Daily energy expenditure in free-living non-institutionalized Alzheimer's patients: a doubly labeled water study

Weight loss is common in Alzheimer's disease (AD), but its pathogenesis is poorly understood. It is unclear whether an elevated daily energy expenditure contributes to the weight loss. We tested the hypothesis that daily energy expenditure is higher in AD patients compared to healthy elderly. Thirty AD (73 +/- 8 years; Mini-Mental State Examination score: 16 +/- 8) and 103 healthy elderly (69 +/- 7 years) were characterized for daily energy expenditure and its components (resting and free-living physical activity energy expenditure) from doubly labeled water and indirect calorimetry. Fat-free mass and fat mass were measured from dual energy X-ray absorptiometry. Fat-free mass tended to be lower in AD patients (45 +/- 9 kg) versus healthy elderly (49 +/- 10 kg; p = 0.07), whereas no differences were noted in fat mass between groups. Daily energy expenditure was 14% lower in AD (1901 +/- 517 kcal/d) compared to healthy elderly (2213 +/- 513 kcal/d; p < 0.001), due to lower resting (1287 +/- 227 versus 1418 +/- 246 kcal/d; p < 0.01) and physical activity energy expenditures (425 +/- 317 versus 574 +/- 342 kcal/d; p < 0.05). No differences in energy expenditure were noted between groups after controlling for differences in body composition. Daily energy expenditure was examined in a subgroup (n = 11) of AD patients who lost significant body weight (5.6 +/- 2.3 kg) within the past year. There was a lower daily energy expenditure in cachectic AD patients (1799 +/- 474 kcal/d) versus non-cachectic patients (1960 +/- 544 kcal/d) and healthy elderly (2213 +/- 513 kcal/d; p < 0.01). Similarly, no differences in energy expenditure were noted between groups after controlling for differences in body composition. We conclude that absolute levels of daily energy expenditure are lower in AD patients due to their lower body mass. However, after taking into account differences in body composition, daily energy expenditure in AD patients is appropriate for their metabolic size. The hypothesis that elevated daily energy expenditure contributes to weight loss in AD is not supported by these findings.