The Body Mass Index is more commonly referred to as BMI and is used as a popular way to determine if you are underweight, normal weight, overweight or obese. Even though it may be popular and you have probably seen it or heard it mentioned in various places, that does not mean it is the best tool for the job. In fact I would suggest you don’t use the calculation at all. There are other ways, better ways.
Why should I not use it, I hear you ask? Well first we shall have a look at what it is used for and how it is calculated.
What Does BMI Tell You
The BMI is used to assess weight relative to height, the calculation used for this is to divide your body weight in kg by your height in meters squared. That’s it, it only uses 2 figures. For those who like to see the formula, here it is:
Weight (in kilograms) ÷ [height (in meters) x height (in meters)] = BMI.
If you use lbs and inches the 2 calculations below will allow you to convert your numbers.
Convert weight from lbs to kgs: Weight (in lbs) ÷ 2.2 = Weight (in kg). Convert height from inches to meters: Height (in inches) ÷ 39.37 = Height (in meters).
Once you have your BMI number you would refer to a published table which contains the different number ranges on it and the corresponding categories i.e. underweight, normal weight, overweight, obese. OK that seems simple and straight forward enough, and it is – it is also not very time consuming (easy and quick – these are the reasons the BMI gets used so often), but the results are not exact, nor are they a reliable indicator of your health.
What BMI Was Used For
The BMI calculation was introduced in the 1800s as a way of determining obesity in the general population (looking at large numbers of people and using averages). It is now used more as a screening tool for weight management but it is not designed to measure body fat percentage in individuals, or measure how healthy a person is.
Why You Shoud Not Use BMI
What this method does not do is determine the difference between body fat, muscle mass or bone density. This means that by using the BMI someone who is athletic or very fit could be categorized as overweight or even obese. This is because a fit, athletic person or athlete is likely to have more lean muscle mass, good bone density and less fat than your average (what is average?) person. It is more accurate to use waist measurements to determine if someone is overweight etc than it is to use the BMI.
Your level of body fat is a good indicator of whether you should lose weight or gain lean muscle mass, so instead of using the BMI (which does not measure level of body fat) you should be using more accurate methods to measure your body fat and fat-free mass (known as body composition). A person’s body composition is one of the measurements used when assessing someone’s level of health and fitness. It is used in conjunction with other methods which include fitness tests and specific health screening tests.
On your quest to get fit, improve your health, lose weight, get leaner etc you should test where you are at the beginning before you start your journey. You need to know where you are to determine what level you should be working at. For example, if you are not very fit and have done little to no exercise for some time you wouldn’t jump straight in with a fitness plan that has been created and designed for someone else who has been going to gym 5 sessions per week for the last 6 months. Or, alternatively if you are already at a good level of fitness you don’t want to follow a fitness program that doesn’t challenge you. Your level of fitness determines what level of intensity you should be working at and how often.
By having starting measurements you will be able to easily track your progress/see if what you are doing works or isn’t working and you have the added benefit that any changes in measurements etc can be used as a motivational tool.
What to Use Instead
Measuring Body Composition – Body Fat
If you want to lose weight, really you are saying you want to lose body fat. So you need to know what your body fat % is. If you want to get leaner, that is an increase in lean muscle mass, again you need to know your body fat/lean muscle mass (also known as fat-free mass) %. Measuring body fat can be done in the following ways:
Using a set of calipers measurements are taken from various areas of your body. The areas that are measured are the chest, midaxillary, triceps, subscapular, abdomen, suprailiac and thigh, although you can also get a reliable figure from only having three areas measured. The next step would be for the appropriate formula to be followed (there are different equations which are dependent on the sites measured, your age and gender) and the final figure will be your body fat %. The accuracy of these results is approx +/- 3.5% but is also dependent on the skill of the person taking the measurements. Calipers can be bought easily along with booklets on how to use them but a fitness professional would be a wiser choice as they should have had the necessary training and practice.
Bioelectrical Impedence Analysis
Technological improvements over the years have made this a more reliable form of measuring body composition than it used to be. Many home scales have this function and you can also buy hand held equipment but the results are unlikely to be as accurate as the equipment used by gyms and professionals. The equipment works by measuring the resistance of body tissue to the flow of a small electrical current sent through the body, the test is non-invasive and very quick. It should be noted that the timing of the BIA measurements can also make a difference to results, i.e. if you are dehydrated, have just eaten or just exercised.
Underwater Weighing (Hydrodensiometry)
This is based on Archimedes’ principle which states that when an object is immersed in water the buoyant force is equal to the weight of the water that is displaced by the object. This method is best left to the professionals to measure for you.
You can also use one of the 2 methods listed below, these don’t tell you what your body fat % is but they can give an indication if you are carrying to much body fat around your middle section. Therefore they are used to help assess health risks, body fat can be found in various areas of the body but where fat is stored can make a difference to the risks associated with high body fat %.
This measurement is a simple way to determine if you are carrying excess fat around your middle and helps assess the associated health risks. According to American Council On Exercise High Risk is >39 inches for men and >35 inches for women
Waist to Hip Ratio (WHR)
This is the circumference of the waist divided by the circumference of the hips, it is a very straight forward and simple method to determine body fat distribution. High Risk is >0.95 for men and > than 0.85 for women.
There are other ways to measure body fat which are more accurate but can be expensive and difficult to access, such as DEXA Scan and Plethysmography (similar to water displacement but using air).
Your body composition can help determine whether you need to lose a few lbs or gain a few lbs but that still may not indicate you how fit you are. To determine your current level of fitness there are various tests that can be undertaken. If you are curious about your level of fitness you can take these tests and find out, and if you are signing up to a personal trainer or gym these may be the tests that they use – so have a look to see what you may be asked to do.
Other Health Tests
Health isn’t just about measuring your weight and testing your fitness level. There are other tests such as heart rate, blood pressure, cholesterol etc that can also help determine your health not to mention tests which your doctor can undertake. It may be beneficial to have a health check-up with your doctor and it is always recommended you speak to your doctor before starting any fitness program.
Other Ways to Help Track Progress
- Circumference Measurements – It is also a good idea to take other circumference measurements to keep track of changes i.e. chest, waist, buttocks and hips. Depending on your goals, you may want to take further measurements such as bicep, forearm, mid-thigh and calf.
- Photos – Another great way of tracking progress of changes to your body is by taking photos. Take photos from both the side angle and front. Try to keep the distance and lighting of the photos the same each time you take a progress photo. Seeing changes in photos over the weeks/months will be clear evidence that what you are working hard to achieve is making a difference. Looking at the photos is very motivating for those days you don’t feel like doing a workout.
- What about Scales? – Many people use scales to measure whether they have lost weight or not, this isn’t always an accurate indicator though, if you start to lose body fat and increase muscle the scale may not actually move that much (or it could show an increase).
So there you have it, it is clear that the BMI is not really a helpful tool to an individual when determining body composition or level of fitness, you can’t get an accurate idea of someone’s health or even their level of fitness by their height and weight alone.
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