Body Mass Index vs Body Fat Percentage

You may know that you are overweight and know how much you actually weight, but how “much” are you overweight? There are generally two ways to gauge how overweight you are: Body Mass Index (BMI) and Percentage Body Fat (PBF). These measurements are not necessarily in opposition. They can actually be used together. To be able to use these measurements, however, you should know what each measures and their respective advantages and disadvantages.

 

Body Mass Index (BMI) is widely used measurement for how overweight someone is. It is calculated by dividing body mass (in kg) by body size (m^2). The BMI scores are then classified based on their values. The below chart summarizes the various BMI scores and their classifications.

BMI Classification Score
Underweight <18.50
Normal Weight 18.5-24.99
Overweigt 25-29.99
Obese >30

Normally, a BMI score of less than 18.50 connotes unhealthy underweight status.There is such thing as being too skinny which poses health risks just as serious as being obese. Any score above 30 suggests that you maybe obese. Of course, you should desire to be in the healthy range of 18.5 to 24.99. But don’t fixate on exact figures. Having a score of 25.50 is not the end of the world. In fact, as you will see later, it may not necessarily imply you’re overweight at all.

BMI is handy tool. All you need is your weight and height, and you have your number. I believe the BMI is useful measure for most people. This belief is backed by scientific studies that show moderate correlation between BMI and other more reliable measurements for body fat. And the reason for this correlation will make sense to you shortly.

 

For most people BMI is reliable measure of excess fat for a given size. However, for athletes and bodybuilders, BMI is not reliable. BMI is a blunt instrument: it assumes all people possess nearly identical muscle mass composition. We intuitively know that is not true. Take the case of two 6 ft men each weighing 240 lbs. However, one has only 8% body fact percentage and is all muscular, gym junkie. The other is a couch potato with 25% body fat percentage. Both of these men would have a BMI of 32.50 rendering both men as obese. Obviously, this is nonsense. The gym guy is in better shape as most of his weight is comprised of lean muscle mass whereas the coach potato dude has a lot less lean muscle mass. But the gym dude doesn’t represent the average man. That is why there is a moderate correlation between BMI score and actual body fat percentage.

 

Body Fat Percentage (BFP) is more accurate measure of body fat. Beyond that, the BFP informs about your overall fitness level. The lower your BFP, the fitter you are. Unlike the BMI, the BFP score incorporates your gender and age into its calculation. BFP eliminates almost all of the disadvantages of the BMI. Table 1 below shows a rough guide for BFP for women and men.

Table 1
Body Fat Classification Men Women
Essential Fat 2-5% 10-13%
Athletes 6-13% 14-20%
Fitness 14-17% 21-24%
Average 18-24% 25-31%
Obese >25% >32%

 

Note that men have lower threshold for obesity and for essential fats than women. Essential fats are the bare minimum your body needs to function properly. So, while we want to shed fat, some of it is actually necessary for our bodies to function properly. Generally, for men, if you want to have six-packs you’ll need body fat percentage below 10%, maybe below 8%. The table shows BFP for different ages of both genders.

Body Fat Percentage

 

 

 

 

 

 

 

 

 

 

 

There are different ways to measure your Body Fat Percentage. For the average person, the best method is to purchase calipers. You can purchase them at most fitness stores. Some weight scales come with purported body fat percentage measuring capabilities. These scales send electrical impulses through your body and then measure the resistance. From this the instrument extrapolates your body fat percentage. The problem is there are a lot factors that can affect the resistance the electrical signal meets. How much water you have in your body, how dirty/clean you feet are, the last time you ate… all affect the reading you get. That is why I prefer using the calipers.

Caliper

Your body fat percentage, once obtained, can be very help with your weight loss goals. First, you can use it to set a target. If you are obese, there is no point in setting an immediate or intermediate target of 10% body fat. You better off setting a more realistic goal of 15% against a current 25% body fat. Needless to say, you need to have a concrete plan in place delineating how you will reach your target goal. Second, once you reach your target, you need to set a realistic “range.” Reality is your body fat percentage will fluctuate. By setting a target, you can maintain a healthy weight using deviations from your target range as a reminder for greater vigilance. Your body fat percentage can also tell you if you need to pack some place. You might be of those “skinny fats,” people have weight normal person weight but are in fact carrying too much fat. The BFP will tell you if you are carrying too much fat.

 

To recap, use the BMI to get a crude estimation of your body fat. Use the Body Fat Percentage to get a more accurate measure of how much fat you’re carrying. Once you know how much excess fat you’re carrying, devise a plan to shed the excess weight.

 

 

Daily Energy Expenditure

People are often surprised to find out that most of the energy they burn in a day doesn’t come from exercise but from doing nothing other than just resting. Even on a day when you exercise, unless you’re a professional athlete who played a regulation game, most of the energy you burn will come from doing nothing more than mundane every day tasks. Reason being your body needs to spend energy just to stay alive.

 

In fact, knowing your daily energy expenditure (DEE) requirement will be instrumental in your weight loss journey. By knowing your DEE, you can create a balanced exercise and diet plan that creates sufficient calorie deficit for weight loss. For example, if your DEE is 2000, you can create 500 calorie deficit through dietary reduction and exercise. Beyond knowing your DEE you should know what affects your DEE and how you can raise it so you can burn energy while doing nothing.

