13C-carbohydrate breath tests: impact of physical activity on the rate-limiting step in lactose utilization.
BACKGROUND
13CO2 breath tests can be used to monitor carbohydrate digestion in the small intestine. However, after ingestion of 13C-substrates, 13CO2 excretion in breath originates from two sources: a digestive/oxidative fraction, derived from the small intestine, and a fermentation fraction, derived from undigested substrate spill-over in the colon. In this study, the determinants of the digestive/oxidative fraction were analysed in order to improve the sensitivity/specificity of the 13C-carbohydrate breath tests.


METHODS
13C-carbohydrate breath tests were performed in healthy adults using 13C-lactose, pre-digested 13C-lactose, 13C-glucose, and 13C-galactose as substrates. The effect of exercise (bicycling, 50 W), increasing the metabolism of digested/absorbed substrate, on the outcome of the test was analysed.


RESULTS
In rest, no difference was observed in the 4-h cumulative percentage dose recovered in breath (4-h cPDR) after administration of glucose, pre-digested lactose, and lactose, which were 20.3 +/- 4.5%, 19.2 +/- 5.5%, and 19.9 +/- 4.9%, respectively. The 13CO2 excretion rate after 13C-galactose consumption was significantly slower than after 13C-glucose consumption. Exercise increased 4-h cPDR of 13C-glucose significantly: 76.0 +/- 1.0% vs. 22.7 +/- 2.3%. This effect was also observed using 13C-lactose as substrate: 66.1 +/- 6.2% vs. 19.6 +/- 3.9%. One subject had non-symptomatic lactose maldigestion indicated by a positive H2 breath test. The 13CO2 breath test of this subject in rest was indistinguishable from that of the others (4-h cPDR 16.6 vs. 19.6 +/- 3.9%), whereas the test was clearly indicative during exercise (4-h cPDR 20.5 vs. 66.1 +/- 6.2%).


CONCLUSIONS
In healthy volunteers in rest, glucose oxidation is the rate-limiting step in lactose conversion into 13CO2. Increase of metabolism (for instance, by exercise) can shift this step to intestinal hydrolysis of lactose, making the 13C-lactose breath test more sensitive.
Stellaard
F
F
Dept. of Paediatrics, University Hospital Groningen, The Netherlands.
Koetse
H A
HA
Elzinga
H
H
Boverhof
R
R
Tjoonk
R
R
Klimp
A
A
Vegter
D
D
Liesker
J
J
Vonk
R J
RJ
D000328:Adult; D001944:Breath Tests / Q000379:methods; D050260:Carbohydrate Metabolism; D002241:Carbohydrates / Q000008:administration & dosage; D002245:Carbon Dioxide / Q000032:analysis*; D002250:Carbon Radioisotopes; D002849:Chromatography, Gas; D015444:Exercise; D005260:Female; D005690:Galactose / Q000008:administration & dosage / Q000378:metabolism; D005947:Glucose / Q000008:administration & dosage / Q000378:metabolism; D006801:Humans; D007421:Intestine, Small / Q000378:metabolism; D007785:Lactose / Q000378:metabolism*; D008297:Male; D011336:Probability; D012680:Sensitivity and Specificity
D002241:Carbohydrates; D002250:Carbon Radioisotopes; D002245:Carbon Dioxide; D005947:Glucose; D007785:Lactose; D005690:Galactose