C carbohydrate breath tests

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 noninvasive 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. 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 C lactose and measured the 13CO2 excretion in breath to diagnose adult hypolactasia patients. 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 C lactose. Combination of the H2 / 13CO2 test results produced a higher sensitivity (85%) and a lower specificity (65%) than the outcome of both tests separately. The use of a combined H2/ 13CO2 breath test can reduce the number of jejunal biopsies needed for diagnosis of hypolactasia, without significant loss of discriminative power. The same group (Stellaard et al, abstract 2) further analysed the factors involved in the 13CO2 excretion in breath after lactose administration. By comparing the 13CO2 excretion rate after administration of C lactose, pre-digested C lactose, C glucose, and C galactose, the rate limiting step in C lactose use was investigated. Exercise increased four hours cumulative percentage dose recovered (cPDR) significantly. It was concluded that in healthy volunteers, glucose oxidation is the rate limiting step in the overall process of lactose use. In mild lactase deficient subjects, exercise shifts the rate limiting step to intestinal hydrolysis of lactose. In hypolactasic patients the rate limiting step is at the level of intestinal hydrolysis of lactose. Fructose ingestion may lead to diarrhoea and abdominal pain in susceptible individuals. By means of a breath H2 test, it has been shown that the absorption capacity of fructose is limited in adults as well as in children. 8 Interestingly, both the percentage of malabsorbers and the peak breath H2 increases were higher in toddlers compared with younger and older children. The relation of apple juice consumption to chronic non-specific diarrhoea (toddler’s diarrhoea) has been well established and attributed to the high fructose content of apple juice. Fructose absorption is facilitated by equimolar doses of glucose and amino acids (especially L-alanine), the mechanism underlying this eVect remains unclear. To study fructose absorption in a more direct way, Hoekstra combined breath H2 studies with breath 13CO2 studies (abstract 3). The results, however, did not clarify the mechanisms behind this stimulating eVect of glucose and amino acids on intestinal fructose absorption. Variation in 13CO2 output in breath can be caused by changes in oxidation rate, absorption rate or, in the case of colonic fermentation, colonic production rate. Without the knowledge of the rate limiting step in the overall utilisation process, the use of the 13CO2 excretion rate in breath for mechanistic interpretation is diYcult. Starch digestion is aVected by several factors. One of these is the physicochemical characteristics of the starch granules which can be influenced by pre-treatment such as cooking and heating and hydrolysis of the starch molecule by the enzyme “á-amylase”. To monitor the digestion rate of corn starch, which is naturally enriched in C, the 13CO2 breath test can be used, as first described by Hiele et al. Especially in the study of the small intestinal digestion of resistant starch (Hylon VII and Novelose), this can be helpful to characterise the digestibility of the product as shown by Vonk et al (abstract 4). The six hour cPDR of raw Hylon VII and raw Novelose is 16.6 (SD 6.0) % and 15.5 (SD 6.0) % respectively. The six hour cPDR of glucose is 30.5 (SD 4.9) %. If we assume that 100% of the glucose is absorbed and the post-absorption metabolism of glucose from glucose itself or from starch is the same, then the six hour cPDR can be compared and used as an index of intestinal absorption. When these data for Hylon VII and Novelose are calculated in this way, both types of resistant starch are digested for 54% and 51% respectively in the small intestine. Colonic 13CO2 excretion, which is defined as 13CO2 output in breath which parallels H2 excretion, starts after about eight hours, which implies that in the time period in which digestion is studied, colonic metabolism does not play a major part in 13CO2 production. Gut 1998;43(suppl 3):S20–S22 S20