 

Regular bodily functions such as breathing, breaking down and digesting food, temperature regulation, ordinary brain and other organ functions all consume energy. These activities use up most of the calories the average person consumes in a day. Removing the energy spent on digesting food, you get what is called Basal Metabolic Rate (BMR). The BMR is the “basic” daily energy requirement your body needs while at rest. There is a variant of this called Resting Metabolic Rate (RMR) which factors in calories burned digesting food. Although people often confuse the two, the difference is often minor for most people. That said, I use the BMR and advise you should do so as well. The BMR is not only the most widely used variant but also, as you will see below, the basis for the Harris-Benedict.

 

To determine your DEE, follow the following steps:

 

Step 1: Calculate your Basal Metabolic Rate (BMR). The BMR can be calculated using the Harris-Benedict Equation below.

 

Men (Metric): Cal./day = (13.75 x Weight) + (5 x Height) – (6.76 x Age) + 66

Men (Imperial): Cal./day = (6.25 x Weight) + (12.7 x Height) – (6.76 x age) + 66

Women: Cal./day = (9.56 x Weight) + (1.85 x Height) – (4.68 x Age) + 655

Women: Cal./day = (4.35 x Weight) + (4.7 x Height) – (4.68 x age) + 655

Height is in cm and weight is kg for the Metric system and in inches and pounds for the Imperial system, respectively. You can either calculate your BMR using the equation above or simply use an online calculator.

 

Step 2: Multiply your BMR by your activity level factor. Refer to the table below for activity level factors. Simply multiply your BMR by the activity level factor that best applies to you.

Activity Level Factor
Sedentary: Little to no exercise 1.2
Mild: Exercise 1-3 times per week for about 30 minutes 1.375
Moderate: Exercise 3-4 times per week for about 30-60 minutes 1.55
Heavy: Exercise 5-7 days per week at about 1 hour each time 1.7
Extreme: Exercise daily (athletes) combined with physical labour job 1.9

A word of caution: most people overestimate their activity level. So, be honest and choose a realistic factor. To be extra safe, I normally discount my BMR by 10% just in case I overestimated my activity level factor.

Now that you have your DEE, you know exactly how many calories you need to consume each day to maintain your current weight. Your next job is to use this calorie figure to create a daily deficit.

 

What affects your DEE?

 

  1. Age: As you get older your metabolic rate lowers and consequently you require fewer calories each day. Aging also causes muscle mass loss. Muscle is metabolically active burning calories even when at rest. The less of it you have, the lower your DEE.
  2. Gender: Women generally have lower DEE than men and I think this is related to differences in size and the proportion of lean mass (muscles) relative to weight. All else being equal, women generally have lower DEE than men.
  3. Weight: The heavier you are the more the higher your DEE, all else being equal. A bigger body needs more energy to sustain itself than a leaner and smaller body. However, you should be careful in how you interpret this. Not all heavy individuals are equal – a 240 lb man comprised mostly of lean mass, with less than 10% body fat percentage, has higher DEE than a comparable 240 lb man with 30% body fat percentage. In this regard, DEE is a blunt instrument. Regardless, the DEE is applicable to the average person.
  4. Miscellaneous factors: The preceding three factors account for most of the DEE value. However, there are other factors that account for small portion of the DEE. These include medical conditions that lower metabolic rate. Temperature affects your metabolism and therefore your DEE as well. Also, there are certain medications that lower metabolic rate. But these factors are very small and therefore can be ignored.

 

Marketplace on Food Labels

This Marketplace episode is more about how companies, through advertising and slogans, mislead consumers about the nutritional value contained in their food products. However, the episode underscores the importance of reading food labels and independently verifying claims instead of taking them at face value. If a food manufacturer claims one of its cereals is a “healthy breakfast alternative,” you should verify the claim by referring to the food label.

 

The challenge for most, including myself when I first embarked on my weight loss journey, is knowing the healthy amount of grams-per-day of each food group (carbs, protein, and fats). I’ll blog about this shortly. In the meantime, enjoy the Marketplace episode on food labels.

 

Food Labels

Food Labels provide nutritional information for packaged foods. They show not only calories per serving (and serving size) but also breakdown of each food group – Proteins, Carbohydrates and Fats – as well as minerals and vitamins. So, being able to properly read and interpret food labels is vital inFDA_Nutrition_Facts_Label_2006losing weight. To lose weight, you need to keep track of how many calories you consume.

 

There are two items on food labels you should be mindful of: Serving Size and % Daily Value.

 

Serving Size

I find the serving size to be the most prone to misinterpretations. Take the label to the right for example. The serving size is 2/3 cup or 55 grams. For that serving, the calorie count is 230. This means that if you only eat 55 grams of this food, you’ll consume 230 calories. It doesn’t necessarily mean the whole package contains 230 calories. This is the biggest misinterpretation of food labels. Let’s say the net weight of the whole package is 110 grams. If you ate the whole package, the total calories consumed would be: 110g/55g = 2 servings x 230 per serving = 460 calories. It is really important to pay close attention to portion size.

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