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Body composition assessed by DXA
DXA is a powerful clinical tool used primarily for measurement of bone mineral content and bone mineral density for the diagnosis of osteoporosis and assessment of fragility fracture risk. In most clinical circumstances, this involves measurement at specific skeletal sites — forearm, lumbar spine and proximal femur.
Less well recognized is the utility of DXA in assessing body composition using a three-compartment model — fat, lean and bone mass. Total body weight assessed by DXA is virtually identical to weight measured on scales with r2 ~0.95. (DXA essentially measures “naked” body mass — something we should not be doing on our clinical scales.)
Both GE Lunar and Hologic, the major providers of DXA technology, offer equipment for measurement of body composition, and their respective websites provide valuable information about their systems. To my knowledge, there is as yet no reimbursement code for DXA body composition — if you have updated information please let me know — but there are a number of clinical situations where the technology is of immense value.
I order this study most of the time when evaluating bone health in children during growth. There are reference data for assessing regional skeletal health in growing children, but it is difficult to fully account for the changes in bone size during growth, particularly since BMD measurements do not account for the third dimension. Used in conjunction with growth charts, whole body DXA can provide invaluable information about growth and development in children with fragility fractures, skeletal dysplasia or diseases associated with nutritional disorders.
Monitoring body composition is also an important adjunct to the management of patients with eating disorders.
A case can also be made for DXA body composition in obese patients as long as you appreciate that the very obese and very heavy (>300 lb) cannot be accommodated on most of the equipment. Besides, there are a number of inexpensive measurements such as BMI, waist/hip ratio and skinfold thickness that can be used as indicators of adverse health outcomes.
One thing I have noticed in patients working hard to lose weight is that body composition often changes before any weight loss is detected. Often a patient will report that their friends complement them on their weight loss, but they are frustrated because this does not show up on their scales. Body size and shape will alter before total body mass decreases as fat mass is converted to muscle mass. This can occur in obese patients with sleep apnea controlled with continuous positive airway pressure or similar, and in hypogonadal men treated with testosterone, and in all overweight patients who undertake a rigorous exercise program. There can be nothing more frustrating than doing everything you can to overcome a weight problem only to find that the scales are not congratulating you. Any positive feedback such as a favorable change in body composition must be of great benefit to our patients.
Given the ever-rising incidence of obesity and associated increased health care costs, it is probably time to push for a reimbursement code for DXA body composition — it would be well worth it if positive reinforcement can be shown to be effective in combating obesity.
There is also an important research study waiting to be done! Chronic steroid therapy reduces lean mass and bone mass, while increasing fat mass. While bisphosphonates can prevent the bone loss, I am unaware of any successful approaches to preventing loss of lean mass and/or gain in fat mass. Body composition using DXA seems an ideal tool for studying potential approaches to controlling this vexing clinical problem.