Objectives This study aimed to measure resting energy expenditure (REE) in institutionalized old persons and to determine factors possibly related to change in REE as a basis for estimating energy requirements. Design and Settings A monocentric cross-sectional study was conducted. Statistical approaches were conducted to determine independent factors associated with REE. Various published predictive equations of REE were compared to our population. Participants 72 residents of a nursing home, mostly women (80.5%) aged 87.4 +/- 6.6 years were included. Measurements REE (indirect calorimetry), body composition (bio-impedance analysis), biological and anthropometric data were collected. Results Mean REE was 1006 +/- 181 kcal/d and was higher in men than in (1227 +/- 195 vs. 953 +/- 131 kcal/d, p<0.05). According to criteria adapted from the Global Leadership Initiative on Malnutrition consensus, 65.3 % of the institutionalized population were malnourished. In multivariate analysis adjusted on gender and age, REE was positively associated with calorie intake, fat-free mass (FFM), functional abilities (French Autonomie Gerontologie Groupe Iso Ressources scale), and elevated CRP level (> 25 mg/l). Significant differences (p<0.05) appeared between measured REE and predicted REE by using various published equations. Conclusion REE of very old nursing home residents is influenced by FFM, calorie intake, functional abilities, and CRP levels and is poorly predicted by classical equations based on age, gender, height, and weight. This suggests a metabolic adaptation to caloric restriction and inflammation and prompts to consider the level of physical activity and muscle loss when assessing caloric requirements in this population.