Even if you don’t know precisely what it is, or how it’s worked out, I’ll hazard a guess that you’ve heard about body mass index, or BMI before.
If you’ve ever had a health check with your doctor or dietitian, they probably made a note of your BMI.
They may even have used it as ‘evidence’ in their case to help you realize that you need to improve your physical health.
But, is it a reliable indicator of your body composition and overall wellness?
What Is Body Mass Index?
Basically, it is a calculation which uses your weight verses your height, to determine if you are underweight, normal weight, overweight or obese.
(You can also take a look at my Body Mass Index page)
The weight ranges, set by the World Health Organization, are as follows:
- 18.4 is underweight for your height
- 18.5 to 24.9 is an ideal weight for your height
- 25 to 29.9 it over the ideal weight for your height
- 30 to 39.9 is obese
- Over 40 is very obese
As you can see, these categories place sharp boundaries between underweight, ideal, overweight, and the obese groups — you could say a person’s health hinges on a single decimal place.
Or, to put it another way, the ‘number’ worked becomes the pivotal factor which determines what advice your doctor or dietitian will give you.
This, as we shall see, is totally ridiculous.
Where Did The BMI Calculation Originate?
In the early 19th century, Lambert Adolphe Jacques Quetelet, a mathematician, produced a formula to provide a quick way to measure the degree of obesity in the general population. It was called the Quetelet Index.
However, the newer term “body mass index,” referring to the ratio calculation, dates to a paper published in 1972 in the Journal of Chronic Diseases, by Ancel Keys.
In this paper, Keys explicitly states BMI is appropriate for population studies, but inappropriate for individual diagnosis.
Yes, you read that correctly. The man behind the body mass index made it clear that the measurement should not be used on a case by case basis.
Yet in spite of that, due to its simplicity and ease of use, the BMI calculation in now widely used for assessing an individual’s health status.
Why BMI Is Not Reliable
As I’ve said, the BMI calculation was designed to define the “average man” in a population.
But, when you hone that definition in on a specific individual, the theory doesn’t apply, due to the danger of gross inaccuracy on an individual basis.
It can, and often does, give a misleading result if it is used exclusively to determine how healthy, or not, an individual is.
Here are a number of reasons why BMI is not a reliable indicator of how healthy you are.
1. BMI Cannot Measure Body Fat Directly
It is impossible for the BMI calculation to make allowances for the relative proportions of bone, muscle and fat in someone’s body.
Bone is denser than muscle, and twice as dense as fat.
So for example, if you are very muscular, you will have a higher BMI score, potentially classifying you as overweight or obese, when you are neither.
As a result, a high percentage of professional athletes are placed in the overweight or obese category, because they tend to be more muscular.
At the other end of the scale, the BMI calculation is likely to provide an inaccurate measurement for those who are less lean.
Take older people as an example.
Generally speaking, they have more body fat than a younger person with the same BMI score. In this case, their BMI score may underestimate body fat, and the resulting healthrisks, making them believe they are “healthier” than they really are.
A study published in the International Journal of Obesity in 2008, found that BMI-defined obesity was present in 21% of men, and 31% of women.
However, when they switched to using body fat percentages, obesity was found in 50% of men, and 62% of women.
As you can see, the inaccuracy is clear to be seen.
2. Where Fat Is Stored Is More Important
Rather than taking time to calculate your BMI, you would be better to simply stand in front the mirror to determine your body shape, and exactly where your fat is distributed.
This “eye ball” method is much more helpful.
For women, body fat is usually stored around the hips. But for men, it is usually around the abdominal area.
If your fat is mostly around your abdomen, it poses a greater health risk than fat that is carried in the thighs or hips.
Of course, BMI does not have the ability to take this into consideration.
The truth is, using the body mass index to estimate one’s health, is liking using a shotgun to kill a fly. It’s ineffective, and can be downright dangerous. There’s a simpler, and much safer way to get the job done.
So, is BMI ever a useful tool?
As I’ve said, BMI can be a useful measurement when used to assess the health risks of a population.
