Body mass index misclassification due to kyphotic posture in Japanese community-dwelling adults aged 65 years and older.

Background: In older adults, kyphosis (slouching posture) can lead to underestimation of height, which may in turn lead to overestimation of body mass index (BMI). We investigated the extent to which inaccurate BMI assessments led to misclassification of elderly people as normal weight (when they were actually underweight) or overweight (actually normal weight).

Methods: Anthropometric measurements were taken in 2005 and 2006 for 842 residents aged 65 years or older (women: 491; men: 351). We calculated BMI from measured height and weight (observed BMI) and then predicted BMI from height as determined by demi-span, which is unaffected by kyphosis (predicted BMI). Kyphosis was assessed by the number of blocks placed under the occiput required for the supine participant to achieve a neutral head position. Participants were classified as underweight (BMI < 18.5), normal weight, or overweight (BMI ≥ 25.0) according to both observed and predicted BMI; classification accordance was investigated by cross-tabulation.

Results: Kyphosis was present in 17.2% of the participants overall and in 23.6% of those aged 75 years or older. Predicted BMI measurements showed that 11% of participants with kyphosis requiring ≥ 3 blocks were misclassified as normal weight and that 10% were erroneously classified as overweight. In those aged 75 years or older, the corresponding figures were 15% and 12%.

Conclusions: Our results suggest that inaccurate BMIs due to kyphosis lead to substantial numbers of older adults being misclassified as normal weight or overweight, which can cause significant distortions in data on the impact of underweight and overweight on health outcomes.