In the study, 209 seventh graders, 111 girls and 98 boys, consumed four ounces of fruit juice containing either 0, 10 or 20 mg of zinc gluconate each school day for 10 to 12 weeks. Students, their parents and teachers did not know who was receiving which, if any, zinc supplementation until the study was completed. At the beginning and end of the study, students performed a battery of tasks designed to measure mental and motor skills, like attention, memory, problem-solving and hand-eye coordination.
Examples include tapping a key on the keyboard as fast as possible, using a mouse to follow an object moving across the screen, searching a group of objects for two of a kind, learning and remembering lists of words or simple geometric patterns, and categorizing objects. Students, their parents, and teachers filled out questionnaires about the students' mental, physical and social abilities and skills, school performance, and problems in any of these areas to provide a measure of psychosocial function. Blood samples measured zinc status before and after the treatment.
Compared to the students who received no additional zinc, students who consumed an additional 20 mg zinc each day decreased reaction time on a visual memory task by 12 percent versus six percent; increased correct answers on a word recognition task by 9 percent versus three percent; and increased scores on a task requiring sustained attention and vigilance by 6 percent versus one percent. Those who received only 10 mg a day, the current Recommended Dietary Allowances (RDA) for this age group, did not significantly improve performance, however.
Supplementation at either the 10 mg or 20 mg did not appear to improve motor and social skills, although girls receiving the placebo experienced a 10 percent increase in conduct problems during the study while the behavior of girls receiving any level of zinc supplementation remained unchanged. If further studies confirm that the mental function, and in particular memory, of adolescents benefit from increasing zinc intakes, says Dr. Penland, then this and other similar studies would provide information that could be used when revising dietary guidelines for zinc in this age group.
Such guidelines ultimately affect school breakfast and lunch menus, the food guidance system, nutrition labels on food packages, and other uses. Zinc is a common essential mineral found in foods, particularly red meats, fish and grains. Previous studies have shown that zinc is needed for growth and immune function and may be important for eye-hand coordination and reasoning in very young children and in memory, muscle strength and endurance in adults.
Dr. Penland's co-investigators are Dr. Henry C. Lukaski, also from the Grand Forks Human Nutrition Research Center (GFHNRC), and Dr. Jacqueline S. Gray, previously with GFHNRC and now with the Center for Rural Health at the University of North Dakota.