BMI & BMR

Body Mass Index (BMI) 

Body Mass Index (BMI) is a value used to get a general sense of a person's overall mass and is calculated using a person's height and weight. It is more often used than weight alone to determine if an individual is underweight, normal-weight, or obese. The following two equations can be used to calculate BMI depending on the units used for height and weight.

 BMI = Weight (kg) / Height2 (m2)

                or

BMI = [ Weight (lbs) / Height2 (in2) ] x 703

Though BMI is often used to help assess for excess weight, it is by no means a perfect representation of a person's body fat percentage. For example, an individual can have a higher than normal BMI but have a normal body fat percentage if they have higher than average muscle mass because excess muscle contributes to a higher weight. The following table shows how different ranges of BMIs are often categorized into underweight, normal weight, overweight, and obese.

BMI 

 Classification of Overweight and Obesity by

                                                            Body Mass Index (BMI)

	Obesity Class	BMI
Underweight		< 18.5
Normal Weight		18.5 – 24.9
Overweight		25.0 – 29.9
Obesity	I	30.0 – 34.9
Obesity	II	35.0 – 39.9
Extreme Obesity	III	>40.0

Since BMI is not a perfect representation of a person's body fat percentage, other measurements like waist circumference are often used to better assess for unhealthy excess weight as it pertains to body fat. Despite not being a perfect representation of healthy and unhealthy weight, BMI is very important to value because it helps health professionals identify people who are at higher risk of developing illnesses like diabetes, hypertension, dyslipidemia (high cholesterol), liver disease, and some cancers. In general, as BMI increases so too does a person's risk of developing those previously stated illnesses. Additionally, regularly calculating a person's BMI can also be used to help track changes in a person's body mass over time.

Basal Metabolic Rate (BMR)

Basal Metabolic rate (BMR) is one of the main components of a person's daily energy expenditure. BMR is defined as the amount of energy that is expended during a given amount of time by a person at rest. In other words, it is the amount of energy a person's body uses to do things like pump blood, maintain proper brain function, breakdown toxins, and ensure other bodily functions. Technically speaking, BMR is the amount of energy the body expends during the following very specific conditions: right after waking up, while in a resting state, and after fasting for 12–14 hours. Sometimes the term Resting Metabolic (RMR) is used in place of BMR but RMR is slightly different in that it is not measured under the previously listed stringent conditions and is about 10% more than BMR.

BMR is directly proportional to a person's lean body mass. In other words, the more lean body mass a person has, the higher their BMR. BMR is also affected by acute illnesses and increases with conditions like burns, fractures, infections, fevers, etc. BMR can be measured via direct and indirect calorimetry; however, it is possible to estimate a person's BMR using one of several equations that use a person's age, sex, height, and weight to calculate a fairly accurate estimate of a person's BMR. Some of the most popular and accurate equations used to calculate BMR are the original Harris-Benedict equations, the revised Harris-Benedict equations, and the Mifflin St. Jeor equation.

BMR Formulas

The original Harris-Benedict Equations are as follows:

BMR (Males) in Kcals/day = 66.47 + 13.75 (weight in kg) + 5.0 (height in cm) - 6.76 (age in years)

BMR (Females) in Kcals/day = 655.1 + 9.56 (weight in kg) + 1.85 (height in cm) – 4.68 (age in years)[11]

The revised Harris-Benedict Equations are as follows:

BMR (Males) in Kcals/day = 88.36 + 13.40 (weight in kg) + 4.8 (height in cm) – 5.68 (age in years)

BMR (Females) in Kcals/day = 447.59 + 9.25 (weight in kg) + 3.10 (height in cm) – 4.33 (age in years)[11]

The Mifflin St. Jeor Equation is as follows:

BMR (Males) in Kcals/day = 9.99 (weight in kg) + 6.25 (height in cm) – 4.92 (age in years) + 5

BMR (Females) in Kcals/day = 9.99 (weight in kg) + 6.25 (height in cm) – 4.92 (age in years) – 161[11]

The Mifflin St. Jeor Equation was found to be the most accurate predictor of BMR compared to BMR measured by direct and indirect calorimetry.