Having too little calcium in the diet increases women's risk of a hormone condition that can cause bone fractures and kidney stones, scientists suggest.

Primary hyperparathyroidism (PHPT) affects around one in 800 people during their lifetime and is most common in post-menopausal women.

Writing in the British Medical Journal, the team suggest increasing calcium intake cuts the risk of the disease.

Adults need around 700mg of calcium a day.

Milk and other dairy foods, nuts and fish such as sardines and pilchards (where the bones are eaten) are some dietary sources of calcium.

Taking too much could cause stomach pains and diarrhoea.

PHPT is caused by overactive parathyroid glands secreting too much parathyroid hormone.

As well as bone and kidney problems, there have also been suggestions it is linked to an increased risk of high blood pressure, heart attack and stroke.
Benefits v risks

The US team from Brigham and Women's Hospital looked at 58,300 women who were taking part in a much broader ongoing piece of research called the Nurses' Health Study.

All were aged between 39 and 66 in 1986, when the study began, and had no history of PHPT.

The women have completed food questionnaires to record how frequently they ate particular foods or supplements - including calcium - every four years, with the latest being completed in 2008.

Over that 22 year period, 277 cases of PHPT were confirmed.

The researchers divided all the women into five groups , depending on the calcium intake.

They accounted for factors such as age, body mass and ethnicity.

It was found those with the highest intake of dietary calcium had a 44% reduced risk of developing PHPT compared with the group with the lowest.

Writing in the journal, the team led by Dr Julie Paik, said: "Increased calcium intake, including both dietary and supplemental calcium, is independently associated with a reduced risk of developing primary hyperparathyroidism in women."