Recent reports have appeared on the occurrence of lactose intolerance and low intestinal lactase activity in otherwise apparently healthy populations. In these individuals, administration of an oral lactose load fails to produce a significant increase in blood glucose concentrations and may be associated with moderate to severe gastrointestinal disturbances. Milk and milk products consumed in quantity may also induce these clinical symptoms of lactose intolerance. Since a high incidence of low lactase activity has been found in American Indians, Mestizo children and in Mexican American adult males, the present study was undertaken to determine lactose tolerance in Mexican American children in San Antonio, Texas, and to correlate this data with selected dietary and anthropometric measurements. After an overnight fast, 282 healthy Mexican American and 51 Anglo American children between the ages of 2 and 14 years were given an oral lactose load of 2 g/kg body weight (up to a maximum of 50 g lactose) as a 20% solution in water. Capillary blood was drawn at 0, 15, 30 and 60 min and assayed for true glucose. A blood glucose rise of 25 mg/100 ml was considered a normal lactose tolerance test. A 24-hour dietary recall was obtained for each child, as were height, weight, head circumference, arm circumference and triceps skinfold measurements. Gastrointestinal symptoms were carefully recorded for a 24-hour period following lactase loading. Overall incidence of lactose malabsorption was significantly greater in Mexican American children than in Anglos. Lactose malabsorption increased with age in both groups. Incidence of symptoms occurring after lactose in lactose malabsorbers also increased with age. Most common symptoms reported in order of frequency were abdominal cramps, bloating and diarrhea. Mean protein intake exceeded the RDA at all ages for both ethnic groups. Mexican Americans consumed less protein, riboflavin, vitamin A, calcium and fewer calories than Anglos. There were no differences in caloric and nutrient intakes or in milk consumption between lactose absorbers and malabsorbers, but Anglos drank significantly more milk than Mexican Americans. Mean height and weight for Mexican American males and females were slightly below the 50th percentile of the Stuart-Meredith standards and slightly above the 50th percentile of the Ten-State Nutrition Surveys means for Mexican American children. Triceps skinfold and head circumference measurements were similar to standards. Fifteen percent of the Mexican American lactose absorbers and 23% of the malabsorbers reported having symptoms after milk ingestion. None of the Anglos recognized symptoms after drinking milk. If lactose malabsorption does have a nutritional effect, the effect is not of sufficient magnitude to be revealed in the growth patterns of otherwise healthy children. Most children consumed small to moderate amounts of milk with no adverse gastrointestinal symptoms. Because milk is a source of high quality protein, calcium, riboflavin and vitamin A milk drinking should be encouraged as tolerated. / Ph. D.