Measuring Body Composition


Bioelectrical Impedance

Bioelectrical impedance analysis is a means of measuring body composition. It is a non-invasive test that involves the placement of two electrodes: one on the right hand and another on the right foot. A low level, imperceptible, electrical current is sent through the body and the speed at which it travels through the body is changed based upon fat/water concentrations. Tissues with large amounts of fluid like blood, have high conductivity and electrical signals pass through quickly, but tissues like bone and fat slow the signal down.

Bioelectrical impedance analysis is how a lot of smart scales, like garmin index and fitbit aria, are able to measure body fat percentage and lean mass.

Caveats to Bioelectrical Impedance

Bioelectrical impedance has many pitfalls. Electricity will always follow the path of least resistance, this is why wires are insulated - so electricity will only flow through the wire and not the insulation. When electricity is passed through the body, it will flow primarily through water dense organs, missing most body fat. It is the least accurate measurement of body fat discussed in this section, other than BMI, but is one of the most accessible in smart home technologies.

Those who have electronic medical devices implanted are advised by the FDA to use caution when using bioelectrical impedance analysis due to electrical interference. Those with implantable pacemakers and pacemakers are especially susceptible to electrical interference.

Body Mass Index (BMI)

BMI is a measure of relative body fat based upon height and weight of a subject. BMI values are useful for a general indicator of health, but should not be a substitute for a real body fat composition test. BMI is calculated by dividing the weight of the subject by the height squared. This number corresponds to a broad category of “underweight”, “normal/healthy”, “overweight”, or “obese”.

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For adults over 20, BMI is determined based upon standard weight categories. These categories are the same, regardless of age and sex. The standard weight status categorized associated with BMI calculations are in the following table.

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Caveats to BMI

There is a correlation between body fat and BMI, however two people with the same height and weight may have significantly different levels of body fat. Athletes are known to have a BMI correlated with being overweight or even obese, but are typically very healthy. BMI is a screening tool, but is not diagnostic of the body fatness or health of an individual. Before being concerned with a BMI score, consult a healthcare professional who can understand how lifestyle can affect BMI scores.  

BMI is also not a one-size-fits-all determinant of health, especially if you are not a descent of Northern Europe. Many have criticized the BMI system for it’s racial bias. The BMI calculations for the weight statuses were developed in Belgium by Lambert Adolphe Jacques Quetelet. These numbers were based upon body types of Northern European people, not taking into account other races and ethnicities. The test has been used for over 200 years as a screening tool, and often doctors will not screen for diseases if a person is in the healthy weight category. A study suggested that Americans of South Asian heritage who have BMIs within the normal range are twice as likely to develop heart disease compared to white heritage Americans who have similar BMIs.

Dual-Energy X-ray Absorptiometry (DXA or DEXA)

DXA or DEXA is a means of measuring bone density that used an enhanced form of x-ray technology to measure bone mineral density most commonly in the lower spine and hips. DXA gives a detailed view of body composition: fat content, bone, and lean tissue. The scans takes 3 to 12 minutes, depending on the size of the person measured and type of machine used.

The DXA scan works by using two x-ray beams to scan the body. One beam is absorbed mainly by soft tissue, and the other by bone. The soft tissue is subtracted from the total and what is left is calculated as bone mineral density. The exam estimates the amount of visceral fat in patients a measure which can predict risk for cardiovascular disease and diabetes.

Caveats to DXA

DXA scans calculate fat mass by subtracting out from the body cell mass that DXA actually measures. DXA has been used to help diagnose abnormal fat distribution, its use as a diagnostic tool for conditions like this is not yet FDA approved.

Skinfold thickness measurement (with calipers)

The skinfold thickness measurement test is one of the most common methods for determining a person’s body composition and body fat percentage. This test estimates overall body fat percentage by pinching the skin and measuring the thickness ar several locations on the body: the back of the upper arm, pectoral, subscapular, midaxilla, abdomen, suprailiac, and quadriceps.

The thickness of the folds is a measure of the fat underneath the skin, known as subcutaneous adipose tissue.

Caveats to skinfold measurement

Medical professionals should perform a measurement. The range of error for an untrained person trying to calculate body fat is very high.

Underwater weighing

Hydrostatic underwater weighing, or hydrostatic testing is a means to measure body composition. It relies on Archimedes’ Principle of displacement. This measure has been used as the gold standard for body composition assessment, however isn’t used as often as you would think due to the specialized equipment needed and new methodologies coming out.

Lean tissue, such as bone and muscle, is denser than water, and fat tissue is less dense than water. Fat, however, is less dense than water. When weighing out of the water, total mass is taken into consideration. Underwater, only bone and lean tissue are measured. This allows for subtraction to calculate the fat content.

Underwater weighing takes into account two measurements: a person’s weight on land and a scale underwater. The procedure is repeated three times and then averaged.

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Caveats to Underwater weighing

The accuracy of this test is thought to be very high. One source of error that may happen is if the person is unable to expel all of the air from their lungs underneath the water. Any air remaining will make the person float and throw off the results. This is why three underwater weights are typically taken to ensure accuracy.

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Hannah Nourie