Body Fat vs. BMI vs. The Scale

Every year, millions of people begin fighting the battle of the bulge and start an exercise and/or nutrition program.  They often begin these programs at the urging of their doctors who have indicated that based on their BMI (Body Mass Index), they are overweight or obese, and therefore, they need to lose a set amount of weight to get that BMI into the healthy range.  As a personal trainer, I welcome all of these people who are beginning the journey to a healthier lifestyle through exercise and good nutrition.  However, I’m often asked why I don’t agree with their doctor on how much weight they need to lose.  It’s both a simple answer and a complicated one.  The simple part is that as a trained fitness professional, I don’t go by BMI or the scale, but rather by Body Fat Percentage, to determine a clients needs for optimal fitness.  The more complicated answer comes with examining each way of looking at the body to determine whether each individual is overweight, over fat, or obese.

 

Since most medical professionals use BMI as their standard of measure, let’s begin by looking at what BMI is and why it is both good and bad.  Body Mass Index is basically a weight to height ratio, where weight in kilograms is divided by height in meters squared.  Since BMI has been used for years, you don’t have to do the math yourself to figure your BMI, just look it up on a chart based on height and weight.  The ease of just looking on a chart to determine BMI makes it a quick, easy, non-invasive way for physicians to determine disease risk in their patients.  In the general population of sedentary people, this is a good indicator and a great way for doctors to be able to identify patients who need to lose weight.  However, BMI doesn’t look at anything other then that weight to height ratio, so it isn’t accurate for all individuals.  In fact, according to Shape Up America (www.shapeup.org), a foundation to promote health and wellness that was started by C. Everett Coop, BMI can misclassify up to 1 in 4 people as far as whether they are underweight, healthy, overweight, or obese.  The reason for this is because BMI doesn’t look at the individual age, gender, or musculature of each person.  Someone who has a high amount of muscle mass will read as a higher BMI, even if they are otherwise healthy.  While most people will say that professional athletes shouldn’t use BMI for this reason, there are a lot more people in the world with above average muscle mass then just athletes.  For the last 5 years I’ve worked exclusively in women’s only gyms with 99% of the members being just your average women, not athletes.  But in that time, I’ve noticed a major trend in women being a lot more muscle mass then average without even realizing it.  While women are often afraid of lifting weights for fear of building bulky muscle in the gym, they don’t hesitate a second to carry their baby, the baby’s car seat, the baby’s diaper bag, their purse, and their groceries into the house—all at the same time—which in fact is a weight workout that will build muscle; Muscle that isn’t accounted for in a BMI reading.  This same population of women are the fitness population that need to focus on retaining and/or building muscle to decrease their risk of osteoporosis as they age, so not tracking their lean body mass is, in my opinion, a disservice to them in taking care of their overall health. 

 

Another device used by most people to determine their fitness level that doesn’t look at age, gender, or musculature is the scale.  The typical bathroom scale is what most American’s use each day to see where they are on their fight against fat.  However, the basic scale doesn’t know the difference between a pound of muscle, a pound of fat, or a pound of feathers; it just sees a pound.  If someone is going strictly by the scale and doesn’t see the change they want there, they will get discouraged and possibly quit their fitness program.  In actuality, this individual is making changes to their health, but just may not see it on the scale.  Two individuals can have the same height and weight, and therefore the same BMI, and still be vastly different in their shape, size, and fitness level.  How can this be?  Simple, they have different body fat percentages.  The people who are seeing changes in their clothes and athletic abilities, but not on the scale, could quite possibly be changing their body fat percentage and their lean body mass but not changing their weight or BMI.  When someone begins an exercise program, they start building muscle simply by overloading that muscle which can make it fairly simple to see a pound or two of muscle build very quickly.  Fat burning, however, takes burning 3500 calories from stored body fat in order to burn off just one pound of fat.  Depending on the type of exercise being performed, body fat may or may not be the primary source of fuel for the exercise, so it becomes quite difficult to burn fat as fast as you build muscle.  When you burn fat at the same or lower rate then you are building muscle, you may not see a change on the scale or you may see an increase on the scale.  This is why I use body composition analysis to show my clients that they have lost X amount of fat and built Y amount of muscle so that they can understand the changes on the scale (or lack thereof) and continue to see their progress and not get discouraged.

 

Body composition analysis looks at the percentage of the individual that is body fat and the percentage of the individual that is lean body mass (muscle, bone, organs, and water) to determine their fitness level.  Persons with a higher percentage of body fat are typically at greater risk of heart disease, diabetes, and certain types of cancer.  Persons with a higher amount of lean body mass are typically at a decreased risk of arthritis, osteoporosis, or muscular injury.  There are several different ways of doing body composition analysis.  Some of them are quite complex and expensive which prevents them from being used by most people.  Others are quite easy and inexpensive, but not always as accurate.  The two most common done in gyms and by personal trainers are with calipers or bioelectrical impedance.  Calipers are great because they are inexpensive and easy to use, but if done by someone who is untrained in their proper use can be quite inaccurate.  Having caliper measurements done by multiple trainers can also prove confusing because the different ways and amount of pressure each person uses to determine the measurements can have a big effect on the reading of body fat.  Another drawback to calipers is that they are only measuring the body fat just below the skin, not the body fat around the internal organs.  Personally, I prefer bioelectrical impedance because it gets a measurement of all body fat, including that around the organs and that closer to skin level.  Bioelectrical impedance uses a low-voltage electrical current to determine body fat based on age, gender, athletic level, height, and weight.  The drawback to bioelectrical impedance is that dehydration can negatively affect the reading; meaning if you are dehydrated, you will get a higher body fat reading then if you are properly hydrated.  Therefore, I try to tell all of my clients to be sure and be well hydrated before their fitness assessment.  For women, water retention during certain times of their monthly cycle will also cause some variance to the readings, so I also like to make sure my clients know when not to do a bioelectrical impedance reading each month.  Due to the fact that it does use an electrical current to determine body fat percentage, bioelectrical impedance shouldn’t be used on clients who are pregnant or who have a pace maker to regulate their heart rate. 

 

Once a body composition analysis has been done, a certified fitness professional can tell you what the reading means as far as how much of the body is fat and how much of the body is lean mass.  They can also state where the percentage falls on the health scale based on the age and gender of the client.  When choosing a certified fitness professional to help you understand your readings, I recommend one who has been certified by one of the top three certification agencies in the United States:  American College of Sports Medicine (ACSM), American Council on Exercise (ACE), or National Strength and Conditioning Association (NSCA).  There are other certification companies out there, but typically they are based on the three above and not as strict in their testing requirements.  A properly trained fitness professional can help their clients to achieve a healthy range of body fat, while maintaining or building lean body mass, leading to a healthier individual even if there is not a change in weight or BMI.