Development and validation of anthropometric predictive equations that estimate the total body water and fat-free mass in Tunisian adults

General characteristics

The physical and anthropometric characteristics and body composition parameters of all subjects and according to sex are presented in Table 2. In total 178 subjects (77 men, 101 women), aged 18 to 59 years participated in this study. The means of age, weight, height and BMI of the participants were, respectively, 29.2 ± 12.3 years, 69.4 ± 12.2 kg, 167.7 ± 10.4 cm and 24.75 ± 4.28 kg/m2. According to the BMI categories, 5.6% were underweight, 48.3% of the subjects were in a normal range, 33.2% were overweight and 12.9% were obese.

Table 2 General characteristics and body composition of the study by subjects, gender and subgroups.Prediction equations tested

The comparison of our results with those predicted equations using a previously published anthropometric equations, is presented in Table 3.

Table 3 TBW determined by DDT in our study and that calculated according to the published based anthropometric prediction equations.

All equations for the estimation of TBW, except that of Slater and Preston, showed significant differences in all subjects, and in men and women. Indeed, TBW predicted by the equations of Hume and Weyers [11], Watson et al. [12], Medoua et al. [14] and Chumlea et al. [15] significantly (p < 0.0001) overestimated TBW in the total sample with a positive bias value (2.50, 1.67, 5.34 and 5.15 kg, respectively for each equation). However, Slater’s equation gave a non-significant difference in all subjects (p = 0.94), but in terms of sex, the Slater inbuilt equation significantly (p = 0.0089) overestimated TBW in women with a bias of 1.06 ± 4.00 kg and (p = 0.0018) underestimate TBW in men.

Body composition in the development and validation groups

Table 2 shows the characteristics of studied participants and subgroups (development and validation groups). There were no significant differences observed in anthropometric variables and different compartments of body compositions assessed by DDT between the development group (n = 89) and the validation group (n = 89). The mean of TBW was 35.3 kg with a range of 23.2 to 55.6 kg, the mean of FFM was 48.1 kg with a range of 30.7 to 76.0 kg and the mean of FM and %FM were respectively 21.3 kg and 30.3% with a range of 4.1–45.7 kg and 7.8–50.0%.

Development of the equations

The prediction equations were derived by stepwise linear multiple regression and the results for this prediction equation are presented in Table 4. The TBW (kg) and FFM (kg) were the dependent variables in their respective equation. Weight, height, sex and age were the significant predictors variables included in the equations. The weight, age, and sex considerably added to the accuracy of our equations and improved r and RMSE.

Table 4 Regression models for the prediction of TBW and FFM.

The prediction equations derived were:

$$}}}\left( }}}} \right) = }}}}}}} + }}}}}}} \times }}} + }}}}}}} \times }}} - }}}}}}} \times }}} - }}}}}}} \times }}};$$

$$}}}\left( }}}} \right) = - }}}}}}} + }}}}}}} \times }}} + }}}}}}} \times }}} - }}}}}}} \times }}} - }}}}}}} \times }}};$$

where TBW in kg; FFM in kg; Weight in kg; Height in cm; sex in dummy code (men = 0, women = 1) and age in years.

In the TBW prediction equation, the values of R2, RMSE, and % RMSE are 0.94, 1.80 kg, and 5.08% respectively, while the same parameters of the FFM prediction equation were of 0.94, 2.44 kg and 5.05% respectively (Table 4).

Validation of the prediction equations

In the validation group, the use of the paired t-test showed that there was no significant (p > 0.05) difference between TBW (35.4 ± 7.8 kg) and FFM (48.3 ± 10.7 kg) measured by DDT and estimated with the new anthropometric equation (35.4 ± 7.7 kg and 48.1 ± 10.5 kg; respectively) (Table 5).

Table 5 Comparison between TBW and FFM measured by isotope dilution using FTIR and by new anthropometric equation in the cross-validation sample.

Bland and Altman plots of TBW and FFM prediction equations are shown in Fig. 1. The bias was expressed as the mean of the difference in TBW or FFM measured by DDT using FTIR and estimated from the new anthropometric equation with a confidence interval at 95% equal to [−0.4847; 0.3790] for TBW and [−0.7298; 0.4168] for FFM. For the models’ pure errors, it was 2.0 kg for the TBW prediction equation and 2.7 kg for the FFM prediction equation (Table 4).

Fig. 1: Bland and Altman plot between the deuterium dilution technique and those determined by the new prediction equation in the validation group.figure 1

The body composition parameters determined by the new anthropometric equations showed a non-significant difference compared to those calculated from DDT for all subjects as well as for men and women (Table 6).

Table 6 Comparison of body compartments based on TBW and FFM anthropometric estimation equation to the body composition compartments measured by deuterium dilution technique.

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