Summer is coming, soon the holidays and the beach. A quick step on a scale is necessary for possible measurements to be taken before the swimsuit. To guide you in your good intentions, new connected electronic scales announce you to weigh not only your weight, but also to determine your percentage of body fat. Faced with this promise, the first reflex of the physicist, even concerned about his silhouette, is circumspection. Wouldn’t this still be an advertising message capable of attracting all those who struggle to find the line? Reading the manual reassures the circumspect: the operation of this scale requires bare feet and wet soles. Everything lights up! Information on body composition (fat, muscle, bone mass, etc.) is obtained by measuring electrical conductivity. What is the principle? They are truthful?
a complex driver
Deducing body composition from a simple electrical measurement seems challenging, as electrical current conduction in the human body is a complex phenomenon. The main reason is the heterogeneity of the tissues that make us up. Furthermore, if we think, for example, of the elongated cells of muscle fibers, we quickly understand that the passage of current depends on the orientation of the fibers with respect to the applied voltage. Finally, let’s add that the electrical properties of our body vary according to our state of hydration, the progress of our digestion… However, the comparison between the body composition announced by these scales and the measurements made by other more sophisticated means, such as like the IRM, it shows the robustness and relevance of the results. Why ? Because these electrical measurements give access to various quantities and, combined with additional information on height, age, and gender, are compared with measurements made on control subjects.
To understand, consider a simplified model of a biological tissue. The conduction of electric current through a material results from the movement of electric charges within the latter. In the case of biological tissues, several phenomena are involved. When an electrical voltage is applied to a tissue, an electrical field appears inside it that sets in motion the ions present in the cells as well as in the interstitial fluid that separates them. The electric current created is therefore all the more important the greater the concentration of ions.
Two obstacles prevent the movement of these ions. First, the collisions with the molecules of the physiological liquid in which they bathe. The result is that they almost instantly acquire a constant average speed, proportional to the electric field to which they are subjected, resulting in an electric current proportional to the applied voltage. This corresponds electrically to the behavior of a resistor.
So for intracellular ions, the matter is complicated by the presence of cell walls. These act as an insulating medium with positive ions accumulating on one side and negative ions on the other. These separate charges create an electric field that opposes the applied field. The movement of the ions slows down and an equilibrium is reached. From an electrical point of view, we find a capacitor.
The cell as a whole, which combines internal conduction and charged walls, behaves like a capacitor in series with a resistor.
This association defines a characteristic time: the time it takes for the capacitor to charge up when a voltage is applied to it or to discharge when it goes to zero.
What are the consequences on conductivity with an alternating voltage? If the current is almost continuous or has a period longer than the charging time, the dominant effect is that of the capacitor and the conductivity is null or almost null: we are dealing with a circuit breaker. On the contrary, with shorter periods, the effect of the capacitor becomes negligible in favor of the resistor.
Electrical modeling of the body.
Now imagine a more complex biological tissue consisting of cells (including fat cells) and interstitial fluid and assume that the ion content is the same in all fluids. By weighing this tissue, we determine the total amount of matter. As fat conducts current very poorly, DC or low frequency conductivity measurement is only sensitive to the amount of interstitial fluid (cells are disrupted), while high frequency conductivity indicates the total amount of fluid, both interstitial and intracellular. Finally, three measurements (weight and the two types of conductivity) reveal three quantities: the mass of fat, the mass of non-adipose tissue, and the mass of water between cells (associated with water retention).
What about the human body, which is even more complex with the presence of bones in particular and knowing that other phenomena are also involved from an electrical point of view? For example, the electric field orients the polar water molecules of a physiological liquid, which has an electrical effect equivalent to that of a capacitor. In fact, the principle remains the same: the weight of the person is measured, then the electrical conductivity in a range of frequencies to distinguish the different physical effects mentioned above.
In the simplest balances, the conductivity between the two feet is measured, which corresponds to a passage of the current in both legs and in the abdomen. More advanced devices attached to the feet and fists can also be found. The measurements then provide separate information on the four limbs and the trunk.
In practice, the device is also informed of the person’s height, age and gender to refine the estimate of body composition. How do these results compare with those obtained by other more sophisticated physical methods? Electrical measurements give very good results for the average value of the quantities measured between the different individuals of a group. For a single person, on the other hand, the values obtained are less precise than those provided by sophisticated measurements. However, they are still enough to assess the situation and determine what needs to be done to, for example, lose weight. Does the person have enough muscle to have enough metabolism to burn fat? In total weight, what is the relative importance of fat compared to water retention? Or, how is visceral fat and superficial fat distributed?
Above all, it is for monitoring that these devices are of interest. It is true that the absolute values given for the different compositions are not precise due to the morphological specificities of each one, but these specificities do not change. Consequently, successive measurements on the same person are particularly significant. They then perfectly indicate whether the observed weight loss is the result of a loss of muscle or fat. Or on the contrary, if a weight that is maintained is due to muscle gain associated with fat loss. You will have no more excuses! The beaches are yours… in Greece.
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