Background:The ability of breast-feeding infants to utilize lactose, the major carbohydrate in breast-milk, is dependent on the presence of the enzyme lactase (E.C.3.2.1.108). Lactase is located in the brush border of the small intestine and because of its exposed position it is extremely vulnerable to pathogenic damage. Breast-fed Gambian infants have poor growth associated with intestinal damage beyond 3-4 months. The aim of this study was to assess the ability of Gambian infants aged 2-15 months (N = 113) to digest lactose and to see how this varied with age, intestinal permeability, and growth performance.Methods:Lactose maldigestion was estimated by monthly measurements of urinary lactose and lactulose following an oral dose of the latter.Results:Both urinary lactose excretion and lactulose maldigestion increased with age (p < 0.0001 ANOVA). Up to 6 months the mean urinary lactose: lactulose excretion ratio was within the quoted normal range (< 0.4). Beyond this age, mean values were hypolactasic. Lactose maldigestion was related to poor growth in both weight and length (r =-0.04, p < 0.0001, after age correction). Althogh a major part of this relationship was a reflection of the previously reported correlation between intestinal permeability and growth, more than 30% of the association was in addition to the permeability effect. Possible explanations are discussed.Conclusions:Moderate-to-severe hypolactasia does occur in breast-fed Gambian infants and is related to poorer-than-expected growth. However, this does not mean that breast milk intake should be reduced as the nutritional and immunological benefits of breast milk continue to outweigh any disadvantages.