Body fat estimation using Near IR method

It is important to estimate the body fat in order to monitor obesity, nutritional plans, training schedule and the overall health status1. With the huge emphasis on health and good nutrition these days, constant monitoring of body fat is valuable. Athletes who are training for different competitions should watch their body fat contents closely since for example some sports such as rowing require a special type of body for the athlete to be successful2. Rowers require both strength and endurance for optimal performance which means they should have a lean body with minimal body fat. Figure 1 shows the suitable lean body for a rower.

Figure 1: Suitable body fat for a rower

The gold standard for body fat estimation is Hydrostatic Weighting (HW). This method is based on the Archemedeces’ principle that a body placed in a liquid will experience a buoyant force upwards equal to the weight of the liquid it displaces. The body density can be measured accurately by the HW method and an appropriate formula can calculate the percent body fat. Due to the fact fat floats on the water whereas muscle sinks, leaner and more muscular bodies register more densities than fatty bodies. Figure 2 shows the apparatus for HW method.

Figure 2: Hydrostatic weighing method of determining body density

Although HW method is very accurate, it is expensive and can not be deployed when a large number of samples are required1. Three other portable methods are suggested here that can replace the HW method. These are Skinfold Measurement (SF), Bioeletrical Impedance Analysis (BIA) and the Near Infrared Analysis (NIA). From a practical point of view, the aforementioned methods are more suitable for clinical applications3.

These three methods will be compared in this article and the accuracy of each method is compared against the gold standard which is the HW method.

Skinfold measurement (SF)

Figure 3 shows how the SkinFold measurement is done.

Figure 3: A typical SkinFold measurement

This measurement uses a gauge and at certain points in the body (such as shown in Figure 3), the skin is grabbed by the operator and the gauge is placed around the grabbed skin. The reading in millimeters is read off from the scale. A formula is then used to calculate the percent body fat from these readings. The cost for the instrument is typically less than $1000 and the accuracy relies heavily on the operator’s accuracy of measurement.

Bioelectrical Impedance Analysis

Figure 4 shows the pose of the patient for Bio-electrical Impedance Analysis method.

Figure 4: The patient is in a standard supine position while the electrodes are connected

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In this non-invasive method, electrodes are placed on the person’s right hand and foot while lying in a supine position. A low-level current is sent though the body. The flow of the current is affected by amount of water in the body. This current passes through blood which is conductive due to its high levels of liquid and electrolyte. However, it is impeded by fat tissues that have low conductivity. As BIA passes through the body, it determines the amount of water from which fat content can be calculated using a set of equations4

NIR method

The NIR method is based on research findings that pure fat has a peak absorption at 930 nm and pure water has absorption at 970 nm5. The Futrex-5000 near infrared spectrophotometer developed in the 1990’s contains two photodiodes at 940 and two photodiodes at 950 nm with four silicon detectors. First the optical density is measured using a Spectralon reference surface and then the optical density from the biceps are measured. The weight, height, gender, physical activity levels, frame size and age are either measured or determined and all the data is plugged into an equation which can determine precent body fat. Figure 5 shows the near IR measurement from the mid-biceps of the body.

Figure 5: Near-IR method of measuring body fat from the biceps6 

Another more advanced instrument Futrex 6100/XL1measures the body diffuse reflectance at 6 different wavelengths of 810, 910, 932, 944, 976, and 1023 nm. Silicon detectors measure the optical density of reflected light. The advantage of measuring at 6 different wavelengths in this range is because it is a closer method to the original method by Conway et.al7 who measured the optical density of reflected light from biceps in the range 700-1100 nm. The optical densities and the measurement of height, weight, age etc. are plugged into a regression equation and the percent body fat is calculated. The results of the measurements using this method suggest that this method is very promising for clinical studies. Although the absolute accuracy of the method is on the border line and can be upto 4% different from the gold standard (HW method)2, the test reliability is very good and is suitable for serial measurements related to diet and weight control management programs2.

Allied Scientific Pro offers the Nirvascan spectro-photometer that works in the range 900-1700 and can measure diffuse reflectance from the skin.

Instead of using many NIR emitters and silicon detectors, the very affordable spectro-photometer uses two internal Halogen-Tungsten detectors and a single element InGaAs detector. This is truly an advantage over the Futrex method since the full spectrum is available for analysis. An app can be developed that can take the readings and convert them into percent body fat using a regression equation.

Fat molecules such as stearic (C17H35COOH), arachidic (C19H39COOH), Oleic (C17H33COOH) and linoleic (C17H31COOH), all contain CH bonds that can be measured in the third and second overtone regions of an infrared spectrophotometer with a 900-1700 nm range. Figure 6 shows the method of measurements from the biceps and the measured spectrum with CH absorption bonds indicated on the graph. 

Figure 6: Measurement of diffuse reflectance using the Nirvascan spectrophotometer from biceps and the resulting spectrum

A calibration method needs to be developed by measuring the body fat content of at least 40 volunteers of varying body builds using for example the SkinFold method. The spectra from their biceps need to be measured as well. The developed Partial Least Square Model (PLS) will be used to predict the body fat content of new subjects.

The following link contains more information about the Nirvascan Spectrophotometer.



1-  Percent body fat estimations in college men using field laboratory methods, a three compartment model approach, J. Moon, Dynamic medicine, 2008, 7:7.

2-  Validity of near-infrared interactance *Futrex 6100/XL) for estimating body fat in elite rowers, D.H. Fukuda et al., Clin Physiol Funct Imaging (2017) 37.

3-   Validation of the Futrex-5000 near-Infrared spectrophotometer analyzer for assessment of body composition, D Nielsen, JOSPT, volume 16, number 6, December 1992.1992


5- Reliability of near-infrared interactance body fat assessment relative to standard anthropogenic techniques. Pamela Schriener et al, J Clin Epidemiol, Vol 48, No 11, 1995.


7-  A new approach for the estimation of body composition: infrared interactance, JM Conway, Am J Clin Nutr, 40(60:1123-1130

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