What is Near Infrared Interactance?

Near Infrared Interactance is one of many measures of body composition, which uses a computerized spectrophotometer with a single, rapid scanning monochromator and fiber optic probe. This method uses the principles of light absorption and reflection to measure body fat. The technology was originally developed by the United States Department of Agriculture to measure the body composition of livestock and the fat content of various grains.

At approximately 928 nanometers, near-infrared light provides a direct measure of subcutaneous fat through absorption of the infrared portion of the spectrum. A monochromator, or light "wand", sends a low-energy beam of near-infrared light into the biceps area that penetrates underlying tissue to a depth of one centimeter. The energy is either reflected, absorbed, or transmitted, depending on the scattering and absorption properties of the biceps. A detector within the wand measures the intensity of the re-emitted light, and shifts in the wavelength of the reflected beam combined with prediction equations compute the percent body fat.

How to Perform NIR Body Fat Testing

Equipment required: Computerized spectrophotometer (such as Futrex-5000 or BodyScript analyzer)

Pre-test: Explain the test procedures to the subject. Perform screening of health risks and obtain informed consent. Prepare forms and record basic information such as age, height, body weight, gender, and test conditions. See more details of pre-test procedures.

Procedure: The measurement is taken on the person's dominant arm. The subject stands with arms relaxed at their sides. First, data including the individual's weight, height, age, gender, frame size, and physical activity level are entered into the NIR device. The fiber optic probe is then placed on the middle biceps of the dominant arm. The monochromator sends a low-energy beam of near-infrared light that penetrates the underlying tissue. A digital read-out displays percentage body fat and lean tissue.

NIR Testing Advantages and Limitations

✓ Advantages

  • Safe and non-invasive
  • Portable and lightweight equipment
  • Requires minimal training to use
  • High repeatability (95.3% reliability)
  • 10-second testing time
  • Not affected by hydration levels like BIA
  • No meal or beverage restrictions

⚠ Limitations

  • May underestimate fat in subjects over 30% fat
  • May overestimate fat in subjects under 8% fat
  • Assumes arm fat proportional to total body fat
  • Affected by probe pressure and technique
  • Skin color may affect readings
  • Black tattoos can absorb NIR light
  • Less validated than DEXA or hydrostatic weighing

Body Fat Standards for Athletes

Research by Robert Wood, MSc Sports Science and founder of Topend Sports, has compiled extensive normative data for athletes across different sports. The following table shows typical body fat ranges for male and female athletes by sport:

Sport Male (%) Female (%)
Bodybuilding (Competition) 3-5% 8-12%
Marathon Running 5-11% 12-18%
Wrestling 5-12% -
Sprinting 6-10% 12-18%
Soccer/Football 7-12% 15-22%
Swimming 8-14% 14-22%
Basketball 7-12% 18-24%
American Football (Backs) 9-12% -
American Football (Linemen) 15-20% -

Comparison with Other Body Composition Methods

Near Infrared Interactance has been validated against various criterion methods. Research shows the following correlations:

Method Correlation with NIR Notes
Deuterium Oxide Dilution r = 0.94 Gold standard reference method
Skinfold Measurements r = 0.90 Common field method
Ultrasound r = 0.89 A-mode subcutaneous fat
Hydrostatic Weighing r = 0.80-0.90 Population-specific equations
Bioelectrical Impedance (BIA) r = 0.85-0.92 Affected by hydration

NIR Equipment and Products

Products: Futrex (Futrex-5000, Futrex-6100) and BodyScript analyzers use Near-Infrared light to directly measure percent body fat. The BodyScript Body Composition Analyzer features NIR technology that provides measurements for muscle, fat, bone, and water content with gold-standard accuracy equal to hydrostatic weighing and DEXA scan techniques.

Frequently Asked Questions

How accurate is Near Infrared Interactance for measuring body fat?

NIR has approximately 95.3% reliability for body fat measurements when readers are properly trained. The method correlates at r=0.94 with deuterium oxide dilution, the gold standard reference method. However, accuracy may vary at extreme body fat levels - NIR may underestimate fat by 4% in subjects over 30% fat and overestimate by 4% in subjects under 8% fat.

What is a good body fat percentage for athletes?

Optimal body fat varies by sport and gender. For male athletes, 6-13% is considered athletic, while female athletes typically range from 14-20%. Marathon runners may be 5-11% (male) while football linemen average 15-20%. Essential fat minimums are 2-5% for men and 10-13% for women.

Why is the biceps used for NIR body fat measurements?

Research demonstrates the biceps site provides the highest correlation (r=0.934) with total body fat compared to other sites like the triceps (r=0.920), abdomen (r=0.902), or thigh (r=0.856). The biceps offers consistent measurement access and assumes subcutaneous arm fat is proportional to total body fat.

How does NIR compare to DEXA and hydrostatic weighing?

While DEXA and hydrostatic weighing remain gold standards with ±1-2% accuracy, NIR offers advantages of portability, speed (10 seconds), and minimal training requirements. NIR is not affected by hydration variability like BIA methods, making it more consistent for repeated measurements throughout the day.

What factors affect NIR body fat measurement accuracy?

Key factors include: pressure applied to the fiber optic probe, skin color and pigmentation, very dark tattoos at the measurement site, and operator training. Unlike BIA methods, NIR is not significantly affected by food intake, hydration status, or time of day, making it convenient for athletic testing.

Can NIR track body composition changes during training?

Yes, NIR has high repeatability with low intra- and inter-observer variability when testers are trained. For best tracking, use the same device, time of day, and measurement conditions. Athletes can expect to see meaningful changes of 1-2% body fat over 4-8 weeks of focused training and nutrition.

Is NIR appropriate for all athletic populations?

NIR has been validated across various athletic populations including wrestlers, swimmers, and track athletes. Population-specific equations improve accuracy. However, athletes at extreme body fat levels (very lean bodybuilders or those with obesity) may see reduced accuracy and should consider DEXA for more precise measurements.

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References

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