Drinking milk as a teen doesn't necessarily prevent hip fractures later in life, according to a U.S. study that raises questions about conventional wisdom for bone health.

Having three glasses of milk a day or equivalent dairy foods has long been recommended for children and adolescents to build up bone reserves. It was assumed that having more bone mass in adolescence could help prevent osteoporosis and bone fractures later in life.

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A dietetic intern talks to a group of high school students about how to make nutritional food choices. Drinking more milk earlier in life may not help to avoid hip fractures later in life. (Mel Evans/Associated Press)

Diane Feskanich, an assistant professor at Harvard Medical School in Boston, and her team tested the assumption by looking at hip fractures among more than 61,000 women and 35,000 men over 22 years.

"We questioned the belief that drinking more milk in earlier life would help to avoid these fractures in older adults," Feskanich said in an interview.

In Monday's issue of the journal JAMA Pediatrics, Feskanich and her co-authors said milk consumption during teenage years was not associated with a lower risk of hip fracture in the group of older adults they studied.

The researchers analyzed findings from white participants from the Nurses' Health Study and Health Professional Followup Study.

Health Canada recommends 1,300 milligrams of calcium for children aged nine to 18.

"It does make you stop and ponder and want to see better evidence for our dietary recommendations," Feskanich said of the study's findings.

During followup, they identified 1,126 hip fractures in women and 490 in men. More than 90 per cent of the fractures occurred when tripping or falling from the height of a chair. Hip fractures are considered an important measurement because they are so costly in health terms.

Since greater height is also a risk factor for hip fracture, the study's authors said more research is needed to better understand the role of milk consumption early in life, height and hip fracture prevention in older adults.

Taller people are more likely to experience a hip fracture, but whether it is because taller people are more likely to fall, or whether once they fall they are more likely to fracture, is unclear, Feskanich said.

The study's authors noted it isn't calcium alone in milk that contributes to bone development. Fortified milk is a good source of vitamin D, which is important for strengthening bones around puberty, and also contributes protein and other nutrients.

Weight-bearing activities that are done on your feet and work against gravity are also recommended for bone health at all ages.

One of the study's limitations is the potential for error from asking participants to recall their milk consumption during their teenage years. The researchers didn't ask about preteen consumption but said that could be more relevant for females since girls reach their peak height about two years before boys. 

A journal editorial accompanying the study pointed to studies suggesting higher rates of fracture in milk avoiders than milk consumers. 

"Practically speaking, does the study by Feskanich and colleagues offer a solution to osteoporosis? Without dairy, dietary quality is compromised," concluded Connie Weaver of the department of nutrition science at Prude University in West Lafayette, Ind. 

The study was funded by the U.S. National Institute on Aging and the U.S. National Cancer Institute.