Obesity is currently defined by the body mass index (BMI). Specifically, the Centers for Disease Control (CDC) consider anyone with a BMI of 30 or greater to be obese. Those with a BMI less than this but greater than 25 are termed “overweight.” These indices are often related to disease and the risk of early death, including the appearance of diabetes and heart disease earlier than expected in the general population. However, BMI was not created to measure an individual’s health, though it is used to summarize the “fatness” of a population.
BMI is calculated from an individual’s height and weight. The assumption is that the more fat a person accumulates the heavier they weigh. However, people who are taller will weigh more due to the proportions they have to maintain a human shape. The index was developed by a statistician in the 1800s to describe changes in populations, but the index was accepted as a standard for an individual’s size in relation to their health in the 1980s. The equation is simple: weight (in kg) divided by height (in inches) squared (just the height is squared, not the whole equation). Muscle mass, bone structure, and gender are not taken into account. Thus, many consider it a flawed and, at best, limited measure.
All criticisms withstanding, many studies have found that an increased BMI is associated with the risk of disease, particularly cardiovascular disease. This is thought to be due to the link between BMI and other factors, particularly one’s weight, and this has been a contradictory measure in studies that include waist circumference measurements (as well as hip-to-waist ratio).
When an individual has what is currently considered an unhealthy diet and/or lifestyle, their body accumulates more fatty tissue due to excess calories. This results in a larger waist circumference due to how fatty tissue is deposited in the human body. This subcutaneous fatty store (also called abdominal fat) contributes to higher BMI because it increases one’s weight. The caveat is that it is visceral adipose tissue (also called deep abdominal fat) that has been found over the last decade to be associated with an increased risk of heart disease. This fatty tissue does not increase waist circumference, but it can contribute to a high BMI value.
Though the apparent correlation is that increased waist circumference causes a person to weigh more, which leads to a higher BMI and the label of obesity, the presence of a high BMI without the increased waist circumference may be an indicator of true obesity when using it as a definition of disease risk. The often contradictory study results regarding obesity and health, especially in relation to BMI, can be better understood if the nuances of fat deposition and its effects on health are taken into account.
