Article # 191. What is the difference between BMI and BMR?

What is the difference between BMI and BMR?

BMI, or body mass index, is a statistical measurement of a person’s body weight that is based on both weight and height. It is used to determine what a person’s ideal weight is. This number is not only an indicator of body fat, but can be used to estimate the probability of various diseases relating to obesity, such as heart disease and diabetes.

In comparison, BMR refers to a person’s Basal Metabolic Rate, or the number of calories a person would burn in a day even if he or she did nothing but sit or lie in one place. The formula to determine BMR is called the Harris-Benedict Equation and is a calorie formula that uses your BMI, age and sex to determine your BMR.

A person’s ideal BMI, for the most part, stays constant—as long as you are at one height, your ideal weight will always fall within a certain range. Actual BMI will change as you lose or gain weight and muscle. Your BMR, however, will decrease as you age. As we get older, our metabolisms naturally slow, making it harder for us to eat whatever we want and still be skinny.

Other things make our metabolisms slow down, as well, including not eating. If your body expects a certain amount of calories to keep everything steady, depriving it of calories put it into a shock of sorts. So, if you start eating less, your body will lower its metabolism rate further in order to keep more of the calories. Sure, if you continually keep eating less than you should, you will lose weight. However, once you begin eating regularly again, you’ll find you gain it all back that much faster.

A regular exercise regimen, however, can increase your basal metabolic rate. Bodies that are moving a lot need more food to be healthy. Do you have any idea the amount of calories that professional athletes are required to eat in a day? As you increase your activity, your metabolism increases and so does the amount of fuel your body needs to function well.

BMI and BMR Calculations and What They Mean

Your BMI is calculated only by your weight and height (for a child, his or her age and sex are also factors). Your BMR is determined by your weight, height, sex, and age. The first can help you determine your immediate health needs (where are you in the range of “health”?) and the second can help you determine what changes you may need to make your diet in order to be where you want to be.

A normal BMI will be between 18.5 and 24.9. You are considered underweight if your BMI is less than that. Anyone with a BMI under 16.5 is considered severely underweight and it is an unofficial criterion for anorexia. The BMI category from 25 to 29.9 is overweight. You are considered obese if your BMI is over 30, and are morbidly obese it is over 40.

American BMI / BMR Standards vs Other Countries

Of course, these are American standards; other countries, such as Japan and Singapore have lowered their BMI ranges. In Japan, one is considered obese if his or her BMI is over 25. In Singapore, you are not considered to be severely underweight unless your BMI is under 15. Other countries base their BMI ranges on health risks rather than weight. The United States tends to have the highest ranges (we also tend to have the most overweight people, in general).

Given all this, if you are a 30 year old woman who is 5’5” and 130 pounds, your BMR is 1385 calories a day to stay at your current size. Your BMI is 21.6, putting you squarely in the “healthy” zone. If you weight 180 pounds, your BMI is 30, and your BMR is 1595. As a BMI of 30 makes you obese, whether you are in the United States or elsewhere, you probably will not want to continue consuming 1595 calories a day with no activity.

Another thing to keep in mind in regard to BMIs in particular is that your BMI is not a perfect measurement. An athlete in his or her physical prime may be considered overweight (or maybe even obese) if he or she has a lot of muscle mass. There are several physical factors that BMIs are not able to take into consideration, so it is not always the most useful tool for individuals to use to determine their ideal weights.

The two seem to work well together, however. In 1972, a study said that BMI was better to determine the overall health of populations at large, and not so good for individuals. However, because it gave doctors a more objective chart, it made it easier to talk with their individual patients about weight in relation to health. While it may not be fool-proof, it is a great tool to get the conversation started. That, paired with the knowledge that your BMR can give you, can get you started on a great path to your ideal self.