Because mucosal glucoamylase is most active against glucose polymers less than 10 glucose units in length, longer chain polymers may not be completely absorbed by young infants. In order to investigate this possibility, the absorption and oxidation ofI3C-rich glucose, short-chain (3 to 8 glucose units in length) and long-chain (average length 43 units) glucose polymers (GP) were compared in 12 healthy, 1-month-old infants. Recovery of the GP and glucose in stool was measured by determining theI3C enrichment of stool. The oxidation of the GP was measured by tracing the increase in breath13CO2after GP were fed. Carbohydrate malabsorbed in the small bowel was assessed by measurement of breath H2, a gas formed from the fermentation of carbohydrate in the colon. Analysis of the infants' stools revealed that one infant excreted 9.7% of the dose of glucose, another 6.7% of the dose of short-chain GP, and five infants excreted 2.6 to 18.5% (mean 8.4%) of the dose of long-chain GP. The percent of the administered dose recovered in breath was similar among substrates (mean=28.7% of the dose fed). A rise in breath H2greater than 20 ppm was found in four of the 12 infants after the feeding of glucose, in five of 12 after the short-chain GP, and in six of 12 after the long-chain GP. None of the infants developed diarrhea. The results suggest that healthy young infants do not absorb long-chain GP as completely as they absorb short-chain GP. In the absence of pancreatic amylase, salivary amylase and mucosal glucoamylase are sufficient in some young infants to allow for complete digestion of long-chain GP.