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Research summaries. Why you should not trust your scale

Why you should not trust your scale

The year has just begun and so we set our new year’s resolutions. So, this is that moment of the year when those who are considering to lose weight join a gym and/or start a new diet. On the other hand, there are those who do not carry any “extra” weight and therefore do not worry much about exercise or diet. In this post, we will talk about body weight and how it can be deceitful.

To roughly determine whether we are healthy or not so healthy we commonly rely on simple measures such as our body weight or our body mass index (body weight [kg] / height2 [m2]).

Body weight does not provide any information about our body composition. ‘What is my fat mass?’ or ‘what is my muscle mass?’ are questions that our bathroom scale is not going to answer. If a skinny woman starts eating more protein and doing resistance training, and after a couple of months she puts on 2 kg, it is highly likely that she has increased her muscle mass. If instead, she starts eating 4 extra doughnuts every day and after a couple of weeks she has put on 2 kg, it is most than likely that she has increased her fat mass. If we know what is going on before and after putting on weight, we may guess what happened, but our scale will not tell us where those 2 extra kg are coming from. Body composition matters and the way body fat is distributed is also important. The image on the left shows two men with a similar percentage of body fat. However, the man on the left has a higher proportion of visceral body fat (note the white areas around chest and abdomen), which is associated with increased risk of metabolic disease.

The body mass index (BMI) allows to compare two different individuals as it represents how heavy someone is accounting for their height. Traditionally a BMI between 18.5 and 25 is associated with a healthy weight and body composition, and values below 18.5 or above 25 are not.

This been said, the BMI is not always a good indicator of the body composition. A bodybuilder with a BMI of 26 and with 7% body fat should not be considered overweight, as the extra weight is due to an increased body mass. In the same way, a sedentary man with a BMI of 24 may or may not have a healthy body composition. This image on the right shows two men with a BMI of 24 kg/m2, yet the one on the right has twice the body fat and 6 times the visceral fat that the one on the left.

To summarize this post, we could say that simple measures such as weight and BMI can be useful when context is taken into account. Body fat % and how fat is distributed is, therefore, more important, and the take-home message is that a healthy lifestyle that includes a correct diet and regular exercise is key to have a good body composition.


Thomas, E. L., Frost, G., Taylor-Robinson, S. D., & Bell, J. D. (2012). Excess body fat in obese and normal-weight subjects. Nutr Res Rev, 25(1), 150-161. doi:10.1017/S0954422412000054

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