May 7 (HealthDay News) -- Children with cystic fibrosis (CF) have early onset of defective bone mineralization in the lumbar spine, the part of the back between the ribs and pelvis, French researchers say.

The problem is not caused by lung disease or poor nutrition, the team added.

CF is a hereditary disease in which the lungs and digestive tract become clogged with mucus. People with CF die at a relatively young age.

The researchers studied 114 children, ages 2 to 18, with CF. They found that the children's ideal weight for height, fat-free mass and fat mass values decreased significantly as they grew older.

Fat-free mass is the total weight of the body (muscle, bone, skin and organs) that is not fat.

The study also identified an increase in pulmonary disease severity, mainly in adolescents.

The finding of reduced bone mineral density in these children suggests an initial CF defect in primary bone, which can be aggravated by additional factors such as low fat-free mass, excessive inflammation, poor nutritional status and low vitamin D levels, the researchers said.

"Defective bone mineral density is becoming an increasingly important clinical issue in adult patients with CF," study author Dr. Isabelle Sermet-Gaudelus said in a prepared statement. She added that much less is known about the illness' effect on children's bones.

The findings were published in the first issue for May of the American Journal of Respiratory and Critical Care Medicine.