How are estimates of body composition derived? Strengths and limitations Populations Further considerations Resources required Instrument library References. Two methods to determine total body water An allowance must be made for residual lung volume the air that remains in the lungs after a maximal exhalation as this air increases the participant's buoyancy.

Dry bodyweight After having measured or estimated residual lung volume, the participant's dry weight is measured wearing a swimming suit or running shorts. Underwater weight The participant climbs into the underwater weighing tank and sits in a seat that hangs from a force load-cell.

After the underwater weight stabilizes approximately 5 seconds , the participant is instructed, though yelling or pounding on the side of the tank, to return to an upright position. During weighing, the participant is always free to stand up or return to the upright position.

Several trials may be required before the participant becomes accustomed to the procedure and consistent readings recorded. Four to eight trials are usually performed. The depth of the water is approximately cm, and temperature maintained between 30 and 34 C.

The density of water is dependent on the temperature and should always be factored into the equation. After each testing session, the water is drained and the tank allowed to dry.

Pre-test Guidelines Participants must not eat or engage in strenuous exercise for at least 4 hours before their scheduled appointment. Body density is required to calculate percentage body fat. Strengths and limitations. An overview of the characteristics of hydrostatic underwater weighing is outlined in Table 1.

elderly Participant must be able to perform complete air exhalation There is a possibility of considerable error introduced by the miscalculation of residual volume RV The test does not identify the exact parts of the body where the fat is located e. unable to assess fat distribution The accuracy of body density measurements can be affected by the consumption of food and carbonated beverages shortly before the test, fluid loss during intensive exercise, fluid retention during menstruation, and the degree of forcible exhalation while submerged Like all the 2 component model methods, it assumes when calculating total body density that the relative amounts and densities of bone, muscle, and water comprising the fat-free mass are essentially the same for all individuals, regardless of age, sex, ethnicity or fitness level Table 1 Characteristics of hydrostatic underwater weighing.

Consideration Comment Number of participants Small Relative cost Medium Participant burden High Researcher burden of data collection High Researcher burden of coding and data analysis Low Risk of reactivity bias No Risk of recall bias No Risk of social desirability bias No Risk of observer bias No Space required High Availability Medium Suitability for field use No Participant literacy required No Cognitively demanding No.

Table 2 Anthrompometry by hydrostatic underwater weighing in different populations. Population Comment Pregnancy Suitable but unable to disentangle the mother-fetal unit Infancy and lactation Not suitable Toddlers and young children Not suitable Adolescents Not suitable Older adults May not be suitable as older individuals have less dense bones, resulting in over-estimation of body fat Ethnic groups Suitable Other athletes Athletes tend to have denser bone and muscle tissue, resulting in under-estimation of body fat Other obesity Suitable.

Further considerations. Resources required. Hydrostatic stainless steel weighing tank Underwater mounted chair and scale Weighted belt and nose clip A simpler set up may include a chair and scale suspended from a diving board over a pool Trained research staff.

Instrument library. A method specific instrument library is being developed for this section. In the meantime, please refer to the overall instrument library page by clicking here to open in a new page. Ball SD.

Interdevice variability in percent fat estimates using the BOD POD. Eur J Clin Nutr ; Brozek J, Grande F, Anderson JT, Keys A: "Densitometric Analysis of Body Composition: Revision of some Quantitative Assumptions", Academic Science ; Ellis KJ: Human Body Composition: In Vivo Methods.

Physiological Rev. Champagne, Ill. Med Sci Sports ; 1: 8 Wells JC, Fewtrell MS: Measuring body composition Archives Dis Child 91; Widen EM, Gallagher D: Body composition changes in pregnancy: measurement, predictors and outcomes Eur J Clin Nutr ; Resources Concepts Dietary Assessment Physical Activity Assessment Anthropometry Instruments.

The Toolkit About What's new Other resources Toolkit Team Contact. Risk of reactivity bias. Risk of recall bias. Risk of social desirability bias.

Because of its simplicity and widespread use, BMI is often used when studying populations. Researchers can compare the BMI of groups of people over time in different areas, to screen for obesity and its related health risks.

BMI does have several limitations. For these reasons, BMI might be used as a screening tool for potential weight-related problems rather than to diagnose certain conditions.

The accuracy of BMI in predicting health risk may vary across different individuals and racial and ethnic groups. Some populations have higher rates of obesity but that do not have corresponding rates of metabolic diseases like diabetes, and vice versa. BMI might be supplemented with other measures such as waist circumference or waist-hip ratio that better assess fat distribution.

When examining the relationship between BMI and mortality, failure to adjust for these variables can lead to reverse causation where a low body weight is the result of underlying illness, rather than the cause or confounding by smoking because smokers tend to weigh less than non-smokers and have much higher mortality rates.

Experts say these methodological flaws have led to paradoxical, misleading results that suggest a survival advantage to being overweight. Some researchers consider waist circumference to be a better measure of unhealthy body fat than BMI as it addresses visceral abdominal fat, which is associated with metabolic problems, inflammation, and insulin resistance.

