To study the effect of age on the association between body fat percentage and maximal walking speed in older people.

Cross-sectional analysis of data collected in the Finnish population-based Health 2000 Survey involving 916 men and 1 222 women aged 55 years and older with complete data on body composition and a walking speed test.

Body fat percentage was assessed using bioelectrical impedance analysis and maximal walking speed based on a timed walking test over a distance of 6.1 meters. Linear regression models were used to study the effect of age on association between body fat percentage and maximal walking speed.

The association between body fat percentage quartiles and maximal walking speed differed significantly between persons of different ages (p for age interaction = 0.027). In the agestratified analyses, the association between body fat percentage and maximal walking speed remained significant among 60–69-year olds and 70–79-year olds, but disappeared among 55–59-year-olds and 80-year and older after adjustment for potential covariates. Body fat percentage explained 11% of the variation in maximal walking speed among 55–59-year-olds, 21% among 60–69-year-olds, 17% among 70–79-year-olds and 11% among 80-year and older.

Association between body fat percentage and maximal walking speed was strongest between the ages of 60 and 79 years. The results suggest that the effects of excess body fatness are especially harmful for physical functioning among adults in their sixties and seventies and they could benefit from interventions.