Previous cross-sectional and longitudinal studies reported significant decline in FFM with aging (8, 21). Since FFM is the major determinant of REE, explaining 60–90% of its variance (22), REE therefore decreases as a person ages mainly due to the decrease in FFM (8, 9). However, this decrease in REE is prevalent even after adjustment for body composition and must therefore have additional causes.
Findings of the present study indicated that REE is positively associated with FFM and negatively associated with the degree of malnutrition on admission. Indeed, REE was significantly lower in patients with lower FFM and severe malnutrition, suggesting that REE is different even at different stages of malnutrition. In addition, expressed as a ratio of FFM, REE values increased with higher score of MNA-SF however, the association did not reach the level of significance likely due to the small number of subjects. In contrary to our results, in a cross-sectional study of 52 older patients (mean age 79 ± 6 years), Schneider et al. (24) indicated that REE/FFM values increased along with degree of malnutrition (from 32.9 ± 4.1 kcal/kg in patients with BMI 18.5–20 to 44.7 ± 9.5 kcal/kg in those with BMI < 16, P = 0.001). The discrepant results may be explained by differences in study population and in methods used for assessing nutritional status. In the study by Schneider et al. (24), only malnourished patients with BMI < 20 were included whereas the majority were very lean with BMI between 16-18.5 and < 16 which is very low compared to the mean BMI in our study population (23.4 ± 4.0 kg/m2). Therefore, the very low BMI and relatively low FM in that study may have resulted in a higher mean REE/FFM in severe malnourished patients. In addition, in that study, BMI was used to evaluate the degree of malnutrition however, we assessed nutritional status using MNA-SF which is considered as a validated tool for screening the nutritional status in older persons across settings and provides true classification of nutritional status compared to the standard BMI.
Energy deficit and weight loss result in metabolic adaptation which leads to reduced energy expenditure and may enhance metabolic efficiency (10–12). Previous studies showed that malnourished patients have a lower REE compared to patients with normal nutritional status (23). In a cohort study of healthy individuals (age 18–83 years) (24), the mean REE measured by IC in older persons age > 70 years was higher compared to the mean measured REE in our malnourished older patients (5 MJ/day = 1194 kcal/day vs. 967 kcal/day, respectively). Accordingly, the low REE in our malnourished patients maybe a consequence of metabolic adaptation to low energy intake. Consequently, REE values may be biased by malnutrition and may not represent patient’s true energy requirements.
The major finding of the present study is that REE, either as absolute values or as a function of FFM, significantly increased after almost 2 weeks nutritional therapy confirming our hypothesis regarding the effect of nutritional treatment on low REE in malnourished persons. It has been previously reported that increased REE or REE/FFM ratio maybe a consequence of hypermetabolic state (25). It is important to determine that in this study; older patients were free from severe acute stress such as infections. In addition, although, almost half of the population displayed moderate inflammation, no significant associations between REE and CRP on admission were observed and CRP did not explain any variance in changes in REE in regression analysis.
We believe that malnutrition decreases REE as an energy saving component of metabolic adaptation, unless there is a specific cause for an increase in REE such as severe inflammation, whereas nutritional treatment normalizes the low REE in malnourished patients. Indeed, after administration of almost 2 weeks individualized nutritional therapy, mean measured REE by IC increased from 968 to 1180 kcal/day which was comparable to the mean predicted REE by the Harris-Benedict Eq. (1190 kcal/day, P = 0.901) in healthy persons and to the mean of measured REE by IC reported in previous studies among healthy older individuals (24, 26). In addition, in this study, the parker mobility score and the activities of daily living as measured by Barthel-index improved during hospital stay as the result of routine rehabilitation program in our geriatric acute care unit, i.e. physical and occupational training. Such activities may also increase REE (27), however, the effect appears relatively minor and was not the target of our study.
To the best of our knowledge, this is the first study showing that the low REE in malnourished older hospitalized patients can be reversed by nutritional therapy. Considering these findings, it appears essential to take into account the impact of malnutrition on REE in malnourished older hospitalized patients. In case of doubt, REE estimated by the Harris-Benedict-formula seems to better reflect the true energy needs of these patients than uncorrected data of indirect calorimetry. Furthermore, increasing REE in malnourished older patients seems to be a sign of an effective nutritional therapy.
This study has some limitations. We assessed body composition using BIA which is highly influenced by hydration status (28). That is why we excluded subjects with edema. However, BIA can be considered as a validated tool for measuring body composition in hospital settings (29). We did not measure nutritional intake, which is difficult to perform in a geriatric population. In addition, our study population is relatively small mainly due to difficulty in using indirect calorimetry in some older patients i.e. patients' fear to use the ventilated canopy hood or patients' discontent to assess REE two times. However, IC which is considered as gold standard, was used to determine energy requirements accurately in our patients on admission and at the time of discharge. Further longitudinal research is required to better understand the individual changes of energy needs.