It’s strange, how does a body fat scale know how much fat I have?

Audit expert: Shen Yingjian

Director of the Nutrition Department of Hebei Yanda Hospital

In recent years, people's living standards have been continuously improving, and more and more people have begun to pay attention to their own health. Not only are various "green" eating methods popular among people, but even the measurement of "body fat percentage" has become very popular.

Many people are keen on reducing their body fat percentage by controlling their diet, increasing exercise, etc. There are also many body fat scales on the market that claim to be able to measure body fat percentage, and they are advertised to help people monitor changes in body fat and develop better fitness plans.

Image source | Baidu Encyclopedia

So, are the calculation results of these body fat scales really accurate? What is the inevitable connection between body fat percentage and our physical health?

1

How to calculate body fat percentage?

Body fat percentage, as the name implies, refers to the percentage of fat tissue mass in the body weight . There are several ways to calculate body fat percentage, including underwater weighing method, ADP method, ultrasonic detection method, bioelectrical impedance method, etc. The underwater weighing method calculates body fat percentage based on the difference in tissue density between fat and muscle. The ADP method, also known as air displacement volume scanning method, places a person in a special measuring instrument, measures the volume occupied by the human body in the instrument, and then calculates the fat ratio.

Image source | Wikipedia

Ultrasonic detection method is to use ultrasound to detect the fat content in the body. Similar methods include infrared, X-ray, etc. Because the above methods are complicated to operate and the instruments are not easy to obtain, they are generally only available in specialized medical institutions and medical research institutes . For ordinary people, the most common method to test body fat is the bioelectrical impedance method, which is also the measurement principle of body fat scales on the market.

2

Detailed explanation of the principle of body fat scale

The body fat scale on the market is an electronic scale that can estimate the user's body fat content in addition to measuring weight. Its measurement principle is bioelectrical impedance analysis technology (BIA).

The body's fat has poor conductivity and a high resistivity, while muscles are just the opposite. Muscles contain more water, have good conductivity, and have a lower resistivity.

When microcurrent passes through fat, muscles, bones, organs and other tissues in the body, it will be subject to different resistances.

Therefore, bioelectrical impedance analysis technology measures the rate and time of microcurrent passing through the human body to determine the resistivity differences of various parts of the human body, and then calculates the proportion of fat and muscle in the body. This technology is used in many body fat scales due to its convenient measurement and low cost. However, it should be noted that when using a body fat scale to measure body fat, the correct operating procedures must be followed.

Image source | Shopping website

The correct way to measure is to take off your shoes and socks, and stand on an already turned on body fat scale with bare feet. At this time, the low-voltage current of the body fat scale will pass through the human body, thereby calculating the body fat percentage. In addition, in addition to foot-standing body fat scales, there are also handle-type body fat scales on the market. The handle-type body fat scale has multiple handles, and the metal electrodes on the handles will be connected to the body fat scale. Compared with foot-standing body fat scales, handle-type body fat scales can measure the bioelectrical impedance of the upper and lower body in sections, so the calculated body fat percentage will be more accurate.

However, no matter what kind of body fat scale you use, it can only calculate an estimated value of body fat percentage, and it cannot be 100% accurate. Why is it an estimated value?

First, body fat scales do not directly measure the fat content in the human body, but estimate the body fat percentage through resistance data; secondly, the instruments and measurement time we use cannot meet the conditions required for accurate data. In 2003, professionals conducted a BIA measurement experiment to measure body fat percentage. The whole process took 40 minutes each time, while we only stand on the body fat scale for a few tens of seconds.

In addition, in experimental BIA measurements, the four-electrode method is generally used, that is, four body surface electrodes are placed on the human limbs to measure the bioelectrical impedance of the whole body. However, the electrodes of household body fat scales are all concentrated on the soles of the feet, and the measurement range is relatively limited.

Image source: "Reliability study of multi-frequency segmental bioelectrical impedance analysis for measuring human body composition"

So, what is the difference between measuring body fat and measuring weight?

3

Body fat measurement vs weight measurement

As can be seen from the above, body fat measurement reflects the percentage of fat tissue in the human body, which can better reflect the proportion of body fat. Human body fat is distributed in a wide area, including subcutaneous fat, visceral fat, etc. Excessive body fat rate can cause many health problems and physical diseases, such as fatty liver, heart disease, diabetes, and obesity. The fat ratio can reflect certain health problems. On the left is the abdominal CT of a person with normal weight, and on the right is the abdominal CT of an obese patient, whose subcutaneous fat thickness is 3.6cm.

Image source | Wikipedia

Body weight measurement refers to measuring the weight of the entire human body, including the total mass of various tissues such as muscles, bones, fat, organs, etc. Body weight is usually measured using a scale, and the change in weight value can be used to preliminarily estimate the increase or decrease in body weight.

However, weight measurement cannot distinguish the detailed differences in body fat, muscle tissue, etc. A person with a large body weight may have more developed muscles or higher body fat. Therefore, body fat and weight are different indicators for observing human health. The measurement methods and meanings of the two are different, and need to be analyzed according to specific circumstances.

It should be noted that body fat measurement is more significant in terms of obesity than simple weight measurement, and is better than simple weight measurement for weight loss.

In general, measuring body fat percentage can help us have a preliminary understanding of our body fat level, but we don’t need to pay too much attention to the specific numbers. Compared with the specific values ​​of body fat percentage, we should pay more attention to its increase or decrease, so as to understand the impact of our recent diet and lifestyle on the body and make appropriate adjustments in time.