Depression may increase the risk of the bone disorder osteoporosis in premenopausal women, a study suggests.
A US study found 17% of depressed women but just 2% of those not depressed, had thinner bone in a part of the hip.
It found depressed women had overactive immune systems, making too many chemicals that promote inflammation including one that promotes bone loss.
The Archives of Internal Medicine study compared 89 depressed women with 44 non-depressed women, all aged 21 to 45.
Osteoporosis affects half of all women, and one in five men, over the age of 50.
It is estimated to cause 60,000 broken hips each year in the UK, costing the NHS ?1.73bn.
After bone mass reaches its peak in youth, bone-thinning continues throughout life, accelerating after menopause.
Hip bones are among the most vulnerable to fracture in osteoporosis patients.
The researchers, from the National Institute of Mental Health, found these bones were particularly susceptible to thinning in depressed premenopausal women.
Dr Richard Nakamura, NIMH deputy director, said: "Osteoporosis is a silent disease. Too often, the first symptom a clinician sees is when a patient shows up with a broken bone.
"Now we know that depression can serve as a red flag - that depressed women are more likely than other women to approach menopause already at higher risk of fractures."
Other risk factors for osteoporosis - such as calcium and alcohol intake and contraceptive use - were similar in the two groups.
The depressed women were taking anti-depressant medications, which have previously been linked to an increased risk of fracture.
However, the current study found no link between these drugs and bone-thinning.
The researchers found 17% of the depressed women had thinner bone in a vulnerable part of the hip called the femoral neck, compared with just 2% of those who were not depressed.
Low bone mass in the lumbar spine, in the lower back, was found in 20% of depressed women, but in only 9% of non-depressed women.
The level of bone loss was at least as high as that associated with known risk factors such as smoking, poor diet and lack of exercise.
Blood and urine samples revealed that the depressed women had overactive immune systems, producing too many chemicals which promote inflammation, and not enough that keep it in check.
One of these chemicals - a protein called IL-6 - is known to promote bone loss.
The National Osteoporosis Society said much was still unknown about the disease, and said it would monitor further developments closely. ( BBC )