Body mass index is a measure that uses a person’s weight and height to calculate how healthy they are. It is used to calculate the supposed amount of fat a person carries compared to their total weight. The actual calculation is as follows:
BMI = WEIGHT (kilograms)/ HEIGHT (metres) x HEIGHT (metres)
The values that you get once you carry out the calculation can then be split into ranges. These are:
|Underweight||Less than 18.5|
|Normal Weight||18.5 - 24.9|
|Overweight||25 - 29.9|
|Obese||30 - 39.9|
|Morbidly Obese||Over 40|
Patients that fall into the “obese” or “morbidly obese” categories are said to be at greater risk of having other health problems such as cardiovascular disease and type 2 diabetes. However, are these ranges accurate? Should you be worried about your weight?
What factors are not considered in the BMI calculation?
Muscle mass – Muscle on average weighs three times more than the same volume of fat. Therefore someone who is muscular will put a larger dent on the weighing scales. When comparing two people of equal weight in the obese category, if one is very muscular and toned and the other has a high volume of fat then the BMI calculation becomes somewhat negligible. It is unfair to say that the muscular person is unhealthy. A more accurate measure of “healthiness” in this situation would be waist size.
Underlying illness - Patients who have certain underlying illnesses are sometimes more prone to obesity. One such illness being Cushing’s syndrome. Secondly patients who contract an illness and are unable to eat very much after will usually benefit from a higher body fat percentage. The extra store of fat can help as an energy source during recovery. So this patients “healthy” weight would naturally be higher than a normal persons.
Distribution of fat – BMI doesn’t factor in where fat is distributed on the body. Fat situated around the belly/midriff area is mainly visceral fat. This type of fat is much more harmful than if the fat is stored elsewhere. Visceral fat surrounds the major organs in the body, it puts physical strain on the organs but it also has a more sinister problem associated with it. It secretes hormones and inflammatory chemicals that can disrupt the body’s natural hormone balance and disrupts how the body manages how much energy it actually requires to function and how much it needs to store. This type of fat can actually make you put on more fat in a viscous circle that continues until the person realises they have to control it. The other problem with visceral fat is, people who seem to look healthy can sometimes have a high amount of dangerous visceral fat surrounding their organs. BMI will not be able to help diagnose these people alone and other diagnostic (blood hormone) tests will be required.
Why is BMI still used even with its limitations?
BMI is used because it is an easy way to help healthcare professionals treat their patients and keep them healthy. In general promoting healthy weight to their patients will help prevent their patients getting any further weight related complications. As of yet there are no simple calculations that can take the other factors into account. In specific cases, healthcare professionals should use other tests i.e. blood hormone tests, waist size etc. But in general the use of BMI isn’t about to diminish. Using BMI as a health indicator is also cheaper than using a more rounded multi-test system. The best thing about using BMI is that for the general population it’s something they can keep an eye on at home and self manage.
If you are concerned about your weight contact your GP as they can help. If you have a BMI of over 28 and wish to review weight loss treatment CLICK HERE.