Interpret a skin and impedance analysis

Who I am
Robert Maurer

Author and references

Determining body composition is a fundamental piece of information when intervening in the sports field to take care of an athlete's physical and nutritional appearance. The balance between the "compartments" that make up the body of an athlete is the basis of an excellent sports performance: for example, an excess of body fat can constitute an impediment to physical training and sports competition, as well as muscle mass deficit and dehydration can affect athletic performance.

Furthermore, over the last decade the interest in improving physical appearance, maintaining physical and body health and achieving higher performance, has also increased significantly in amateur sports, giving rise to a multimedia phenomenon (internet, television, magazines, etc.) and economic (fitness, beauty, slimming centers, etc.) increasingly vast and developing.

This "mass" event also fully involved the scientific environment, in particular, for example, science applied to sport, wellness and fitness, inducing international research centers, medical industries or pharmaceutical companies to invest money and time on these issues, to prove innovative methods, tools and techniques in order to provide increasingly precise and useful analyzes.

The Body Composition

In a very general way when we talk about body composition we can use an analytical model that divides body weight into compartments.

At the molecular level, body weight (BW) can be expressed as BW = TBW + PM + MM + Gn + FM that is, total water + protein mass (protein mass) + mineral mass + glycogen and fat mass, actually the sum of water, proteins, minerals and glycogen constitutes the lean mass (FFM, free-fat-mass) and the bicompartmental model of the human body is therefore constituted by the sum of these two constituents FFM + FM. In the case of the two-compartment analysis, the hydration state fixed at 73,2% is considered.

Other analyzes allow to analyze the body composition on three different compartments: fat mass (FM), cell mass (BCM) metabolically active component of our organism in which oxygen exchanges take place, glucose oxidation and containing potassium, and extracellular mass (ECM ) which include plasma, interstitial fluids, transcellular water, tendons, dermis, collagen, elastin and skeleton.

Although there are several other methods for dividing body weight into compartments and analyzing body composition, we have specifically introduced these concepts in order to go on to analyze in detail the strengths and weaknesses of two of the most used analyzes for the assessment of body composition: plicometry and body composition. bioimpedance analysis, focusing on the interpretation of relevant data from an analysis of this type. These methods are defined indirect because they are based on estimates instead of analyzes such as DEXA, hydrodensiometry and plestymography defined direct.


Let's start with plicometry, a technique useful for the quantification of subcutaneous fat, one of the indirect indicators of the subject's state of health / fitness. The folds provide a good measure of subcutaneous fat; since there is a relationship between subcutaneous fat and total body fat, it is believed that the result of the measurement of the skin folds is a good indicator of body composition and density. In fact, some authors believe that the sum of the various folds can be used to estimate total body fat.

First of all, this analysis will allow us, by analyzing the individual skin folds, to define the topography of the subcutaneous fat. In fact, several studies report how identifying the localization of body fat in specific areas of the body can be an indication of adopisity linked to hormonal pathways and different constitutional characteristics (). To give an example without going into specific biochemical, anatomical and endocrinological features, just think of the basic distinction between android biotype and gynoid biotype.

The gynoid type (gluteal-femoral obesity) manifests itself in a very specific way, accumulating fat (and / or subcutaneous water and metabolites) in the lower areas of the body, especially in the thighs and buttocks, as well as in the lower abdominal area and in the triceps, in the back of the arms, the accumulation prevails in the subcutaneous fat reserves (subcutaneous adipose tissue), with the characteristic of appearing softer to the touch (Bouchard, 1997).

While the gynoid is associated with a lower risk of contracting serious diseases than the android (visceral fat associated with cardiovascular disease and metabolic syndrome for example), the fat distributed in the typical ginoid areas is much more difficult to mobilize.

This greater difficulty in mobilizing fat in the lower subcutaneous areas compared to the visceral one seems to be due to a greater sensitivity to the lipogenetic action of insulin, that is, the insulin action in these subjects will more markedly suppress the release of fats (lipolysis ).

In fact, there are several studies that show how insulin inhibits the lipolytic action more markedly in relation to the subcutaneous fat localized in the lower areas of the body compared to the visceral one, in addition it seems to be subject to a lesser extent to the action of catecholamines ( adrenaline and noradrenaline in particular) associated with lipolytic action.

At a visceral level, in fact, adipocytes are more sensitive to beta-adrenergic action (majority of beta receptors compared to alpha receptors associated with the mobilization of fat deposits on hormonal action) than subcutaneous fat present in the lower areas of the body (Tchernof et al. ). In conclusion, gynoid fat, deposited in the lower regions, is much more resistant to slimming processes because it is more receptive to the molecules that cause fat accumulation (insulin) and more deaf to the molecules that promote its mobilization (catecholamines).

In addition to providing support for the development of analyzes such as the one just described and therefore framing nutritional and training therapy for the subject, the skinfold can be used using the sum of the folds, or rather, predictive equations that associate the values ​​of the subcutaneous folds to body fat. total were developed using both linear and quadratic regression models.

A large number of population-specific equations exist for predicting body density (D) from various combinations of skin folds, circumferences and bone diameters (Jackson and Pollock, 1985. Slaughter et al. 1988. Lohman 1986). Once the body density value has been obtained, it is possible to calculate the percentage of body fat through different formulas. Once the percentage of fat is known, it is possible to determine the body fat mass with the following equation: FM (kg) = (FM% x BW) / 100 By difference it is therefore possible to obtain the lean mass: FFM (kg) = BW - FM ( kg), where: BW = body weight.