In people who do not have overweight, increasing waist size over time may be an even more telling warning sign of increased health risks than BMI alone. Wear thin clothing or no clothing.

Stand up straight and wrap a flexible measuring tape around your midsection, laying the tape flat so it crosses your navel belly button. The tape should be snug but not pinched too tightly around the waist.

You can repeat the measurement times to ensure a consistent reading. According to an expert panel convened by the National Institutes of Health, a waist size larger than 40 inches for men and 35 inches for women increases the chances of developing heart disease, cancer, or other chronic diseases.

Like the waist circumference, the waist-to-hip ratio WHR is used to measure abdominal obesity. It is inexpensive and simple to use, and a good predictor of disease risk and early mortality.

Some believe that WHR may be a better indicator of risk than waist circumference alone, as waist size can vary based on body frame size, but a large study found that waist circumference and WHR were equally effective at predicting risk of death from heart disease, cancer, or any cause.

The World Health Organization has also found that cut-off points that define health risks may vary by ethnicity. For example, Asians appear to show higher metabolic risk when carrying higher body fat at a lower BMI; therefore the cut-off value for a healthy WHR in Asian women is 0.

Stand up straight and follow the directions for measuring waist circumference. Then wrap the tape measure around the widest part of the buttocks. Divide the waist size by the hip size.

The WHO defines abdominal obesity in men as a WHR more than 0. Waist-to-height ratio WHtR is a simple, inexpensive screening tool that measures visceral abdominal fat. It has been supported by research to predict cardiometabolic risk factors such as hypertension, and early death, even when BMI falls within a healthy range.

To determine WHtR, divide waist circumference in inches by height in inches. A measurement of 0. Equations are used to predict body fat percentage based on these measurements. It is inexpensive and convenient, but accuracy depends on the skill and training of the measurer.

At least three measurements are needed from different body parts. The calipers have a limited range and therefore may not accurately measure persons with obesity or those whose skinfold thickness exceeds the width of the caliper.

BIA equipment sends a small, imperceptible, safe electric current through the body, measuring the resistance. The current faces more resistance passing through body fat than it does passing through lean body mass and water. Equations are used to estimate body fat percentage and fat-free mass.

Readings may also not be as accurate in individuals with a BMI of 35 or higher. Individuals are weighed on dry land and then again while submerged in a water tank. This method is accurate but costly and typically only used in a research setting.

It can cause discomfort as individuals must completely submerge under water including the head, and then exhale completely before obtaining the reading. This method uses a similar principle to underwater weighing but can be done in the air instead of in water. It is expensive but accurate, quick, and comfortable for those who prefer not to be submerged in water.

Individuals drink isotope-labeled water and give body fluid samples. Researchers analyze these samples for isotope levels, which are then used to calculate total body water, fat-free body mass, and in turn, body fat mass.

X-ray beams pass through different body tissues at different rates. DEXA uses two low-level X-ray beams to develop estimates of fat-free mass, fat mass, and bone mineral density. It cannot distinguish between subcutaneous and visceral fat, cannot be used in persons sensitive to radiation e.

These two imaging techniques are now considered to be the most accurate methods for measuring tissue, organ, and whole-body fat mass as well as lean muscle mass and bone mass. However, CT and MRI scans are typically used only in research settings because the equipment is extremely expensive and cannot be moved.

CT scans cannot be used with pregnant women or children, due to exposure to ionizing radiation, and certain MRI and CT scanners may not be able to accommodate individuals with a BMI of 35 or higher.

Some studies suggest that the connection between body mass index and premature death follows a U-shaped curve. The problem is that most of these studies included smokers and individuals with early, but undetected, chronic and fatal diseases.

Cigarette smokers as a group weigh less than nonsmokers, in part because smoking deadens the appetite. Potentially deadly chronic diseases such as cancer, emphysema, kidney failure, and heart failure can cause weight loss even before they cause symptoms and have been diagnosed.

Instead, low weight is often the result of illnesses or habits that may be fatal. Many epidemiologic studies confirm that increasing weight is associated with increasing disease risk. The American Cancer Society fielded two large long-term Cancer Prevention Studies that included more than one million adults who were followed for at least 12 years.

Both studies showed a clear pattern of increasing mortality with increasing weight. According to the current Dietary Guidelines for Americans a body mass index below But some people live long, healthy lives with a low body mass index.

But if you start losing weight without trying, discuss with your doctor the reasons why this could be happening.

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Search for:. Home Nutrition News What Should I Eat? Role of Body Fat We may not appreciate body fat, especially when it accumulates in specific areas like our bellies or thighs. Types of Body Fat Fat tissue comes in white, brown, beige, and even pink. Types Brown fat — Infants carry the most brown fat, which keeps them warm.

It is stimulated by cold temperatures to generate heat. The amount of brown fat does not change with increased calorie intake, and those who have overweight or obesity tend to carry less brown fat than lean persons.

White fat — These large round cells are the most abundant type and are designed for fat storage, accumulating in the belly, thighs, and hips. They secrete more than 50 types of hormones, enzymes, and growth factors including leptin and adiponectin, which helps the liver and muscles respond better to insulin a blood sugar regulator.

