sucrose. All brands of honey contain fructose at higher levels than glucose. Over the past 10 years, information has been accumulating to suggest that many people have problems digesting fructose adequately. Fructose that isn't absorbed from the small intestine into the bloodstream passes on into the large intestine, where it is consumed by the resident bacteria. The end result? Wind and diarrhoea

Greek researchers recently undertook the first randomised investigation of incomplete carbohydrate absorption after eating honey. Twenty students and staff from Athens University were given differing amounts of honey after an overnight fast. Breath hydrogen measurements were used as a way of diagnosing whether the carbohydrate had been completely absorbed or not; subjects were also asked to report any intestinal discomfort or diarrhoea after eating the honey

When given a 100g dose (equivalent to 3 tablespoons), 65 per cent of people showed signs of incompletely absorbed carbohydrate, and 30% reported loose stools. For a 50g dose, 40% didn't fully absorb the sugars, and 15% reported loose stools. The authors cheerily conclude that honey and water, a traditional Greek remedy for constipation, should be tested on patients.

For athletes with the opposite problem, however - eg, runners' diarrhoea - it could be worth cutting out honey and seeing if there is any improvement in symptoms. If you are fructose-sensitive, cutting out sources of excess fructose should show an immediate benefit. Apples, pears, and their juices also have a high level of excess fructose

('Honey may have a laxative effect on normal subjects because of incomplete fructose absorption', Ladas et al, American Journal of Clinical Nutrition vol 62, pp1212-5)