The estimation equations, however, have the limits related to the factors listed above, for example a girl with accumulated fat or extracellular water predominantly localized (very high quadriceps and calf folds) in the lower part of the body and free of fat / water retention in the part. higher could reveal overall body fat% greater than 20%. If we went into detail to analyze the upper district we could detect% even <15% while in the lower district> 25%. It goes from that this data will have to be interpreted, in order to plan the due nutritional and training strategy with these considerations.

One of the limitations related to skinfolding is that the caliber of the skinfold meter is usually limited to thicknesses between 45 and 55 mm therefore unusable for example in obese subjects. Even if some instruments have special gauges, it will still be very difficult in obese patients with large adipose accumulations to apply this analysis.


Let's move on to bio-impedancemetry, BIA (bioelectrical impedance analysis), a fast and non-invasive method in which a low voltage alternating current passes through the body of the subject, the impedance (Z) is measured in this way, that is the resistance to the passage of current . Without going into the technical specifications, let's focus on the key points of this analysis.

The passage of current occurs thanks to the electrolytes present in the water. The resistance to the passage of the current will therefore be greater in the adipose tissue and less in the lean mass. In fact, biological tissues behave as conductors or as insulators; the lean mass contains large quantities of water and electrolytes making it better than the fat mass (hydrophobic) in the conduction of electric current.

Using these principles, an impedance analysis will provide various parameters relating to the body composition and hydration status of the subject under examination. Total Body Water (TBW: Total Body Water) is the basis, the starting point, from which to estimate the other parameters of "fitness". Some manufacturers (Akern) have developed clinically tested solutions to determine the state of hydration on the basis of the BIA values, using normograms such as the “BiaVector” or by relating the liters of water to the height. Another distinction linked this time to the distribution of body fluids is their compartmentalization between intracellular (ICW) and extracellular (ECW), once these data are in hand it will be possible to make various considerations.

A basic index of well-being depends on the correct distribution of total water in the two compartments. In fact, it is not so important to know that our body contains for example 50 Lt of water, as its distribution; many studies confirm that normohydration is present only in healthy and well nourished subjects: EXW: plasma, lymph, saliva, eye fluids, digestive juices, sweat, liquid of the space surrounding the cells, ...).

Most of the liquid lost with sweat comes, for example, from the extracellular compartment, in particular from the plasma. For example, moreover, by crossing the results of the plicometry with those of the BIA relating to hydration, taking as a reference a hypothetical gynoid girl described above, we could have more information / confirmations / denials on the fact that the plicometric measurements have reported fat or subcutaneous water in the buttock, quadriceps and calf area. This is done by evaluating on the one hand the distribution of fluids and on the other by comparing the estimates on body composition related to BIA.

In particular

We describe some other important indices related to this analysis. The phase angle for example PA (relationship between resistance and reactance) can vary in a normal subject between 6 ° and 7 °, however very thin people will have lower values ​​and very high muscular individuals. In healthy subjects a low Phase Angle represents a low cell mass and therefore malnutrition (Energy Protein Malnutrition).

A high Phase Angle value is associated with high Reactances and can indicate, if greater than 10 °, states of dehydration or higher than normal BCM quantities, as in athletes. The possibility that the instrument indicates the Phase Angle value, as well as Resistance and Reactance, allows the professional who treats nutrition and training, to precisely plan and monitor the evolution of both the state of nutrition and hydration, making the right corrections. We have mentioned another parameter, the cell mass, BCM (body cell mass) or the metabolically active cellular tissues, component of the body composition that contains the tissue rich in Potassium, which exchanges Oxygen, which oxidizes glucose and added to the Extracellular Mass (ECM) , results in the weight of the Lean Mass.

Important indices related to this parameter are: BCMI (body cell mass index) indicates the Cell Mass ratio (BCM), with the height of the subject that will help us determine the nutritional status of the subject and the ECM / BCM ratio, extracellular mass / cell mass that will provide us with a data on the "quality" of lean mass. This data allows to parameterize the evolution of the nutritional status over time, especially in slimming, protein-energy malnutrition, in sportsmen under an intense period of training, and to make the necessary changes if necessary.

Under normal conditions this ratio occurs between 0,9 and 1, in catabolic situations the BCM will decrease and this index will increase above 1 as well as in states of water retention, in case of dehydration this value could instead fall below the 0,9-1. Other estimates that the BIA will provide us, albeit less reliable, are those relating to the basal metabolic rate (obtained indirectly through the BCM values, estimated consumption between 8-10 ml of oxygen x kg BCM, 2,7-3,6 kcal / hx kg BCM). Another value always linked to the state of hydration is the sodium-potassium ratio on average between 0,85 and 1 based on the sex and specifics of the subject and decreasing in states of dehydration.

The measurement of the bioelectrical impedance is however influenced by numerous factors and its standardization will therefore be essential for a fruitful use of the method.


With this article we have dealt in a very general and descriptive way, without going into the specific applications of these tools, an overview of the main data that can emerge from an analysis of body composition through plicometry and bio-impedancemetry.

We have chosen these two tools because they are among the most used. As always, the variables and possible errors are many, it will be up to the nutritionist or trainer to take into consideration all the possible variables and to interpret the results appropriately according to the situation.

[Doctor in food science and technology; Master's degree in Biological Sciences (Nutrition and Functional Food); CSEN certified Instructor and Personal Trainer]

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