Drinking milk has long been promoted as positive for building stronger bones. But it seems drinking milk as a teenager does not reduce the risk of hip fractures later in life and can even increase the risk for men, according to new research published in JAMA Pediatrics.
The 2010 Dietary Guidelines for Americans from the US Department of Agriculture and the Department of Health and Human Services recommend consuming three cups of milk a day or equivalent milk products as a good way of getting a dietary intake of nutrients, such as calcium and vitamin D.
Calcium is a chemical element that is known to build and maintain strong bones. Researchers from Brigham and Women's Hospital, led by Diane Feskanich, say that drinking milk during adolescence is recommended in order to achieve optimum bone mass, and it is expected to help reduce the risk of hip fractures and osteoporosis later in life.
However, they note that evidence of these associations has been unclear, and that higher milk consumption is known to contribute to height growth - a risk factor for hip fractures.
With these points in mind, the investigators analyzed more than 96,000 men and women from the Nurses' Health Study and the Health Professionals Follow-Up Study in order to determine whether teenage milk consumption has any effect on hip fractures later in life.
Participants from the Nurses' Health Study reported their milk consumption as teenagers in 1986, while those from the Health Professionals Follow-Up Study reported theirs in 1988.
All study participants were followed for more than 22 years. During this period, women reported 1,226 hip fractures while men reported 490.
'Increased risk of 9%' for every glass of milk
Of those who consumed four or more glasses of milk a day as teenagers, men were on average 1.9 cm taller as adults than men who consumed two or fewer glasses a week, while women were 1.7 cm taller.
Overall, men reported higher consumption of milk in their teenage years than women, with 2.1 glasses per day, compared with 1.6.
However, milk consumption in men between the ages of 13 and 18 was associated with higher risk of hip fractures later in life, and every additional glass of milk each day increased this risk by 9%.
The researchers note that the increased risk of hip fractures in men was partially influenced by their height.
Explaining their findings, the researchers say:
"Milk consumption in early life not only builds bone mass but also increases height, and height is a risk factor for hip fractures in later life.
This was true in our cohorts, where risk was increased by approximately 5% per centimeter. A mediating effect of height can partially account for our observed positive association between teenage milk consumption and hip fractures in men, as risk was attenuated when height was added to the model."
The investigators found no association between milk consumption in women and increased risk of hip fractures.
Concluding their study, the researchers write that higher milk consumption during teenage years did not translate into a reduced risk of hip fracture for older adults, and a positive association was observed in men.
From these findings, they note that further research is warranted to "clarify the role of early milk consumption and height in prevention of hip fractures later in life."
Late last year, Medical News Today reported on a study suggesting that two cups of milk a day is enough to provide children with the right amount of nutrients.