BMI, Body Composition, and Physical Functioning in Older Adults

(SILVER SPRING, MD, July 17, 2007)- New research published in the July issue of Obesity suggests that fat mass seems to be a more important risk factor than muscle mass in determining walking speed and functional impairment in older adults living in the community, even after body mass index (BMI) is considered.

According to Jean Woo, MD, the Chinese University of Hong Kong and principal author of the study, "these findings emphasize another adverse effect of obesity in the elderly, apart from increased risk of various diseases-that of functional limitation. Declining grip strength with increasing fat mass suggests that muscle function may be adversely affected and may partly account for functional limitations."

Recent studies have emphasized the importance of muscle and fat mass in relation to age-related decline in physical function. The objective of this study was to determine whether BMI as a measure of fat mass could be used to measure risks for physical functioning in older adults and whether measuring body composition by DXA show any advantage over BMI. BMI has been widely used to measure mortality and morbidity in diverse populations with different ethnicities. DXA bone density studies of the spine and hip are used to diagnose osteoporosis and follow changes in bone density over time.

Two thousand men and women over age 65 living in the community participated in the cross-sectional study. Participants responded to a questionnaire asking about their diseases, activity levels, and instrumental activities of daily living (IADL). IADL questions covered difficulty in walking two to three blocks outside on ground level, climbing up 10 steps without resting, preparing own meals, doing heavy housework (scrubbing floors) or washing windows, and doing own shopping. Subjects were grouped into different BMI categories, separately for men and women, to represent underweight (<18.5 kg/mē), normal-weight (18.5 to <23 kg/mē), overweight (23 to 24.9 kg/mē), and obesity I (25 to 29.9 kg/mē) and obesity II (= 30 kg/mē).

The wide BMI range among the subjects in this study provide a unique opportunity to examine the relationship between physical function, fat body mass index, fat and muscle mass in elderly people living in the community. "Although declining muscle mass is expected to contribute to functional performance, the study highlights the importance of fat mass as well, as a risk factor for functional impairment," notes Jean Woo.

Subjects in the two obese categories had a greater number of IADL impairments compared with under-weight and normal-weight groups. The underweight and obese groups have slower walking speed compared with the groups with normal BMI.

The study confirmed that the decrease in muscle mass and increase in fat mass with age and being female are underlying factors for differences in frailty. Fat mass seems to be a stronger determinant of minimum time walking or speed in walking than muscle mass, supporting the idea that BMI may not fully reflect fat mass.

In obese elderly people, an exercise component must be included in order to maintain or increase lean muscle mass and bone mineral density. Exercise has been shown to increase physical performance and improve quality of life.

The Obesity Society is the leading scientific society dedicated to the study of obesity. Since 1982 The Obesity Society has been committed to encouraging research on the causes and treatment of obesity, and to keeping the medical community and public informed of new advances. For more information about The Obesity Society and obesity, visit www.naaso.org or contact:

Nancy Olins
The Obesity Society
nolins@naaso.org