We are constantly hearing about the obesity epidemic in the United States. For a variety of reasons, including genetics and lifestyle, we as a country are getting heavier and less healthy.
From a National perspective the results of this trend are frightening. From a personal perspective they can be tragic.
What is obesity, how do you know if you are obese, and, why worry about it?
What is Obesity?
Obese is a funny sounding word with serious consequences. I think most of us know what obesity looks like, but what is the difference between being overweight and being obese.
Overweight is defined as a body weight that exceeds an average or that exceeds what is generally considered healthy.
Obesity is a complex disorder involving an excessive amount of body fat.
While many of us can subjectively define “overweight” or “obese”, The Centers for Disease Control (CDC) uses the body Mass Index, or BMI, to define overweight and obesity ranges.
The definitions for adults are:
- An adult who has a BMI between 25 and 29.9 is considered overweight.
- An adult who has a BMI of 30 or higher is considered obese.
The following table provides an example.
|5′ 9″||124 lbs or less||Below 18.5||Underweight|
|125 lbs to 168 lbs||18.5 to 24.9||Healthy weight|
|169 lbs to 202 lbs||25.0 to 29.9||Overweight|
|203 lbs or more||30 or higher||Obese|
The CDC link above will give you for more information on BMI and weight trends in the United States.
Whats your number?
BMI does not directly measure the amount of fat that your body has. Body Mass Index (BMI) is a number calculated using a person’s weight and height. No other factors are taken into account.
While the BMI is quick and easy to calculate, there are more accurate methods to measure body fat.
The BMI calculation is a good place for you to start, even though it does not take into account your gender, ethnicity, frame size or muscular development. For most of us it is a reasonably accurate measure to start with and can be done at home.
If your BMI calculation shows that you are overweight or obese, speak with your doctor. She may perform other procedures or refer you to a specialist who can perform more precise measurements of your body fat composition.
Methods to determine body fat percentage
Calipers (Anthropometry, or Skin fold Measurements)
Using a caliper, skins folds are measured in 3-7 locations on the body. Sometimes the measurements are taken several times and averaged to get a more accurate number. These numbers are then put into an equation to determine your body fat percentage. Over 3500 equations have been validated for different body types, age, gender, ethnicity etc.
Like the BMI, skin fold measurements are inexpensive to perform. However, their accuracy depends greatly on the skill of the person doing them and the quality and accuracy of the calipers being used.
The Dunk Tank (Hydrodensitometry, or Underwater Weighing)
In this method a person is weighed outside of the tank and then submerged into a tank of water. They are weighed again in the water and their body density is calculated. This process involves total submersion in the water and exhaling to empty the lungs as completely as possible. This is not a comfortable experience for some people. Using standard equations body fat percentage is calculated from these measurements.
Hydrodensitometry is considered one of the “Gold Standards” of body composition measurement. Done correctly, this method has been shown to be very accurate.
In clinical studies subjects often need to be measured several times and then the results are averaged. There is also the problem of getting all or nearly all air out of the lungs while submerged and keeping the water as still as possible.
This is a method that uses air displacement instead of water to measure body composition. You sit inside of the “Bod Pod” for 8-12 minutes and get a variety of measurements. They claim this method is more accurate than the water displacement method.
In this measurement technique a 50kHz current is applied through electrodes attached to the hands and bare feet. Impedance is the measure of the opposition to the passage of a current when a voltage is applied.
Water is a good conductor of current so it has low impedance. Fat tissue contains 10-20% water so it has greater impedance than fat-free mass in the body which can be 70-75% water.
The results of these tests are then put into equations to figure body mass. When combined with height and weight measurements and body type it is possible to calculate the percentage of body fat, fat-free mass, hydration level and other composition values.
Since the volume of water in the body is being measured, dehydration can affect the results. In preparation for the test care must be taken to maintain similar levels of hydration over a series of tests. Higher accuracy is achieved if the test is performed several times. Some clinicians use this test to measure the level of hydration in athletes or the chronically ill.
This is a painless, simple test, that is inexpensive to perform and under controlled conditions it can be quite accurate. When it was originally available commercially in the 80’s this method was considered un-reliable. Over the years, equipment developed for the clinical and medical practices have become highly accurate.
In 1992, Tanita, introduced a scale that not only measures your weight but also has BIA technology built in. This scale will automatically measure your weight and impedance and calculate your body fat percentage. Today they have a variety of models that range from home to professional use and can get quite sophisticated.
Tanita has some useful charts on body fat, BMI and the health risks of excess fat on their website. This link will download the pdf file for you.
Sometimes you can find these machines at health clubs or physical therapy offices. The accuracy of this measurement method and device have been questioned.
This test uses a fiber optic probe pressed up against the skin. Various sites on the body can be used but typically the bicep is used. Infrared light from the optic probe penetrates the tissues and bounces back off of the bone.
Studies have shown that optical densities are linearly related to subcutaneous and total body fat. The NIR data is entered into an equation with the person’s height, weight, frame size and level of activity, to estimate the body fat percentage.
As with all tests there are variables that can affect the test such as skin color and hydration levels. The amount of pressure from the probe against the skin can also affect results. This is another simple, fast and inexpensive test but more research may be needed.
This is a relatively new technology which can measure total body mineral, fat-free mass and fat tissue mass. DXA is based on research that shows bone mineral content is directly proportional to the amount of photon energy absorbed by the bone being studied. DXA is most widely used to diagnose and monitor osteoporosis.
