To the best of our knowledge, this is the first study that proposes a valid anthropometric model to simultaneously estimate FM, ALST, and BMC in older adults from a multicompartmental approach. DXA was used as a reference method due to its advantages in estimating all components by a single scan (63). Our proposed model with three anthropometric variables plus sex showed high prediction coefficients and low errors to simultaneously predict ALST, FM, and BMC. Since the BC is affected by sex (64), and changes in BC due to aging occur differently between men and women (65), the inclusion of the variable sex was made arbitrarily in the models generated in this study. Therefore, the current prediction equations are useful for estimating and monitoring ALST, FM, and BMC in older adults of both sexes.
Current anthropometric models to estimate BC in older adults have several limitations, causing errors in the estimation of BC. Furthermore, they have been developed using a bi-compartmental model (2-C) that determines FM and FFM (27, 28, 66), and this model is based on there is a linear relationship between subcutaneous fat, total fat, and BD. However, this is not true, because during the aging process there is age-related adipose tissue redistribution that is, an accumulation of visceral and abdominal fat occurs (67). Additionally, these equations do not evaluate ALST and BMC which are components that change during aging. The Lean equations (68) to estimate % body fat showed a coefficient of determination (r2) of 0.77 and 0.70 and standard error of estimate (SEE) of 4.1% and 4.7% for older adults men and women, respectively. However, our results for FM determination showed higher coefficient of determination (r2 = 0.83) and lower errors (SEE = 3,16 kg).
Progressive and metabolically unfavorable changes in BC have long been observed with aging (69). In a prospective study that investigated age-dependent changes over two decades, the main results found were an increase in BM, BMI, and FM until the age of approximately 70 and 75 years, after these parameters start to decrease (70). Regarding the changes in the SMM, the studies have shown a greater reduction in men than in women, with a more accentuated decline between 70 and 79 years old in both sexes (56, 69). However, the pattern and rate of age-related changes in BC may vary by sex, ethnicity, physical activity level, and caloric intake (71).
DXA is the most popular technique for measuring BC (42) and it has shown to be a reliable method of FFM during aging (72). Furthermore, DXA may be considered the current reference technique for assessing SMM and BC in research and clinical practice (42). A high correlation (r = 0.97) between DXA-measured ALST and SMM measured by magnetic resonance imaging (MRI) was reported for both men and women (18–92 years) (8). In the same way, DXA-derived LST was found to be significantly correlated with MRI-measured SMM (r = 0.94; p ≤ 0.001) in older women (73). In comparisons between DXA-measured FM and MRI-measured adipose tissue the associations were also high and significant ( r = 0.99; p ≤ 0.001) for older women (73). The principle of DXA depends on the property of X-rays to be attenuated in proportion to the composition and depth of the material the beam is crossed. The DXA scanner emits two different energy beams (40 and 70 keV). From the number of photons that are transmitted concerning the number detected the quantity of BMC and soft tissue (fat and FFM) can be determined (42). Therefore, DXA can be used as a reference method to propose equations using anthropometry for clinical and professional practice (74). The anthropometric measurements are performed in both the geriatric nutritional assessment and epidemiological studies because they are painless, safe, non-invasive, simple, and low-cost procedures, which permit the estimation of the body components and also the calculation of nutritional indicators using predictive equations (32). The main anthropometric measurements used in the older adults for this purpose are weight, height, the calf and waist circumferences, as well as the triceps, biceps, subscapular and suprailiac skinfolds (32).
The current investigation has several strengths. As far as we know, this is the first study that proposes equations to estimate the main components of BC from the same anthropometric variables for older adults. This implies a reduction in the prediction error and facilitates its use in epidemiological studies. Another positive point is that we included the variable sex in the generated models, facilitating the application in large groups of both sexes. Despite all the research efforts in this study, there were still some limitations: for example, DXA is not a gold standard for older adults’ body composition. However, the current state-of-the-art method for body composition measurement in the four compartments model (4-C models) at the molecular level, as it includes the evaluation of the main FFM components, thus reducing the effect of biological variability. Nonetheless, it requires sophisticated and highly specialized technical equipment; it implies the propagation of measurement errors, difficult to apply in certain population groups, and is time-consuming. Furthermore, it has high costs, making it difficult to use on large samples (75). Nevertheless, DXA represents a reference method for the assessment of human BC in the research field (63, 76) and it is widely considered the gold standard for BC assessment in clinical practice because of its advantages (74). Moreover, reference values of BC assessed by DXA on adults over 60 years old are available from the National Health and Nutrition Examination Survey 1999–2004 and other studies on the local population (77). Although it is a program designed to assess the health of adults and children in the United States, these reference values should be helpful in the evaluation of a variety of adult abnormalities involving fat, LST, and bone.
As it was hypothesized, using a multivariate regression model, simple anthropometric measures can be used to simultaneously estimate body components (ALST, FM, and BMC) in older adults of both sexes. As a practical simulation, an older adult male “A” with measurements of weight (66.3 kg), HAS (80.5 cm), TrSk (16 mm), and sex (0), when applied to our model, would have the estimated values of 18.1 kg, 21.3 kg and 2.2 kg for ALST, FM, and BMC, respectively. Their true measured values (DXA) were 18.2 kg, 20.8 kg, and 2.2 kg. If the equation is applied to an older adult woman “B” with values of weight (58.6 kg), HAS (81.5 cm), TriSK (26 mm), and sex (1) the estimated values for ALST, FM, and BMC would be: 13.1 kg, 23.5 kg and 1.9 kg, correspondingly. As noted, the values are close to the measured DXA values for ALST (13.2 kg), FM (23.4 kg), and BMC (2.0 kg). These values can be compared with the reference values National Health and Nutrition Examination Survey (NHANES) (77) and be useful for many applications in clinical and field practice. For example, using the criteria proposed by the FNIH (cutoffs < 19.75 for men and < 15.02 for women) we can classify both older adults with sarcopenia (78). Since older adult “A” presented predicted ALST values of 18.1 kg and older adult B of 13.1 kg. These findings are highly relevant as they allow permanent following/monitoring of excessive accumulation of FM, declines in BMC and ALST, as risks to older adults throughout the life course (79, 80). Thus, keeping the balance rate of fat, muscle and bone are essential to preserving metabolic homeostasis, and health status and positively contributes to successful aging (74). For this reason, the assessment of BC in older adults is critical and could be an additional preventive strategy for age-related diseases (74), which may result in sarcopenia (9, 48, 81), osteoporosis (14) sarcopenic obesity (64) osteosarcopenic obesity (2) and osteosarcopenia (11). This should impair muscle strength, and functional capacity, as well as greater morbidity and mortality in older adults (4, 5). Therefore, the current prediction equations could increase the available options for the estimation of body composition in older adults. To ensure dissemination and accessibility, an assessment of the main body components based on our predictive models can be found in an excel file (Additional file 1) in the following link (http://posgraduacao.eerp.usp.br/files/Model_BodyComposition_OlderAdults.xlsx). Lastly, future studies should evaluate the efficiency of these equations applied in longitudinal and intervention studies.