What Exactly Does BMI Measure?

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BMI: three simple letters that can mean so much. BMI stands for Body Mass Index and it’s becoming a universal tool to measure body “fatness” even though it doesn’t actually measure body fat like using a caliper or underwater weighing (also known as hydrostatic testing.) However, despite it’s imperfections, BMI is a fairly good indicator of body composition for most people. And as such BMI has become a prevalent health screening method. Understanding what exactly BMI measures, the reliability of BMI results and how to interpret those results, will go a long way in providing the information you need to take steps to improve your health.

What exactly does BMI measure?

If BMI doesn't really measure body fatness, what exactly does it measure? BMI measures the mass of the body in relation to height and weight. And by taking this calculation one can make a relatively accurate assumption of body fat percentage. So for example, if you calculate your own BMI you will learn whether your current weight in relation to your height is considered underweight, normal, overweight or obese. (See example below.)

In addition to learning what weight classification you fall in, a BMI number can (hopefully) give you the motivation needed to lose weight: the only way to lower your BMI is to lose weight! To lose weight you need to have a calorie deficit each week of either 3500 (one pound lost a week) or 7000 (two pounds a week lost). Any more than two pounds and it's really just water.

To determine your overall calorie deficit you’ll first need to know your BMR. BMR stands for Basal Metabolic Rate. This is the amount of daily calories burned when a person is at rest. In other words, if you lay in bed all day, this is the amount of calories you would need to consume to maintain your current weight. Then your BMR and your current activity level are considered to determine your TDEE. TDEE stands for Total Daily Energy Expenditure.

So for example, if your BMR is 1596 and your TDEE is 2752, to lose one pound a week you need to eat no more than 2752 calories a day and for you to lose two pounds a week you need to eat no more than 1752 calories daily. Once you know these numbers you can chart a path forward and see how long it will take to reach your goals. (See example below.)



Diet-to-Go does not recommend going below 1200 calories a day. If you want to expedite your weight loss, don't cut more calories out, burn more. 

How reliable is BMI?

BMI is good but it’s not perfect. BMI is determined by dividing a person’s weight in pounds (lbs) by their height in inches (in), squared and then multiplied by 703. The formula:

BMI = weight / [height (in inches)]² x 703

You’ll notice this formula doesn’t take age or gender into consideration. It also doesn’t account for the amount of muscle mass an individual may have; many athletes with a large percentage of muscle mass, may measure a high BMI even though they do not have excessive amounts of body fat. It’s also important to note some other variations:

• At the same BMI, women tend to have more body fat than men. (Sorry ladies, it’s just the way we’re built.)

• At the same BMI, older people, on average, tend to have more body fat than younger adults.
• Athletes may have a high BMI because of increased muscularity rather than increased body fatness.

What does a high BMI mean?

A BMI of 18.5 to 24.9 is considered healthy, a BMI of 25 to 29.9 is considered overweight and a BMI of 30 or more is considered obese. If you score a BMI of 25 or above means you are at a much higher risk of suffering from a whole host of diseases including:

• Diabetes
• Heart disease
• Stroke
• Cancer (breast, colorectal, endometrial and kidney)

Is BMI the last word?

The good news is, just because you have a high BMI, you are not necessarily going to have a stroke or experience any ill affects. The CDC, National Heart, Lung and Blood Institute and other nationally respected institutions contend that BMI is only one factor impacting the risk for weight-related diseases. The two other main predictors include waist circumference and other existing risk factors (e.g., high blood pressure, smoking, etc.).

How to lower your BMI?

Lose weight. Assuming you are not one of those highly muscled athletes and you are registering a high BMI, the only way to lower this number is by losing weight. We all know losing weight can be difficult, and there is no magic pill, but by making small, sustainable changes to your daily eating habits, you can and will lose weight.

To expedite your weight loss, exercise. Not only can exercise help you lose weight faster but it also helps you maintain the weight you've lost. When you engage in exercise your body burns calories during the activity as well as after the activity. And as you increase your muscle mass you will burn even more calories as muscle tissue burns more calories at rest than does fat.

Remember that losing weight and keeping it off means making smart choices and establishing sensible eating and exercise habits for the long haul.

Check out your BMI now and see how you measure up.

 

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