This technique has the subject lay still on a table for 10 to 20 minutes. The scan uses two low-dose x-ray beams of different energy levels to measure the entire body in 0.5 cm intervals. This test method has been used and studied extensively and has the ability to show exactly where fat is distributed on the body. Because of this method’s precision with only one measurement, it is becoming the new “gold standard”.
I had a DXA or DEXA scan performed at the Cenegenics Medical Institute in Boston. To learn more about the scan and my experience, check out this post.
Converting BMI to body fat percentage
These other techniques do measure body fat, but they need to be done by a trained professional, may be expensive and can be hard to find. If you have done one of these other methods then you know how many of your total pounds are fat, you know your number.
If you are like me and have only done a BMI calculation there is one more step that you need to take to determine your body fat percentage and how many pounds of fat you are carrying around. At livestrong.com I found a formula to convert BMI to percentage of fat.
|Multiply BMI by 1.2|
|Multiply your age by 0.23|
|Add results of steps 1 & 2|
|Subtract 5.4 from this total|
|This is your body fat percentage|
|Multiply BMI by 1.2|
|Multiply your age by 0.23|
|Add results of steps 1 & 2|
|Subtract 16.2 from this total|
|This is your body fat percentage|
As an example I will use my numbers.
I am 6’ 1” and as of this morning I weigh about 173lbs and I am 48 years old. According to the BMI calculators on the CDC’s web site my BMI is 22.8
Step 1 22.8 x 1.2 = 27.36
Step 2 48 x .23 = 11.04
Step 3 27.36 + 11.04 = 38.40
Step 4 38.40 – 16.2 = 22.2%
According to these calculations my body is 22.2% fat and I am carrying 38.406 pounds of fat around with me. That seems like a lot of fat, but my BMI is well within the normal range of 18.5 to 24.9, which is based on population averages
No one is claiming that the BMI is the most accurate method to measure body composition. It also stands to reason that the indexes will shift higher over time as the trend of society to grow heavier continues. But it is free and can be done at home with reasonable accuracy.
The scales that we use to weigh ourselves are the most significant variable. The scale in my bathroom is different from the scale at my gym and different from the scale at my doctor’s office. My doctor’s office scale usually has me 10lbs heavier than my bathroom scale. If I use my doctor’s data I am 183lbs and my BMI becomes 24.1. Using the same calculations from livestrong.com, my body fat percentage goes to 23.76. This is still within the normal range.
I use my bathroom scale as the constant. Whatever it’s inaccuracies; they are the same every day. My doctor uses a clinical tool in his office. You are all familiar with the black and chrome weight and height scale in just about every doctor’s office in America. I have to assume that his equipment is more accurate than mine. To monitor your weight it is a good practice to use the same scale each time.
Looking at my numbers, the BMI seems to correlate fairly accurately to my body fat percentage. Using my bathroom scale, my BMI is 22.8 and my body fat percentage is 22.2%, so the correlation is .97. Using my doctor’s scale my BMI is 24.1 and my body fat percentage is 23.76, or a .98 correlation. This may not be accurate enough for science and engineering, but we’re not trying to land on the Moon. We just need a fairly accurate measure of our body fat. Using the other techniques we can gain greater precision.
Why should you care about your number?
Now that you know the different ways to determine your body fat percentage, now what?
Why should you care and is it really that important?
A 2012 report by the CDC’s National Center for Health Statistics showed that in 2009-2010, 35.7% of all adults 20 years and older were obese. For women 60 years and older the percentage rises to 42.3%. Here is the link to the pdf of the full report.
Obese means a BMI of 30 or higher. If my BMI were 30 my body fat percentage would be 30.84% and I would be carrying around over 56lbs of body fat! That is 13lbs more than my doctor’s scale shows I am currently carrying.
But what does this mean to you? What would a BMI of 30 mean to you? According to the CDC: Research has shown that as weight increases to reach the levels referred to as “overweight” and “obesity,” the risks for the following conditions also increases:
- Coronary heart disease
- Type 2 diabetes
- Cancers (endometrial, breast, and colon)
- Hypertension (high blood pressure)
- Dyslipidemia (for example, high total cholesterol or high levels of triglycerides)
- Liver and Gallbladder disease
- Sleep apnea and respiratory problems
- Osteoarthritis (a degeneration of cartilage and its underlying bone within a joint)
- Gynecological problems (abnormal menses, infertility)
- · In 2008, medical costs associated with obesity were estimated at $147 billion; the medical costs for people who are obese were $1,429 higher than those of normal weight.
According to the American Diabetes Association medical costs for a diabetic are 2.3 times the cost of a person without diabetes. Diabetes causes many chronic diseases that are expensive to treat such as: Heart disease and stroke, high blood pressure, blindness, kidney disease, nervous system damage and amputations. In 2007 the total cost for diagnosed diabetes was $174 billion.
80% of people with Type II diabetes are overweight or obese. A CDC study showed that people with a BMI of 40 or higher are 7.37 times more likely to be diagnosed with Type II diabetes than those with a normal range BMI. As your BMI increases beyond the normal range your risks for diabetes also increases.
A major factor in getting Type II diabetes is your weight. Type II diabetes increases your risk of the diseases listed above. Your weight and fitness level are within your control for the most part. By increasing your activity level and making moderate dietary changes you may be able to avoid Type II diabetes and the cascade of diseases that result from this condition.
This is why you should be concerned about obesity and your own levels of fitness and body fat. Genetics does play a role in obesity but for most of us it does not condemn us to a lifetime of obesity and illness. Discovering your BMI or body fat percentage is the first step in taking control of your health and your future.
So, whats your number?