"Although calcium intake is often cited as the most important factor for healthy bones, our study suggests that exercise is really the predominant lifestyle determinant of bone strength in young women," said professor Tom Lloyd from the Penn State College of Medicine.
"There was only a small positive relationship between calcium intake and bone variables, but a significant association between sports exercise score and young adult bone mass and strength," he added.
The study comes as marketers of calcium supplements are enjoying the impact of increased consumer awareness of bone health, and calcium's role in bone strength.
Calcium supplements were worth around $875 million in the US in 2002, about 60 per cent of mineral sales. Mineral sales were up about 9.5 per cent that year, according to Nutrition Business Journal.
The renewed interest may be in part thanks to pharma companies with osteoporosis products, with many launching major marketing campaigns to raise awareness of the disease.
One of these is small UK biotech firm Strakan, which saw sales of its new vitamin D/calcium supplement rise 100 per cent over 2003.
Meanwhile the bone health sector is set for compound annual growth of 7.6 per cent, according to Datamontor research, which claims the category will be worth £86.4 million in the UK alone by 2007.
But the new study, published in the June issue of Journal of Pediatrics, suggests that calcium supplements during adolescence may have little impact on reducing the risk of fracture or future onset of osteoporosis, the world's leading healthcare problem after cardiovascular disease, according to the World Health Organisation.
Studies have shown that most of a female's bone mass is built between the ages of 13 and 15 and then slowly lost in the last four decades of her life. Therefore, attaining optimal bone mass and bone strength in adolescence may offer the best protection possible against osteoporosis, which puts 30-40 per cent of women worldwide at risk of fracture.
The study compared how calcium intake, oral contraceptive use and exercise affected the development of peak bone mass and young adult hip bone bending strength. Bone bending and torsion strength offers a better representation of bone strength than bone mineral density.
The researchers tracked a number of factors, such as body composition and hip bone measurements, in 80 women taking part in the Penn State Young Women's Health Study.
They also used recently developed software to calculate specific measures of bone strength and geometric properties from dual X-ray absorptiometry (DXA) data - considered a better measure of bone strength than bone mineral density.
The team also collected calcium intake information by recording three-day diet records every six months for the first four years of the study and then yearly thereafter.
Physical activity was determined using a sports exercise questionnaire and a cumulative exercise score was calculated from the 10 years of data.
The results revealed no significant relationship between average daily calcium intake and total bone gain from ages 12 to 22 or in young adult hip bone mineral density.
Oral contraceptives also appeared to have no effect on bone density.
However exercise played a major role in bone health. "Our statistical analysis of sport-exercise in adolescence showed that exercise is responsible for between 16 per cent and 22 per cent of the variation in hip bone mineral density and bending strength," Lloyd said.
"Overall, data from the Penn State Young Women's Health Study indicates that daily calcium intake greater than 500 mg/day during adolescence does not result in clinically appreciable increased body bone accrual or increased adult bone mass."
The findings are contrary to the advice of Swiss researchers, who have found a link between calcium supplementation before puberty and earlier menstruation. This could help prevent osteoporosis later on in life, they told a world congress on osteoporosis last month.