A study in nonhuman primates with implications for adolescent girls showed that moderate deficiency of the trace mineral zinc in the diet can limit adolescent growth and bone mass, that growth restriction occurs without a decrease in food intake, and that growth slows before reduced zinc concentrations appear in the blood. The research is published in the September issue of the American Journal of Clinical Nutrition.
"This data on bone mineralization in zinc-deprived adolescent monkeys could be very relevant to the long-term health concerns of women," said senior author Mari S. Golub, Ph. D., Adjunct Professor of Internal Medicine, University of California, Davis, who, with 7 associates, conducted the research which was published in the monthly peer-reviewed journal of the American Society for Clinical Nutrition.
"Peak bone mass is attained by females during adolescence," said Dr. Golub, "and the extent of the increase in bone density during this time can be directly related to the occurrence of osteoporosis in later life. In girls, peak bone mass is reached 2 years after first menstruation (menarche) and bone mineralization slows dramatically after that. Our data show less bone mineralization in zinc-deficient monkeys in the post- menarchal period than controls, with substantial differences recorded in the lumbar spine."
According to Dr. Golub, periods of rapid growth such as the third trimester of pregnancy, infancy, and adolescence are particularly vulnerable to zinc deprivation.
Zinc is a trace mineral needed for growth, wound healing, healthy skin, normal levels of vitamin A in the blood, and as a helper to certain enzymes in the body. Rich sources of zinc include shellfish, meat, eggs, nuts, milk, peanut butter, and whole grains. The current recommended dietary allowance (RDA) for zinc in adolescent girls is 12 milligrams per day. Recent data indicate that 81% of adolescent girls consume less than the RDA, with 59% below 77% of their RDA.
Dr. Golub and her fellow California Primate Research Center investigators evaluated 10 female rhesus monkeys and 10 matched control primates from the onset of puberty (18 months of age) through the adolescent growth spurt and menarche (27 to 33 months of age); subsets of 4 test primates and 4 control animals were followed to the age of full reproductive maturity (34 to 45 months of age). The test period in primates was equivalent to ages 10 to 16 in girls.
"The zinc deprived diet we used was 2 micrograms of zinc per gram of food," said Dr. Golub. "It would be termed 'moderate' because clear signs of zinc deficiency only appeared in association with rapid growth."
"Although zinc has frequently been identified as a limiting factor in linear growth in infants and children," she said, "that conclusion was based primarily on nutritional rehabilitation studies rather than on an association with inadequate dietary intake near the time of growth restriction.
"In our study," she continued, "we recorded slower skeletal growth and bone mineralization for the zinc-deprived monkeys in the post-menarchal period, along with delay in some indicators of sexual maturity."
According to Dr. Golub, a comparison of plasma zinc levels during different adolescent ages of the monkeys showed that values were similar in the two groups before the growth spurt but were lower in the zinc-deficient group during the growth spurt.
The primate investigator said that the overall gain in bone length for the tibia and ulna from ages 18 to 45 months was significantly lower in the zinc-deficient monkeys. Also, data showed the zinc-deficient animals had significantly lower skeletal maturation scores at 33 and 42 months.
In an editorial on the study in the same issue of the AJCN, Janet C. King, Ph.D., of the Department of Nutritional Sciences, University of California, Berkeley, said that "although there is no doubt dietary calcium is a factor in influencing skeletal development, focus on that nutrient alone may be too simplistic."
She observed: "Osteoporosis is a major public health problem in the United States affecting 25 million people and costing society an estimated $10 billion annually. Because the disorder is easier to prevent than to treat, the goal should be to ensure that each individual reaches their potential peak bone mass through regular exercise and good dietary habits. The study reported by Golub and coworkers, as well as other research, showed that intake of zinc should be considered along with vitamin D, protein, magnesium, fluoride, and other trace elements. Nutritional surveys show that the zinc intake of adolescents is frequently below the recommended standard and intakes in 1976 - 1980 tended to be higher than those observed in more recent surveys. Future efforts to achieve peak bone mass in adolescence and thereby reduce the risk of osteoporosis in later life should promote the intake of foods rich in zinc as well as other nutrients."
The operation of the California Primate Research Center and the research of Dr. Golub and her associates are supported by grants from the National Institutes of Health.