## 4 Discussion  
The main objective of the present investigation was to analyze the differences and agreement between the most commonly used formulas for lipid estimation between themselves and in comparison with [PERSON]'s adiposity formula, once its lipid content was estimated, and to compare the results reported by the different formulas with those found with [PERSON]'s formula. This is important because the present research is the first to take into consideration that, while most anthropometric equations estimate lipid mass by addressing body composition from a molecular fractionation point of view, the only exception is the Kerr formula, which seeks to estimate adipose mass from the tissular model. As the estimation of these formulae is not comparable, the lipid content of the [PERSON] formula was calculated from the [PERSON]'s formula [49], formulated by linear regression on cadavers and therefore with a very high validation power. The results indicated that there were significant differences between most of the formulas, ranging from 8.90 \(\pm\) 2.17% to 17.91 \(\pm\) 2.84% for men, and from 15.33 \(\pm\) 2.94% to 28.79 \(\pm\) 3.30% for women. In the same vein, most of the formulas showed poor concordance (CCC \(<\) 0.900). In fact, the Bland-Altman test also showed differences in the results reported in most equations with respect to [PERSON]'s formula. These results agree with those found in the literature, supporting the idea that there are differences when using different skinfold-based equations for lipid mass estimation in active adults [5]. These differences could be due to the heterogeneity of the samples with which the equations were validated (Table 1), as very different inclusion and exclusion criteria were used in the design [19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. Another possible reason is that all formulas, with the exception of [PERSON], performed a linear regression analysis to estimate lipid percentage from anthropometry [21; 23; 24; 26; 27; 31]. This is the estimation of the body density from the skinfolds included a process carried out by regression considering the skinfolds and the density given in the validation sample of each formula [9]. From this density value, the lipid mass present is estimated. This statistical strategy makes each formula specific to the study population included. The formulas only consider lipid levels and bodies similar to those included during their validation [9]. This could explain why the [PERSON] and [PERSON] formulas, validated on elite athletes, showed the lowest mean values in this research [21; 23]. Moreover, the method of validation of these formulas was not uniform, which could affect the equations reported by the different formulas (Table 1). In this regard, while [PERSON]'s formula was validated by dissection of cadavers [19], most of the others used an indirect method such as hydrodensitometry for their validation [20; 21; 22; 25; 26; 27]. Hydrodensitometry is an indirect method for estimating body composition that has several limitations, such as not being validated on cadavers [9]. Another of its limitations is that, as different densities are involved in the lipid estimation by this method, the estimation is influenced by the total body water. Thus, we find contaminating factors for the estimation of body composition by hydrodensitometry factors, such as renal and hepatic pathologies, but also diet, exercise, or the consumption of certain supplements such as creatine [6; 7]. In turn, hydrodensitometry requires the assignment of constant values for fat mass (0.900 g/cm\({}^{3}\)) and fat-free mass (1.100 g/cm\({}^{3}\)) [6; 7]. However, cadaveric studies have shown that fat-free mass density varies widely between subjects, as it encompasses a wide variety of tissues with very different characteristics [51]. These limitations were evident in previous studies, where American football players were analyzed by hydrodensitometry, and some of them, being very lean and African-American, yielded negative body lipid values [9].