Summer is approaching, soon the holidays and the beach. A quick passage on a scale is necessary for possible measures to be taken before the swimsuit. To guide you in your good resolutions, new connected electronic scales announce 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 preoccupied by his silhouette, is circumspection. Wouldn’t this still be an message able to attract all those who struggle to find the line? Reading the manual reassures the circumspect: the operation of this scale requires barefoot and the soles of the feet wet. Everything lights up! Information on body composition (fat, muscle, bone mass, etc.) is obtained by measuring electrical conductivity. What is the principle? Are they reliable?
A complex driver
Deducing body composition from a simple electrical measurement seems a challenge as the conduction of electric current in the human body is a complex phenomenon. The main reason is the heterogeneity of the tissues that make us up. Moreover, if we think for example of the elongated cells of muscle fibers, we quickly understand that the passage of the current depends on the orientation of the fibers with respect to the voltage applied. Finally, let us 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 measurements made by other more sophisticated means, such as the IRM, shows the robustness and relevance of the results. Why ? Because these electrical measurements give access to several quantities and, combined with additional information on height, age and sex, they are compared with measurements made on control subjects.
To understand, consider a simplified modeling of a biological tissue. The conduction of electric current by a material results from the movement of electric charges inside the latter. In the case of biological tissues, several phenomena are involved. When an electric voltage is applied to a tissue, an electric field appears within it which sets in motion the ions present in the cells as well as in the interstitial fluid which separates them. The electric current created is therefore all the more important as the concentration of ions is high.
Two obstacles impede 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 instantaneously acquire a constant average speed, proportional to the electric field to which they are subjected, resulting in an electric current proportional to the voltage applied. This electrically corresponds to the behavior of a resistor.
Then, 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 conduction within it and charged walls, therefore behaves like a capacitor in series with a resistor.
© Illustration of Bruno Vacaro
This association defines a characteristic time: the duration that the capacitor takes to charge when a voltage is applied or to discharge when it becomes zero.
What are the consequences on the conductivity with an alternating voltage? If the current is almost continuous or has a period greater than the charging time, the dominant effect is that of the capacitor and the conductivity is zero or almost zero: we are dealing with a circuit breaker. Conversely, with shorter periods, the effect of the capacitor becomes negligible in favor of the resistor.
Electrically modeling 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 fabric, 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 circuit breakered) 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 the 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 in addition, from an electrical point of view, other phenomena are involved? For example, the electric field orients the polar water molecules of 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 in order to distinguish the different physical effects mentioned above.
© Illustration of Bruno Vacaro
In the simplest balances, the conductivity is measured between the two feet, which corresponds to a passage of the current in the two legs and in the abdomen. One can also find more advanced devices connected to the feet and fists. The measurements then separately provide information on the four limbs and the trunk.
In practice, the device is also told the person’s height, age and sex in order to refine the estimation 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 sufficient to assess the situation and determine what needs to be done to, for example, lose weight. Does the person have enough muscle to have a sufficient metabolism to burn fat? In the total weight, what is the relative importance of fat compared to that of water retention? Or, how are visceral fat and superficial fat distributed?
Above all, it is for monitoring that these devices are of interest. Admittedly, the absolute values given for the different compositions are not precise because of the morphological specificities of each, but these specificities do not change. Consequently, successive measurements on the same person are therefore particularly significant. They then perfectly indicate whether the observed weight loss results from a loss of muscle or fat. Or conversely 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.