Hypometabolizers: characteristics of obese patients with abnormally low resting energy expenditure

Weight gain or loss is determined by the difference between calorie intake and energy expenditure. The Mifflin metabolic equation most accurately predicts resting energy expenditure (REE) in morbidly obese patients. Hypometabolizers have a measured REE that is much less than predicted and pose the greatest challenge for weight loss induced by restriction of calorie intake. We studied 628 morbidly obese patients (467 female and 161 men, aged 52.5 ± 15.7 years, body mass index [BMI] of 42.6 ± 7.6 m/kg(2) [mean ± SD]). REE was measured using the MedGem® device (REEm) and the percentage variance (ΔREE%) from the Mifflin-predicted expenditure (REEp) was calculated. Patients with ΔREE% more than 1 standard deviation from the mean were defined as hypometabolizers (REEm greater than 27% below REEp) and hypermetabolizers (REEm less than 13% above REEp), respectively. Hypometabolizers had greater REEp (1900 ± 301 vs 1719 ± 346 calories, P = 0.005) and lower REEm (1244 ± 278 vs 2161 ± 438 calories, P < 0.0001) than hypermetabolizers. Hypometabolizers, when compared with hypermetabolizers, were taller (167.2 ± 8.4 vs 164.0 ± 10.9 cm, P = 0.04), heavier (123.6 ± 22.2 vs 110.2 ± 23.1 kg, P = 0.006), and had increased BMI (44.1 ± 6.5 vs 40.8 ± 6.5 kg/m(2), P = 0.04). Other measured anthropometrics were not different between hypo- and hypermetabolizers. Hypometabolizers were less likely to be diabetic (23 vs 43%, P = 0.03) and more likely to be black (25 vs 5%, P = 0.002) than hypermetabolizers. This study defines hypometabolizers as having variance in REEm more than 27 per cent below that predicted by the Mifflin equation. We could not identify any distinguishing phenotypic characteristics of hypometabolizers, suggesting an influence unrelated to body composition.