In paediatric practice proper digestion of carbohydrates is of major importance for adequate growth and development and for prevention of intestinal complaints. Several carbohydrates can be considered in this respect: (a) lactose: the main carbohydrate in the nutrition of early life and also in later life for many white people; (b) starch: the main energy supplier in our food; (c) fructose: a constituent of fruit juices and increasingly used as a sweetener; and (d) galactose: a component of lactose. The fate of this sugar will be considered in more detail in section five, dealing with the measurement of liver function.

Lactose maldigestion due to hypolactasia can be diagnosed in several ways. The non-invasive test most commonly used is the breath hydrogen test after lactose provocation. The reliability of this test, however, is disputed owing to non-H₂ producing flora,1 ,2 or extra intestinal influences that can disturb the test. Moreover, this test has to be done with unphysiological high doses of substrate to get a measurable hydrogen response in breath.3 ,4 This prompted investigators to develop diagnostic tools which permit the analysis of lactase enzyme activity non-invasively in a more direct way and therefore hopefully also in a more accurate and quantitative way. Hiele et al used naturally enriched ¹³C lactose and measured the¹³CO₂ excretion in breath to diagnose adult hypolactasia patients.5

In the BIOMED programme SIGN this principle has also been used by Koetse et al (abstract 1) in paediatric patients. In 27 patients, lactase activity in jejunal biopsy specimens were compared with H₂ and ¹³CO₂ breath tests after administration of ¹³C lactose. Combination of the H₂ /¹³CO₂ test results produced a higher sensitivity (85%) and a lower specificity (65%) than the …