Why is body mass index already an obsolete measure?

The World Health Organization (WHO) defines obesity as an abnormal or excessive accumulation of body fat that involves health risks and has reached epidemic proportions.
Epidemiological data suggest that both the incidence and prevalence of overweight and obesity follow an increasing trend. In fact, by 2030 it is expected that 1 in 5 women and 1 in 7 men will be obese (BMI ≥ 30kg/m2), which is equivalent to more than one billion people worldwide.
Traditionally, obesity has been defined from a universal parameter: the Body Mass Index (BMI). It relates body weight (in kilograms) to the square of height (in meters). According to the BMI, and following the WHO criteria, overweight is defined when this indicator is equal to or greater than 25 kg/m2 and obesity when it is equal to or greater than 30 kg/m2.
Athletes are not “obese”, even if their BMI is high
Although BMI is a widely used parameter for classifying body weight, it is not without limitations. Among other things because it does not distinguish between muscle mass and lean mass (that is, that which is not fat). In addition, the latter can also vary considerably between individuals of the same size.
In fact, people with a high proportion of muscle mass, such as athletes, can be misclassified as overweight or obese based on BMI. Conversely, a low BMI may be due to insufficient lean and muscle mass.
It should be remembered that during aging fat mass increases percentage-wise, while there is progressively a concomitant loss of lean mass and bone mineral density associated with age, physical inactivity and functional limitations. Therefore, this indicator is not suitable for the diagnosis of obesity, especially when the loss of bone mass means that, for the same BMI, a young person has less fat than the older adult.
In addition, this data may not be equally valid based on race and ethnicity. Specifically, ethnic groups differ in the complexion and relative length of the legs (height relative to sitting), which also has an impact on the reliability of BMI to be considered as a universal parameter.
Evaluate obesity through various indicators
Quantifying the prevalence of overweight and obesity depends on what measures we use. That is why it is not trivial to reflect on the indicators.
The aforementioned cut-off points – overweight when this indicator is equal to or greater than 25 kg/m2 and obesity when it is equal to or greater than 30 kg/m2 – are applicable only to healthy adults. In fact, some authors set different cut-off points for different age groups, increasing the cut-off point for excess weight as we age, although there is still no scientific consensus on this criterion.
However, the assessment of childhood obesity is primarily determined by the BMI percentile to assess growth and development in the pediatric population. Consequently, percentile BMI tables have been developed in boys and girls that are used interchangeably both nationally and internationally. In fact, it is currently recommended that, in the absence of a widely shared consensus, prevalence studies consider various reference values. Among them the International Obesity Task Force (IOTF) or the one proposed by the (Orbegozo Foundation), in addition to the WHO reference values, mentioned above.
It doesn’t matter only if there is fat, we must also identify where
Another fact that we must take into account is that BMI is not able to provide information about the distribution of body fat. This poses a problem because fat, when located around vital organs, is a risk factor for cardiovascular disease and, more broadly, metabolic syndrome. Something that does not happen if it only accumulates in the buttocks area.
In this sense, we have the results of a systematic review that includes data from more than 300,000 individuals from diverse populations around the world. His main conclusion is that using complementary and alternative indices that evaluate abdominal adiposity, waist circumference and the relationship between waist and height allows to discriminate the cardiometabolic risk related to body fat.
In addition, the analyses conclude that the relationship between waist and height has a higher poder discriminatory than waist circumference. Therefore, in the absence of consensus and the limitations of BMI, it is recommended to use the percentage of body fat measured by bioimpedance, a safe, economical, accurate and non-invasive method that provides data on a person’s body composition. It consists of an electric current of very low intensity that runs through the organism allowing to assess its resistance, which is inversely related to the lean mass.
An estimate of subcutaneous fat in the skin folds can also be made whenever possible, although it may not be feasible due to lack of equipment (impedanciometers or plicometers).
However, BMI is still used at the population and individual level as it is a fast, easy to apply and low-cost method. In this sense, given the limitations of BMI, alternative measures, such as those mentioned above, could be used to identify central adiposity as accepted predictors of cardiometabolic risk and mortality in the population.
Studies suggest that they are more specific than BMI and that they have greater scientific robustness in predicting cardiovascular disease risk. As we can see, the search for simple, valid and low-cost diagnostic tools is a challenge that must continue.
Teresa Partearroyo, Professor of Nutrition and Bromatology, CEU San Pablo University; Ana Maria Puga Giménez de Azcárate, Adjunct Professor of Nutrition and Bromatology, CEU San Pablo University; Ana Montero Bravo, Full Professor. USP-CEU Group of Excellence “Nutrition for life”, ref: E02/0720, Department of Pharmaceutical and Health Sciences, Faculty of Pharmacy, Universidad San Pablo-CEU, CEU Universities, Universidad CEU San Pablo; Gregorio Varela Moreiras, Professor of Nutrition and Bromatology, CEU San Pablo University; Maria de Lourdes Samaniego Vaesken, Adjunct Professor, Universidad CEU San Pablo y Mar Ruperto López, Nutrition, Universidad CEU San Pablo
This article was originally published on The Conversation. Read the original.

Original source in Spanish

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