Results
The survey was mailed to 1061 subjects (668 women, 396 men). Responses were received from 564 subjects (53%). [Table 1] shows the distribution of the respondents by age and sex, and the mean values for BMI and scores on the PCS-12 and MCS-12. The mean BMI did not vary with age or sex. Scores on both scales of the SF-12 were higher among men than women.
In performing the statistical analysis it was assumed that although the distribution of BMI or SF-12 scores among respondents and nonrespondents might differ, the relationships among these variables observed in respondents is generalizable. The regression results for the MCS-12 and PCS-12 are shown in [Table 2]. Interestingly, while the mean PCS-12 scores declined with age, there was a significant increase in the MCS-12 scores with age. Men had higher scores than women, but there was no interaction between BMI and sex. Both MCS-12 and PCS-12 scores increased with rising income. After adjusting for age, sex, and income, there were significant nonlinear relationships between MCS-12 and PCS-12 scores and BMI. The figure shows the relationships between QOL scores and BMI adjusted for purposes of illustration to age 60 years and to an income of Ž$80,000 per year. For both scales, there was a peak in QOL scores for BMI values in the range 20 to 25 kg per m2. For the PCS-12, there was a steady decline in QOL from the peak, with a drop of approximately 5 points (SD=0.5) at a BMI of 30 kg per m2. The relative change in MCS-12 scores was less, with no further decline for BMI greater than 30 kg per m2. The wide confidence limits preclude a confident assessment of the shape of the curve for the MCS-12 in the range of BMI greater than 30 kg per m2.
Discussion
The scales on the SF-12 reflect a self-assessment of well-being, pain, limitations, and energy. Self-reported QOL varied with BMI, with a peak in the range of 20 to 25 kg per m2. The PCS-12 scores declined monotonically from the peak with increasing BMI, consistent with reports of bodily pain as a significant comorbidity of obesity.3,5 There was a proportionally smaller decline from the peak in the MCS-12 scores; scores did not continue to decline for those with BMI >30 kg per m2.
In 1998, the National Heart, Lung, and Blood Institute (NHLBI) of the National Institutes of Health published evidence-based clinical guidelines for the identification, evaluation, and treatment of overweight and obesity in adults.14 On the basis of BMI they use 5 classes of increasing severity consistent with the idea of graded risk. Subjects with a BMI in the range of 18.5 to 24.9 kg per m2 are classified as having normal weight; those with a BMI of 25.0 to 29.9 kg per m2 are classified as preobese; and those with a BMI greater than 30 kg per m2 are assigned to 3 categories of obesity. The upper cutoff point for healthy weight at 25 kg per m2 is consistent with that recommended by a steering committee of the American Institute of Nutrition15 and an expert committee of the World Health Organization.16 Evaluation of a lower cutoff for healthy weight is complex, because the leanest group in a population is a mix of smokers, persons who have lost weight as a result of underlying disease, and persons who have maintained a lean weight by balancing physical activity and caloric intake.17 In this study, the number of subjects (N=23) with a BMI less than 20 kg per m2 was too low to carefully investigate the lower end of the BMI-QOL relationship.
The observations reported here from a survey of primary care patients suggest that achieving a healthy weight as recommended by the NHLBI will maximize the patient’s subjective sense of well-being.
Acknowledgments
Support from a grant from the American Academy of Family Physicians Foundation is gratefully acknowledged.