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Different countries, different normal ranges

Determining body composition by means of a bio-impedance analysis provides a differentiated assessment of the nutritional and health condition of a person and plays an ever-increasing role in medicine and fitness. Normal ranges are essential in order to assess the measured results correctly. They define the body composition in healthy adults and enable individual values to be classified as well as detecting deviations. But what is healthy – i.e. "a normal state" – and what is unhealthy? 

On the one hand, a healthy "normal condition" can be defined as a range in which a large part of the measured values of a healthy reference population are found, i.e. a mean value. On the other hand, a normal range can be defined as the range in which only minor health risks are to be expected. For example, the WHO defines the BMI between 18.5 kg/m² and 25 kg/m² as the range in which the risk of obesity-related concomitant diseases such as cardiovascular diseases and diabetes mellitus type 2 is low. Consequently, this range is considered healthy or normal when assessing the nutritional condition by means of the BMI.

Normality is therefore not absolute, but depends largely on the population under consideration (mean value) or on what is considered healthy (determined e.g. by a health organisation).

Apart from the fat-free mass (ESPEN criteria for malnutrition) and the BMI, there are no firmly defined threshold values for body composition. To be able to specify meaningful normal ranges for the seca mBCA for all parameters such as muscle mass or fat mass, the body composition of around 1,050 healthy test subjects was recorded. The mean values determined thereby include other influencing factors such as age, height and gender. For example, the normal ranges of muscle mass are related to body size. After all, a man who is 1.90 m (6’ 2”) tall has on average more muscle mass than a man who is 1.75 m (5’ 7”). Consequently, different normal ranges are required for both men if the amount of their muscle mass is to be evaluated. A single normal range for all body sizes would give a false classification of the measurement results for particularly tall or short people.

Different normal ranges for skeletal muscle mass – left: a 1.93 m tall Caucasian man, 25 years. Right: a 1.78 m tall Caucasian woman, 23 years old. In this example, the normal cut-off range for the entire SMM differ by more than 10 kg. The green ranges relate to muscle mass, which is 90% of the reference population.

Not only age, gender or height, but ethnicity, too, influences body composition. Individual ethnic groups differ significantly in height and weight as well as in the proportion of fat and skeletal muscle mass. For the seca mBCA, three further complex normal range studies were carried out with different ethnic groups for this reason. The results were recently published in the international journal Applied Physiology, Nutrition and Metabolism. The overall body composition of 3,069 Germans, Japanese and Mexicans was analysed and compared. The height, weight and hip circumference of all study participants were ascertained, and a BIA was performed with the seca mBCA.

The investigations revealed significant differences between the three populations. The BMI was the highest for Mexican test subjects (women: 27.0 kg/m², men 27.8 kg/m²) on average and the lowest for Japanese (women: 21.0 kg/m², men 23.1 kg/m²). Accordingly, the Mexican group had the highest proportion of overweight people, while the prevalence of underweight was highest among the Japanese participants. 

The tolerance ellipses show the areas in which 50% of the measurement results for the fat-free mass and the fat mass fell. The fat-free mass (FFM) and the fat mass (FM) were related to the body height for this presentation (body composition chart) and set in relation to each other. The body composition chart allows a more accurate estimation of weight than the BMI, as it distinguishes between fat mass and fat-free mass.

Furthermore, the studies showed that the participants differed in their body composition despite having the same BMI. Above a certain BMI, the German test persons showed a higher proportion of fat-free and a lower proportion of fat mass than Mexicans and Japanese. With increasing BMI, however, the Japanese phenotype resembled the German one. The graph above illustrates these differences and thus the need to determine normal ranges on an ethnic basis.

Furthermore, differences were shown in the distribution of skeletal muscle mass. While German test persons showed the highest percentage of skeletal muscle mass in the upper extremities and trunk, the Japanese showed the lowest percentage there. With regard to the lower extremities, Germans and Japanese were similar, whereas Mexicans had weaker muscles in this area.

Ultimately, it was evident that there are demonstrable differences in the body composition of different populations. They are based on multifactorial causes. Besides genetic disposition, they are presumably due to country- and population-specific nutrition and lifestyles.

The results emphasize the importance of adapting the normal ranges to the respective population groups. Thanks to the now published, multi-ethnic normal range studies, the measurement results of the seca mBCA can always be compared with suitable normal ranges which are also scientifically proven and meaningful. In this way, the seca mBCA will in future provide even better support in classifying the state of health. 

The diagrams stem from the study: Jensen, B., Moritoyo, T., Kaufer-Horwitz, M., Peine, S., Norman, K., Maisch, M. J., ... & Fonz-Enríquez, E. (2018). Ethnic differences in fat and muscle mass and their implication for interpretation of bioelectrical impedance vector analysis. Applied Physiology, Nutrition, and Metabolism, 44(6), 619-626. 

Study: https://www.nrcresearchpress.com/doi/10.1139/apnm-2018-0276#.XOZZIUxuKAh

Licence: https://creativecommons.org/licenses/by/4.0/  

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