Intake of calcium is important for the development of the skeleton and to reduce the risk of fracture and osteoporosis in later life, and the peak calcium buildup rate is at age 12.5 for girls and 14 for boys.
The article, published in the journal Pediatrics (Vol. 117, pp. 578-585), reported that about 30 per cent of boys and only 10 per cent of girls were achieving the recommended daily intake (RDI) of calcium.
The RDI for 9 to 18 year olds is 1300 mg per day, and yet the average intake in this age group is about 850 mg per day.
"Sub-optimal intakes of calcium in children and adolescents may be related to the replacement of milk intake by soft drinks and fruit juices and/or fruit drinks," wrote the authors.
This dietary change, away from milk products and towards fruit or soft drinks, is due to parents not drinking enough milk themselves, and so being poor role models, say the researchers.
Four eight-ounce glasses of milk could provide the RDI for teenagers, but some adolescent girls appear reluctant to have a large dairy intake because of misconception that dairy products are fattening.
"For children and adolescents who cannot or will not consume adequate amounts of calcium from preferred dietary sources, the use of calcium supplements should be considered," said the researchers.
The researchers stressed however that a calcium supplement does not offer the benefits of the other 15 minerals and vitamins that a glass of milk could provide, including vitamin D which is well established to aid calcium absorption from the diet.
Recent studies have suggested that calcium supplements could increase bone mineral content of children, but short-term supplementation of up to two years did not indicate any long-term benefits (Nutrition Reviews Vol 58, pp 253-268).
If calcium supplements are to aid bone mineral content, current research suggests their use during the teenage years. A recent study in the Journal of Nutrition (Vol. 135, pp. 2362-2366) claimed that calcium supplementation of women aged 20 to 25 had no measurable effect on bone density.
Osteoporosis affects 55 per cent of the US over-50 population and direct costs are estimated at $18 billion (2002). Eighty per cent of osteoporosis sufferers are women.