Overweight versus obesity

Is there a difference between being very overweight and obese?

Both terms define an abnormal or excessive accumulation of body fat which has adverse effects on a person’s state of health.

A simple index of weight-for-height, known as the body mass index (BMI) is used to classify a person’s weight. BMI determines a person’s weight divided by the square of their height.

• In adults: A BMI that is equal to or greater than 25 indicates a higher than normal / healthy weight. A person is classified as overweight when their total body weight is between 10% and 20% higher than the average standard. A BMI that is equal to or higher than 30 indicates obesity, which means that a person’s total body weight is in excess of 20% or more. There are three different classes of obesity as related to BMI scores. A BMI of between 30 to 35 is known as ‘class 1’. A BMI of between 35 to 40 is known as ‘class 2’. A person will be classified as morbidly obese (extreme or severe) when excess body weight clocks at between 50% and 100% more than the normal standard (i.e. having a BMI of 40 or higher).
• In children: The composition of a child’s body varies during stages of growth, and is also calculated differently as a result between the sexes. BMI is thus typically calculated according a set standard (sex-specific BMI for age chart) for children of the same age and sex (specific percentile). This indicates whether or not a child is within a healthy range. Body fat in children also changes with age, making certain growth stages such as puberty tricky to determine an accurate BMI. Body fat also varies between boys and girls during childhood and must be factored in to measurements taken. Standard charts have been developed in order to make a relative comparison and assign a BMI rating. If a child scores 85%, they will be classified as overweight (85 out of a 100 as compared with children of the same age and sex). A score of 95% or higher will be considered obese.

Calculating BMI

BMI can be calculated using any of the following formulas:

• Metric formula: Weight (kilograms) / height (in either centimetres or metres squared) – weight in kilograms is divided by height in metres or centimetres squared (height in centimetres can be divided by 100 to determine metres). Example – A person with a height of 1.73 metres and a weight of 90 kilograms would have a BMI of 30.1 (classified as obese) calculated as follows:

Weight (Kg’s) / Height (square cm)
90 / (1.73) x (1.73)
= 90 / 2.9929
= 30.07
Rounding up: 30.1

• Imperial formula: Weight (pounds) multiplied by 703 / height (inches) – weight in pounds and inches multiplied by 703, and divided by height in inches squared. Example - A person with a height of 68.1 inches and a weight of 198 would have a BMI of 30 (classified as obese), calculated as follows:

(Weight (lbs) x 703) / Height (square ins)
(198 x 703) / (68.1) x (68.1)
= 139194 / 4637.61
= 30.01

Understanding BMI

It is important to understand that although BMI scores can give a person a good idea as to where on the scale they stand when it comes to overall health and weight, body fat is not measured directly in order to determine this rating.

A BMI is, however, a useful general population-level measure, that provides a fairly accurate guide for indicating excess weight. A BMI score for children will likely change from month to month for each sex, as they age. Weight ranges will change as a child’s height increases.

A BMI is not a diagnostic tool, but fares best when used as one for screening in order to determine problems with weight.

Age, sex, ethnicity and muscle mass are all factors that can influence the relationship between body fat and a BMI score. BMI does not factor all of these specifics into a measurement. Women tend to have more body fat than men (with the same BMI). Likewise, older individuals have more body fat than those younger too, also with the same BMI score. A very muscular individual can have a high BMI based on weight and height calculations, but in reality, not be classified as obese, this is why the BMI index is not always an accurate measure for performance athletes and bodybuilders with high muscle mass. Muscle weighs more than fat but this is not a factor in BMI calculations. An inactive or even frail individual can also have a low BMI but can have more body fat that leads to health concerns and ailments.

Thus, a BMI score cannot be used to distinguish between excess fat (or the distribution of fat), bone mass, or muscle mass. A doctor must assess the true nature of health in order to determine the most effective means of treatment.

There are other means of directly measuring body fat, which can be obtained from skinfold thickness assessments (with callipers), waist-to-hip measurements, underwater weighing methods, bioelectrical impedance, dual energy X-ray absorptiometry (DXA), isotope dilution and more. All directly measure body fat obtaining an accurate percentage.

When assessing weight versus health risk, it is best to take into account a person’s BMI and waist circumference. Abdominal (visceral) fat is known to pose the most debilitating health risks, such as heart disease or type 2 diabetes. A waist circumference that is greater than 35 inches (or 88.9 centimetres) for women and 40 inches (101.6 centimetres) for men is the benchmark for when health risks associated with weight typically begin.

How will a BMI be used in the medical field?

A healthcare provider will make use of BMI calculations during initial assessments as part of a weight issue consultation. A doctor will likely recommend and perform other evaluations in order to determine more accurate measurements and assess any adverse effects caused by excess body fat.

These include:

• Diet and physical activity evaluations
• An assessment of medical / health-related family history
• Skinfold thickness measurements
• A variety of other health screenings and tests as appropriate to an individual’s general condition (ultrasounds, blood tests, urine samples and more)