While it is well known that heavier people develop stronger bones, a burning issue for osteoporosis researchers over the last 20 years has been whether muscle or fat has the greater impact on bone mineral density. Australian and Vietnamese scientists have now shown that the impact of 'lean mass' is 3 times higher than the impact of 'fat mass', finally concluding the protracted debate.

The discussion started in July 1992 when a prominent group from the University of Auckland published a paper in the Journal of Clinical Endocrinology and Metabolism, demonstrating that body fat mass is the most significant predictor of bone density. The work has since been heavily cited, and has spawned dozens of other studies trying either to validate or disprove its findings.

Professor Tuan Nguyen from Sydney's Garvan Institute of Medical Research, in collaboration with Dr Lan Ho-Pham, Head of Rheumatology at People's Hospital 115 in Ho Chi Minh City, undertook a 'meta-analysis' of 44 studies, concluding that 21% of differences in bone mineral density can be explained by lean mass, 8% by fat mass. Ironically, their findings are also published in the Journal of Clinical Endocrinology and Metabolism.

"In the presence of conflicting findings, meta-analysis is a good method of finally resolving the issue, and is a particularly popular tool in clinical medicine," said Professor Nguyen.

"Conflicting findings in the medical literature are the norm rather than the exception, because studies are based on different populations and use different methodologies. The variability in findings between studies can be 'ironed out' by the meta-analysis, which weights the relative importance of each study by its information content - sample size and quality of the data." "While it may seem as if we are splitting hairs by trying to determine which aspect of body weight plays the more influential role, the finding is actually very important for public health - because it will guide osteoporosis prevention. If muscle mass is critical, it makes sense to recommend improvement in physical activity and muscle-building exercise."

The analysis also points out the differences in body composition and bone density between premenopausal women and postmenopausal women. In premenopausal women, lean mass is more important than fat mass, but in postmenopausal women the effect of lean mass is comparable with the effect of fat mass on bone density. It implies that hormones and nutrition are relevant factors for preventing postmenopausal osteoporosis. In Australia, approximately 30% of postmenopausal women have osteoporosis.

Another factor that determines bone density is genetics, which explain up to 75% of variance.