The “Why” of BMI and other Weight Assessment Tools

There are several weight assessment tools used today by health professionals.  Weight alone is not an accurate measurement of health status, but it is an important part of an overall assessment.  Today I want to discuss the “Why” behind BMI and other weight assessment tools that are often used in the health community.

There seems to be a lot of confusion about some of these tools and how to interpret the result, and rightly so because there is more to it than just looking at the number.

Today I thought I would define and break down some of the common anthropometric tools health professionals use to determine health status based on weight and body composition as well as the strengths and weaknesses of each.

Anthropometry: the science of measuring the size, weight, and proportions of the human body

Why Weight Alone Isn’t Enough

We all have a specific number on the scale that we like to see, but the number on the scale is not an accurate indicator of how healthy you are.  There are other factors to consider, so please do not obsess over your exact weight!

Things to consider when interpreting anthropometric measurements are gender, ethnicity, environment, and familial factors.  You also need to think about your weight related to your height, where your body fat is on the body, and your overall health and risks for weight-related problems (again, think family history and then other indicators like blood work).

Weight alone is not a good indicator of health because a person could look thin but not exercise and eat fast food everyday for most meals.  That person would most likely be unhealthy!  On the other hand, a person could be carrying extra weight and exercise daily and eat the recommended amount of fruits and vegetables.  Even with the extra weight, the person is likely to be healthy due to the healthy habits!

When looking at BMI, waist circumference, and waist-to-hip ratio, it is important to remember to look at the person after taking the measurements.  Does the person have a lot of developed lean muscle, or is there extra body fat causing the numbers to be higher?  Does the person have a larger frame?  These are legitimate factors.

Body Mass Index (BMI)

BMI is a validated measure of nutrition status that uses the height and weight of a person.  Health professionals use this measurement as a tool to assess nutritional risk of weight-related diseases.  It is calculated by taking a person’s body weight in kilograms and dividing it by the square of their height in meters.


-Inexpensive and easy-to-perform measurement for nutritional screening

-BMI can be used as a screening tool for weight categories that tend to have higher risk of certain health problems

-Highest correlation with independent measures of body fat for adults (like skinfold thickness tests, dual x-ray absorptiometry (DXA), and underwater weighing)

-Based on relationships between body weight, disease processes, and mortality


-Not diagnostic of health of an individual

-Not gender specific (men and women are measured the same)

-Ideal weight measurement may be too restrictive for older adults

-Does not measure body fat directly

-Athletes and the elderly do not always have accurate assessments based solely on BMI

Waist Circumference

Waist circumference is performed by measuring the distance around the smallest area below the rib cage and above the belly button with the use of a non-stretchable measuring tape.  It assesses abdominal fat content.  Women who have a waist circumference higher than 35 inches and men with a measurement higher than 40 inches are considered at higher risk for disease.


-Independent risk factor for weight-related disease.  As your waist size increases, so does your health risk

-Works well with BMI but is more accurate in predicting risk when measured in people with a BMI less than 35


-Not useful in people with a BMI of 35 or greater (the results don’t show any increased relative risk when compared to BMI at this point)

-Results may not be useful for people less than 5 feet tall

-Results may vary depending on the experience of the person taking the measurements

Waist to Hip Ratio

Waist to hip ratio identifies fat distribution and the related health risks.  This is where the “apple” (android) shape and the “pear” (gynoid) shape come into play.  To obtain a waist to hip ratio, a person must be measured at the smallest abdominal circumference and then again at the largest hip circumference.  Take those measurements, and divide waist/hip.  If the number is greater than 1.0 for men and 0.8 in women, then it is associated with the android shape and indicates increased risk of diabetes, heart disease, stroke, high blood pressure, and breast cancer.


-A person can visually see in the mirror where they seem to carry more weight (abdominal region for android or hips and buttocks for gynoid)

-Inexpensive to perform


-Results may vary depending on the experience of the person taking the measurements

Subcutaneous Fat (Skin-Fold Thickness)

Skin-fold thickness uses calipers to measure and assess the amount of total body fat in an individual.  It bases its measurement off the assumption that 50% of total body fat is subcutaneous.


-Useful in clinical settings


-Validity of results depend upon accuracy of person doing the measurements

-Changes take 3-4 weeks to occur

Underwater Weighing

Like the name implies, underwater weighing is when a person is submerged completely in water, and the volume of water the object displaces equals the volume of the object submerged (you).


-A direct measure of determining whole-body density

-Once considered the gold standard


-Not practical

-Person performing test must have extensive training

-It’s not comfortable for the person having the test performed on them (you have to be submerged in the water and be still long enough for the measurements to be taken)

Bioelectrical Impedance Analysis (BIA)

BIA is based on the principle that lean tissue has a higher electrical conductivity and lower impedance than fat because of its electrolyte content.  There is more water in your muscles than there is in fat, so the electrical impulse isn’t impeded as much in the lean muscle mass.  The electrical conductivity is measured by placing electrodes on various parts of the body and passing a small electrical current through those.


-Safe and noninvasive

-Portable machine

-Quick results

-Found to be a reliable measurement when compared to BMI and other standards


-Results negatively affected if the person is dehydrated, has exercised in the past 4 to 6 hours, and has consumed alcohol, caffeine, or diuretics in the previous 24 hours

-Machines vary, so person performing the test needs to understand how to operate it

Dual-Energy X-Ray Absorptiometry (DXA)

DXA (or sometimes seen as DEXA) measures bone mineral density and can be use to measure fat and boneless lean tissue as well.  It uses an x-ray tube that contains an energy beam, and the amount of energy loss depends on the type of tissue the beam is passing through.  The energy beam emits low levels of radiation, so it is safe!  I actually got to have my percent body fat measured when I was in one of my lab classes at BYU.  It was a cool experience!


-New gold standard; very accurate!

-Usually available in a hospital setting

-Patient does not have to do much other than lay there


-Not readily available to the general public


-Hydration status can affect results


There are even more methods of measuring body fat that I did not go into today.  Is it any wonder that there can be confusion when determining health based on weight?  There are so many factors to consider!  While weight alone is not a reliable indicator of health, when combined with other factors, it can help you know what your risk of weight-related disease may be.  It’s important to recognize that there is health at every size and to not focus solely on the number you see on the scale while at the same time taking seriously the risk that extra weight has on your overall health.  Talk to your doctor and then a dietitian to assess your risk and find the healthy balance that fits you!

Let’s chat!

-Have you ever used a DXA, BIA, or skin calipers?

-What do you think about these assessment tools?

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