Just Being Naturally Thin May Raise Risk Of Osteoporosis In Women

Constitutional thinness refers to young women with no identified eating disorder who have a low body mass index (<16.5 kg/m²) yet continue to have a close-to-normal fat mass percentage, normal physiological menstrual cycles, and normal energy metabolism.

“Constitutional thinness is such a rare entity that subjects are frequently misdiagnosed as anorexics and socially stigmatized,” said Bruno Estour, M.D., of Centre Hospitalier Universitaire de Saint-Etienne (CHU) in Saint Etienne, France. “Research has been severely limited in this area. Until now, low bone density related to low body weight in young women has been described only in patients with anorexia nervosa.”

This study followed 25 constitutionally thin and 44 anorexic young women ages 18 to 30. Femoral neck and lumbar spine bone mineral density were measured by dual energy X-ray absorptiometry (DXA) while distal radius and distal tibia were evaluated by three-dimensional peripheral quantitative computed tomography. Fat and lean body mass were determined using the same DXA device.

“Almost fifty percent of anorexic patients present with a decreased bone mass and a very increased fracture risk, explained by the multiple hormonal and nutritional abnormalities,” said Estour. “In constitutionally thin young women we found an unexpected similar percentage of low bone mass difficult to explain in a context of hormonal, energetic, and bone turnover normality.”

Osteoporosis is mostly found in women following menopause. However, young women have an equivalent degree of osteoporosis when their bone mineral density (BMD) falls within a certain range. Researchers used a manufacturer-supplied reference dataset of healthy young adult female BMD values and identified a Z-score (a score expressed in standard deviation units from a given mean of age-matched controls) < -2.0 as an equivalent of osteoporosis. In this study 44 percent of constitutionally thin subjects presented with a Z-score < -2.0.

Estour and his colleagues hypothesize that mechanisms related to genetics and/or insufficient load on key weight-bearing bone regions may be responsible for impaired bone quality in constitutionally thin young women.

Other researchers involved in this study include Bogdan Galusca, Natacha Germain, Cecile Bossu, Delphine Frere, and Francois Lang from CHU Saint Etienne in France, and Mohamed Zouch, Maire-Helene Lafage-Proust, Thierry Thomas, and Laurence Vico from Institut National de la Santé et de la Recherche Médicale (Inserm) also in Saint Etienne, France.

A rapid release version of this paper has been published online and will appear in the January issue of Journal of Clinical Endocrinology & Metabolism, a publication of The Endocrine Society.