Body Mass Index vs. Body Fat Percentage

Posted: December 12, 2010 in Uncategorized

Body composition is the term used to describe the different components that, when taken together, make up a person’s body weight. The human body is composed of a variety of different tissue types including lean tissues (muscle, bone, and organs) that are metabolically active, and fat (adipose) tissue that is not.

BMI and body fat percentage are weight-loss terms thrown around with little explanation, so if you’re confused about what they mean or which you should pay more attention to, you’re not alone. Here’s a brief primer to help.

BMI stands for Body Mass Index. It’s a simple measure of the weight of a person scaled according to their height. You can calculate it by dividing your weight in kilograms by your height in metres squared. For example, a person who is 1.70m and has a body weight of 65kg would have a BMI of  22.5. These are the cut-offs for BMI. This is the revised BMI cut-offs because the Health Promotion Board has found out that many Asian populations, including Singaporeans, have higher proportion of body fat compared to Caucasians of the same age, gender and BMI.



Body fat percentage  is the weight of a person’s fat divided by the person’s weight. It lets you see how much of your body is made up of fat, and how much is bone, muscle, blood, and organs. There are several ways to calculate your percentage. Your skin folds can be gently pinched with calipers, some high-tech scales can measure body fat, and there’s also underwater weighing. Many people get their body fat percentage measured if they are into sports or trying to measure their progress while losing weight.


Which you you be more concerned with?

I would say body fat percentage. Why? Lets look at a couple of scenarios.

Scenario 1: We have a healthy young man weighing 60kg and standing at 1.65m but has excess stored fat around his waist. His BMI is 22 and that in the healthy range right? However, if we were to use a body fat percentage analysis, he might be of the overfat category.

Scenario 2: A bodybuilder has a weight of 90kg and also stands at 1.65m. Using the BMI, he would have a value of 33!  That’s way over the high risk. But if were to use a body fat percentage analysis, he would probably have about 5% body fat.


Hence, using a body fat percentage analysis would be better than the BMI scale. BMI doesn’t take into consideration how much of your weight is muscle and bone and how much of it is fat.



Ways to measure your body fat percentage would include:


DEXA (Dual energy X-ray absorptiometry) scan

The most accurate way to measure body fat is to use a DEXA scan. This is a special x-ray device that can measure body fat. This test is used to measure bone density, but it also measures body fat percentage as well as where most of your fat is. DEXA uses a whole body scanner and two different low-dose x-rays to read bone mass and soft tissue mass. It takes about 10 to 20 minutes to do a body scan. It provides a high degree of precision with a 2 to 3 % margin of error. This is considered a gold standard for measuring body fat and bone density. It is painless too.


Bioelectrical Impedance (Home Body Fat Scales)

Body fat scales claim to measure body fat quickly and conveniently. These body fat scales use the Bioelectrical Impedance method to measure your body fat percentage. A low-level electrical current is passed through the body and the “impedance” or opposition to the flow of current is measured. The result is used in conjunction with your weight and other factors to determine your body fat percentage. This method can be accurate (4% margin of error) but the results are affected by hydration, food intake and skin temperature. If you are dehydrated, your body fat percentage will read higher than it is.


Skinfold Calipers (Callipers)

This method is also known as Pinch Test. Measuring body fat with a set of body fat calipers is the most cost effective way to determine fat levels. Calipers are a pincher-like device that are used to pinch your skin in specific locations on your body, such as your waist. The American College of Sports Medicine (ACSM) says that when performed by a trained, skilled, tester, they are up to 98% accurate.


Can I change my body fat percentage?

Yes. To increase or decrease your percent of body fat you need to create the right balance between the calories in and calories out. The best way to do this is to decrease daily calories by about 500 and increase your exercise. Aerobic exercise along with strength training is ideal. If you are beginning a new exercise program, you are advised to first consult your physician.


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