The findings, published in the Journal of Clinical Endocrinology and Metabolism (10.1210/jc.2004-0924), are important because it is estimated that 80 percent of all women have used oral contraceptives. If the pills effect on their bone mass is not offset with higher calcium intake in their youth, the peak bone development years, we may see an increase in the incidence of osteoporosis once they reach the menopause. According to the National Osteoporosis Foundation, 10 million Americans suffer from the disease, which causes bones to become fragile and ups susceptibility to fractures, and a further 34 million more have low bone mass, which increases their chances of developing it.

The contraceptive pill was first introduced in 1960, but composition has changed considerably over the last 45 years, most markedly in that dosage is now greatly reduced. More than 45 different brands are now available.

The study group comprised 135 healthy young women aged between 18 and 30 years, all of whom had a dietary calcium intake of less than 800mg per day and 57 of whom reported using oral contraceptives.

These women were randomly assigned to one of three diets: a medium dairy diet, where calcium intake was increased to between 100 and 1100 mg per day; a high dairy diet, with 1200 to 1300 mg of calcium per day; and a control diet, where calcium intake remained below 800mg per day.

A 200ml glass of whole milk contains 243mg of calcium, and the same amount of semi-skimmed milk contains 247mg.

The researchers found that, over the one-year period, an increase in dietary calcium positively impacted the percentage change of total hip body mass density (BMD), body mass content (BMC) and bone area.

Increased intake of dairy products also prevented a negative percent change in total hip and spine BMD in the women taking oral contraceptives.

Overall, the extent of the differences in bone mass with increased dietary calcium noted in this study was between 1 and 2 percent, which translates to a reduced risk of osteoporosis of between 3 and 10 percent over one year. A 5 to 10 percent increase in bone mass is estimated to reduce the risk of fracture by between 25 and 50 percent.

The study authors placed the onus for encouraging young women to up their dairy calcium intake on health care professionals:

Physicians and public health professionals need to encourage young women, particularly those using oral contraceptives, to consume recommended levels of calcium (1000mg/day) in their diets to prevent compromising bone mass.

Those with an aversion or intolerance to dairy products might bear in mind that certain green vegetables, such as broccoli, kale, turnip greens, and bok choy are also good sources of calcium, as are soft-boned fish, dried beans and almonds. Intake can also be increased by taking dietary supplements.