Introductory Chapter: A Historical Index Widely Used in Medicine – Body Mass Index

1. Introduction

Body Mass Index (BMI) which is calculated as a person’s weight in kilograms divided by the square of the body height in meters, and is expressed in units of kg/m², is still commonly used in the practice of medicine, especially in public health and family medicine, due to its free and easy application [1, 2]. BMI was first defined by Keys and et.al in 1972 and it has not lost its popularity since then [3]. BMI gives a simple numerical result of a person’s thinness or thickness thus allowing healthcare professionals to more objectively discuss and monitor weight issues with their patients. For this reason, it seems that it will remain current in medical practice for many years [3, 4].

2. WHO classification of BMI

The World Health Organization (WHO) prescribed BMI and categories to define obesity, overweight, normal weight, and underweight as in below:

Overweight (Pre-obese): 25.0–29.9, Obese (Class I): 30.0–34.9, Obese (Class II): 35.0–39.9, Obese (Class III, Morbid Obese) ≥ 40.0, Normal range: 18.5–24.9, Underweight (Mild thinness): 17.0–18.4, Underweight (Moderate thinness): 16.0–16.9, Underweight (Severe thinness): < 16.0 [5].

3. Evaluation of BMI

BMI is a monitoring method, not an exact measurement of the presence and distribution of body fat. There are, of course, individual differences in these evaluation, and BMI is inadequate as the only way to classify a person as obese or malnourished. In certain populations, such as athletes and bodybuilders, high weight as a result of increased muscle mass falsely increases their BMI. For this reason, special regulations should be provided for BMI monitoring [6, 7].

4. Exact way of measuring body fat - bio-electrical impedance

BMI does not directly detect body fat but is as strongly correlated as more direct measures of body fat such as Bio-electrical Impedance Analysis (BIA) which evaluates the body fat makes simultaneous tetrapolar measurements by touching its electrodes [8]. The measurements of the BIA include; lean body mass, skeletal muscle mass, soft tissue mass, total body water amount, intracellular water amount, extracellular water amount, excess body fat, body fat percentage, visceral fat surface, waist-hip ratio, organ fat mass, subcutaneous fat mass, abdominal fat, basal metabolic rate, total energy amount, internal resistance, overweight target control, excess fat target control, and edema parameters and also body mass index. The ideal analysis of the presence and distribution of fat tissue in the human body is, of course, done with such devices [8]. However, these devices are not available everywhere.

5. Healthy body fat and BMI

BMI is one of the tools used to calculate health risks such as physical activity level, smoking, alcohol and substance use, lifestyle, risks of the living environment, blood pressure, cholesterol level, and blood glucose level [2, 3, 9]. Conditions that reduce life expectancy and are associated with mortality, such as overweight, obesity, and diabetes, can also be expected to be associated with high BMI. In addition, it is known that people who are underweight throughout life have a lower premature death rate. However, the optimal BMI at which the risk of death is lowest is unknown [10, 11, 12].

6. Obesity and BMI

Obesity, which is chronic systemic inflammation of adipose tissue, when prolonged, eventually activates the innate immune system in adipose tissue. As a result, oxidative stress (OS) increases, which triggers the acute phase response and leads to an inflammatory reaction in the body [13]. Thus, obesity and overweight constitute important risk factors for many acute and chronic diseases, from metabolic and mental diseases to cancer. Obesity not only causes health problems, obesity-related health problems also lead to treatment costs and workforce loss [13, 14]. For this, the presence of fat tissue and its distribution in the body must be evaluated and measured accurately. Excess fat accumulation is usually measured by BMI, which is calculated by weight and height ratios. However, in the presence of abdominal (central) obesity, BMI may appear within normal limits. Therefore, reliable measurement of fat accumulation is necessary, especially in children and the elderly. Healthcare professionals should evaluate BMI while being aware of hidden obesity [15, 16].

The prevalence of obesity is increasing rapidly in every age group and in almost all countries around the world, mostly among low-income and low-educated people. Childhood obesity is a problem that needs to be paid more attention because it is difficult to reverse [13]. In adults, BMI is a useful indicator for assessing health risks throughout life because it is easily calculated and widely available. However, interpreting BMI as an indicator of health risk in children is possible by comparing the child’s measurements with age- and gender-specific reference ranges Underestimating the prevalence of obesity, especially in children, as assessed by BMI, may lead to less attention to the problems and inadequate prevention and management. Therefore, a reliable measurement of fat accumulation requires elaborate tools [16].

7. Underweight and BMI

Low body fat, as well as excess body fat, is a risk to body health. People may have low body fat for many different physical (GI disorders, malignancy, infection, thyroid problems, HIV or lung disease, endocrine problems, etc) and mental (such as bulimia, anxiety and stress, dementia, etc.) reasons. In addition to these medical conditions, economic and sociological reasons can also make people underweight because it makes it difficult to access food Detecting and monitoring low body fat ratio is as important as monitoring overweight and obesity. In this case, the most suitable and accessible tool is BMI [11, 17].

8. Limitation of BMI

BMI is an indirect assessment of body fat and therefore already has several deficiencies. Moreover, if it is calculated based on the person’s self-reported height and weight, it cannot accurately determine the body fat percentage. For this reason, height and weight measurements must be made by health professionals under optimum conditions and follow-ups must be the same. BMI is a widely used screening tool, but it does not provide clear data on a person’s body fat percentage and distribution. As it is known, muscle mass decreases with age. In this case, the shortening of height and decrease in muscle mass with age can be misleading in most cases. Additionally, the relationship between body fat percentage and BMI differs in men and women. For this reason, measurements of waist-hip ratio and skinfold thickness along with BMI lead to more accurate results for body fat ratio [18].

9. Conclusion

BMI is a widely used screening tool to determine body fat percentage. However, it should not be a diagnostic tool. When BMI is to be used due to the lack of a device that accurately measures body fat, these conditions should be taken into consideration in children, the elderly, gender differences, and individuals with excess muscle mass. Although obesity is preventable, it is gradually becoming a pandemic due to the lack of implementation of preventive and sustainable public health approaches. Unfortunately, it is becoming increasingly difficult to acquire and maintain a healthy nutrition culture and life habits. Moreover, naturally obtained nutrients are rapidly decreasing. As obesity continues to increase all over the world, BMI will continue to be used because it is easy to apply.