What is Body Mass Index (BMI)?
The body mass index (BMI) was introduced in the 19th century by Quetelet to help estimate the healthy body weight of individuals, depending on their height. BMI is obtained by dividing the body weight (in kilogrammes) to the square of one's height (in metres). Today, BMI calculators are freely available on the Internet.
BMI is applicable to men and women aged 18 to 65. In general, in adults with normal body constitution and a normal level of physical activity, BMI scores in the range of 18.5 - 25 indicate a normal weight. Scores under 16.5 denote underweight, and values over 30 are indicative of obesity. Values over 35 are generally related to multiple health risks, such as increased risk of developing cancer, diabetes, arthritis, and other medical conditions.
The scores of the body mass index have to be interpreted in a different manner in children or geriatric people. In adolescents, BMI scores are compared to ratios obtained from children at the same age, using special growth charts. In elderly people, values of 25 to 27 are considered normal because of the lesser amount of lean body mass and the loss of height through aging. For them, BMI increases without any parallel increase in body weight.
BMI could be misinformative with regard to body fat estimation, as it depends only on the weight and the height. The score does not account for the distribution of muscle tissue and bone mass. For instance, BMI overestimates adiposeness in people with high muscle mass (body builders or athletes) and underestimates the body fat deposit in people with certain medical illnesses (such as paraplegia, anorexia, etc.).
With BMI reference ranges, the race of individuals has to be taken into account. As Asians are known to have a higher body fat percentage (and are in general smaller), the cut-off point for overweight in them is set at 23 and for obesity - at 27. For Pacific Islanders, the body mass index for overweight varies from 26 to 32 and for obesity - it is over 32. The threshold is 32 for Afro-American women because of the heavier bones, different bone mineral content and lean mass density.
These limitations have to be considered when BMI is used as the single factor for predicting health risks related to obesity. Instead, it is recommended to interpret BMI scores together with other, more accurate indices, such as waist circumference, blood glucose, "bad" cholesterol, and triglyceride concentrations. Rather than relying on BMI only, it is a good idea to consult with your physician who is in the position to evaluate your individual situation.
It comes as a surprise that increased BMI values are related to better health outcomes for certain groups of people. This phenomenon is called the obesity survival paradox. It has been reported that among people with heart failure, those with BMI within the range 30.0-34.9 exhibited lower mortality rates than individuals with a normal weight. Similar findings have been observed for other cardiac diseases.
Studies carried out in Europe and the United States revealed a tendency toward higher BMI scores during the last twenty years. There is a marked increase in the highest percentiles, probably due to the more sedentary life and work patterns, together with the consumption of high-calorie foods. Emotional factors such as depression and reduced self-esteem further contribute to overweight and obesity in adolescents.
In sum, the body mass index may be used in large population surveys to predict health and mental risks. BMI may help to outline trends in health care expenditures related to adverse effects of overweight, obesity, and sedentary lifestyle.