But, on an individual level, it is flawed, and ought to be revised.
It is important to remember that health risks are about a lot more than simply how much someone weighs.
How To Access Your Health Risks
So, now that I’ve told you why the BMI calculation is useless, how can you accurately assess your health risks?
To get a clear picture of your overall health, it is important to look at specific measurements, such as blood pressure and waist circumference, not forgetting lifestyle factors like how much you exercise, and if you smoke or drink alcohol.
Research suggests that people with “apple-shaped” bodies (carry more weight around their waist) have increased health risks, compared to those with “pear-shaped” bodies (carry more weight around their hips).
To get a clear picture of your health risks, my best advice is to use a combination of multiple assessments.
1. Waist-To-Height Ratio (WHtR)
A 2010 study, which followed 11,000 individuals, found that BMI is not a good measure for the risk of heart attack, stroke or death.
They concluded, a better measurement is the waist-to-height ratio.
Waist-to-height ratio can be calculated by dividing your waist measurement, by your height.
So, what results should you aim for?
According to this study:
- For people under 40, a WHtR of over 0.5 is critical
- For people aged between 40 and 50 the critical value is between 0.5 and 0.6
- For those over 50 the critical values start at 0.6
2. Waist-To-Hip Ratio (WHR)
The waist-to-height ratio should not be confused with that of the waist–to-hip ratio.
This is a measurement which compares the size of your waist to that of your hips.
A healthy waist-to-hip ratio is:
- 0.8 or less for women
- 0.9 or less for men
In fact, research shows that a waist-to-hip ratio of 0.7 for women, and 0.9 for men, has been shown to correlate strongly with general health and fertility.
3. Waist Circumference
In addition to the waist-to-height and waist-to-hip ratios, a very rough guideline is simply to use waist circumference alone.
- Men should try to keep their waist circumference well under 40 inches
- Women should try to keep their waist circumference well under 35 inches
To measure your waist circumference, place the tape measure around the smallest area of your abdomen below your rib cage and above your belly button.
4. Body Fat Percentage
The body fat percentage calculation takes into consideration both essential body fat, and storage body fat.
Essential fat is that which is necessary to maintain life and reproductive functions. This is the part that is higher in women, due to the demands of childbearing and other hormonal functions, for example.
Storage body fat is that which accumulates in adipose tissue.
Because there is a strong correlation between a higher body fat and negative health outcomes, calculating your percentage body fat tells you a lot more about your overall fitness, than body weight, or body mass index alone.
Your body fat percentage can be calculated by taking the total weight of fat, and dividing this by your weight.
In other words, your lean body mass to your body fat.
A simple way to work out body fat percentage is to use a tape measure to work out the circumference of your waist, hips, forearm and wrist circumference.
These measurements can then be added to an online calculator, like the one I’ve provided below.
If you have access to skinfold calipers, they will give you a more accurate result than simply using a tape measure.
You can then add the results into the calculator provided on the American Council on Exercise website to get your result.
Either way, body fat percentage will give you a pretty good indication of your body composition.
You should definitely be aiming to keep your body fat under 30 percent, with a lower reading obviously being better for your overall health.
Body Fat Percentage Guidelines
The American Council on Exercise provide guidelines on established body fat percentage norms divided into the following categories:
Body Fat Percentages For Women
- Essential Fat 10-13%
- Athletes 14-20%
- Fitness 21-24%
- Acceptable 25-31%
- Obesity >32%
Body Fat Percentages For Men
- Essential Fat 2-5%
- Athletes 6-13%
- Fitness 4-17%
- Acceptable 18-24%
- Obesity >25%
There are obviously more technical and accurate methods, such as underwater weighing or bioelectrical impedance analysis. These are often expensive, and not a viable option for most of us.
We need not worry, though. For most of us, the calculations I have mentioned are adequate to quickly and easy determine how healthy you are.
Over to you…
What’s your experience with BMI? Have you been told by an ‘expert’ to change your lifestyle habits based solely upon your body mass index? Do you use it to keep track of your health?
If you’ve any questions, let me know in the comments below.