Isotope Dilution Space Ratio Declines Above the Age of 60 Years, Potentially Impacting Estimates of Total Energy Expenditure by the Doubly Labeled Water Method1-3, *, ˆ.

Doubly labeled water is gold standard for measuring total energy expenditure (TEE). Measurements using the method are sensitive to the isotope dilution space ratio (DSR). Accuracy and precision of the method might be improved if we could identify factors influencing DSR. We evaluated the potential associations of age, sex, ethnicity, anthropometry, body composition, turnover rates of the isotopes, and geographical elevation with DSR. We used univariate regression analysis to explore the relationships between the continuous variables and analysis of variance to test the relationships between the categorical variables with DSR. Subsequently, we used General Linear Modeling (GLM) and One-way ANOVA to evaluate the simultaneous associations of age, sex, ethnicity, fat-free mass (FFM) and fat mass (FM) on DSR. From 5,678 measurements complied from studies around the world with diverse ethnicity and living at various elevations, the average DSR was 1.0364 ± 0.0141 (mean ± SD). No meaningful physiological effect of any of the continuous and categorical variable on DSR was detected. GLM analysis revealed no effect of FFM and FM (P > 0.33) on DSR, but DSR decreased with age (P < 0.001) among those 60 years of age and older regardless of sex. Among the White who were younger than 60 years of age, DSR was not related to FFM and FM (P = 0.73) but was affected by both age and sex (P < 0.001). Previous estimates of age-related decline in TEE may have overestimated TEE at age 90. Validation studies on older participants are required to confirm this finding.

Greater male variability in daily energy expenditure develops through puberty.

There is considerably greater variation in metabolic rates between men than between women, in terms of basal, activity and total (daily) energy expenditure (EE). One possible explanation is that EE is associated with male sexual characteristics (which are known to vary more than other traits) such as musculature and athletic capacity. Such traits might be predicted to be most prominent during periods of adolescence and young adulthood, when sexual behaviour develops and peaks. We tested this hypothesis on a large dataset by comparing the amount of male variation and female variation in total EE, activity EE and basal EE, at different life stages, along with several morphological traits: height, fat free mass and fat mass. Total EE, and to some degree also activity EE, exhibit considerable greater male variation (GMV) in young adults, and then a decreasing GMV in progressively older individuals. Arguably, basal EE, and also morphometrics, do not exhibit this pattern. These findings suggest that single male sexual characteristics may not exhibit peak GMV in young adulthood, however total and perhaps also activity EE, associated with many morphological and physiological traits combined, do exhibit GMV most prominently during the reproductive life stages.