Scales that measure fat

Summer is coming, soon the holidays and the beach. A quick run on a scale is necessary to take any action before the swimsuit. To guide you in your good intentions, new connected electronic scales announce that they will not only weigh your weight but also determine your body fat percentage. Faced with this promise, the physicist’s first reflex, even when occupied by his silhouette, is prudence. Wouldn’t this still be an advertising message that could appeal to all those struggling to find the line? Reading the operating instructions reassures the prudent: operating this scale requires bare feet and wet soles. Everything glows! 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

Deriving body composition from a simple electrical measurement seems challenging because the conduction of electrical current 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 of the elongated cells of muscle fibers, for example, we quickly understand that the flow of current depends on the orientation of the fibers with respect to the applied voltage. Finally, let’s add that our body’s electrical properties vary according to our state of hydration, the progress of our digestion, etc. The comparison between the body composition displayed by these scales and measurements taken by other, more sophisticated means, such as IRM, shows the robustness and relevance of the results. Why ? This is because these electrical measurements provide access to several variables and, combined with additional information on size, age and gender, are compared with measurements on control persons.

To understand, consider a simplified modeling of a biological tissue. The conduction of electric current through a material results from the movement of electric charges in that material. Several phenomena are involved in biological tissues. When an electrical voltage is applied to a tissue, an electric field is created in it, which sets in motion the ions present in the cells and in the interstitial fluid separating them. The electrical current generated is all the more important because the concentration of ions is high.

Two obstacles impede the movement of these ions. First, the collisions with the molecules of the physiological fluid in which they bathe. The result is that they almost instantaneously assume a constant average speed proportional to the electric field they are subjected to, resulting in an electric current proportional to the applied voltage. Electrically, this 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, accumulating positive ions 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, combining conduction within itself and charged walls, therefore behaves like a capacitor in series with a resistor.

© Illustration by Bruno Vacaro

This association defines a characteristic time: the length of time it takes for the capacitor to charge when a voltage is applied, or to discharge when it becomes zero.

What effects does an AC voltage have on the conductivity? If the current is almost continuous or has a period greater than the charging time, the capacitor dominates and the conductivity is zero or almost zero: we are dealing with a circuit breaker. Conversely, for 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 composed of cells (including fat cells) and interstitial fluid and assume that the ionic content is the same in all fluids. By weighing this substance, we determine the total amount of matter. Because fat conducts electricity very poorly, the DC or low-frequency conductivity measurement is only sensitive to the amount of interstitial fluid (cells are severed), while the high-frequency conductivity indicates the total amount of fluid, both interstitial and intracellular. Finally, three measurements (weight and the two types of conductivity) yield three quantities: fat mass, non-adipose tissue mass, and intercellular water mass (associated with water retention).

What about the human body, which is made even more complex by the presence of bones and knows that other phenomena are also involved from an electrical point of view? For example, the electric field aligns the polar water molecules of a physiological fluid, which has an electrical effect similar to that of a capacitor. In fact, the principle remains the same: the person’s weight is measured, then the electrical conductivity in a range of frequencies to distinguish the various physical effects mentioned above.

body conductivity

© Illustration by Bruno Vacaro

With the simplest scales, 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 provide separate information about the four limbs and the torso.

In practice, the device is also told the person’s height, age and gender to refine the body composition assessment. How do these results compare to those obtained with other more sophisticated physical methods? Electrical measurements give very good results for the mean value of the quantities measured between the different individuals in a group. For a single person, on the other hand, the values ​​obtained are less accurate than those provided by sophisticated measurements. But they are still enough to assess the situation and determine what needs to be done, for example, to lose weight. Does the person have enough muscle to have an adequate metabolism to burn fat? What is the relative importance of fat in total weight versus water retention? Or how are visceral fat and superficial fat distributed?

These devices are particularly interesting for monitoring. Admittedly, the absolute values ​​given for the different compositions are not exact due to the morphological peculiarities of each, but these peculiarities do not change. Consecutive measurements on the same person are therefore particularly meaningful. They then perfectly indicate whether the observed weight loss is due to muscle or fat loss. Or vice versa, if a maintained weight is due to muscle gain coupled with fat loss. You will have no more excuses! The beaches are yours… in Greece.

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