But if there are excessive white cells, these hormones are disrupted and can cause the opposite effect of insulin resistance and chronic inflammation.

Beige fat — This type of white fat can be converted to perform similar traits as brown fat, such as being able to generate heat with exposure to cold temperatures or during exercise. Pink fat — This type of white fat is converted to pink during pregnancy and lactation, producing and secreting breast milk.

Essential fat — This type may be made up of brown, white, or beige fat and is vital for the body to function normally. It is found in most organs, muscles, and the central nervous system including the brain. It helps to regulate hormones like estrogen, insulin, cortisol, and leptin; control body temperature; and assist in the absorption of vitamins and minerals.

Very high amounts of subcutaneous fat can increase the risk of disease, though not as significantly as visceral fat.

The density of the whole body, therefore, depends upon the relative size of these two components. As bone and muscle are denser than water, a person with a larger percentage of fat free mass will weigh more in the water, and have a lower percent body fat.

Conversely, fat is less dense than water. Therefore, a large amount of fat mass will make the body lighter in the water and have a higher percent body fat. Body density is expressed as mass per unit volume, where mass is the weight of the body in air Ma and volume is the difference between the weight in air and the weight of the body submersed during underwater weighing Ma — Mw.

Once body density is calculated, it is possible to convert this to a percentage body fat. The process is divided into three steps: 1 measurement of residual volume; 2 measurement of dry bodyweight; and 3 measurement of underwater weight.

Figure 1 provides an illustration of one method used for hydrostatic underwater weighing. An allowance must be made for residual lung volume the air that remains in the lungs after a maximal exhalation as this air increases the participant's buoyancy. It is measured using methods such as closed circuit dilution including helium dilution or nitrogen washout.

If indirect measurement cannot be conducted, it can also be estimated from vital lung capacity or using published equations that are based on age, height. Hydrostatic underwater weighing typically requires the participant to be completely submerged underwater while exhaling maximally to minimize the effect of buoyancy from lung air.

Differences in residual volume determination have been reported to contribute the largest sources of variation. After having measured or estimated residual lung volume, the participant's dry weight is measured wearing a swimming suit or running shorts.

Participants must not eat or engage in strenuous exercise for at least 4 hours before their scheduled appointment. They should avoid ingesting any gas-producing beverages for at least 12 hours before the test.

Participant must wear swim suits. However, as it is mostly only available in research laboratories or university settings, it is generally used for research purposes and not applicable in clinical practice or in large-scale population studies. It is used less frequently since the introduction of air displacement plethysmography as this technique is better tolerated by participants.

Figure 1 Illustration of measurement of underwater body weight. The most commonly used equations for estimating the percent of body fat from density are as follows:. Considerations relating to the use of hydrostatic underwater weighing for anthropometry in specific populations are described in Table 2.

Refer to section: practical considerations for objective anthropometry. About About the DAPA Measurement Toolkit What's New Other resources Toolkit Team Contact.

Introduction Validity Reliability Error and bias Feasibility Data processing Statistical assessment of reliability and validity Harmonisation. However, the person administering the test needs to understand the principles and do the calculations correctly.

A source of error in the test is the need to get all air out the subject's lungs when weighing underwater, as any air remaining will make them float and throw off the results. This is one reason that three underwater weights are taken.

Many universities offer hydrostatic weight tests for research purposes, and several companies offer testing for the public. Some, such as Body Fat Test , provide a mobile hydrostatic body composition testing service. The lack of easy access to this testing makes is less than ideal to use, especially since other methods of measurement are widely available.

Hydrostatic weighing is considered to be accurate, but there are other technologies being developed that may replace it. One is the Bod Pod, which uses air displacement in much the same way hydrostatic weighing uses water displacement. Another is dual energy X-ray absorptiometry DEXA , which measures bone mineral content as well as estimating body fat and muscle.

Bioelectric impedance body fat monitors are widely available for home use and at health clubs. They use a low voltage electric current to assess body composition. However, these monitors are less accurate than hydrostatic weighing and dependent on several variables.

Using fat calipers is another accessible, easy method. Kirkendall DT, Grogan JW, Bowers RG. Field comparison of body composition techniques: Hydrostatic weighing, skinfold thickness, and bioelectric impedance. J Orthop Sports Phys Ther. Lee SY, Gallagher D.

Assessment methods in human body composition. Curr Opin Clin Nutr Metab Care. Lee DH, Keum N, Hu FB, et al. Development and validation of anthropometric prediction equations for lean body mass, fat mass and percent fat in adults using the National Health and Nutrition Examination Survey NHANES Br J Nutr.

Duren DL, Sherwood RJ, Czerwinski SA, et al. Body composition methods: Comparisons and interpretation. J Diabetes Sci Technol. Because of this, the lean body mass is overestimated and the body fat is underestimated for many Blacks.

Also, after age 45 to 50, substantial changes in bone density, especially in women, invalidate the use of an assumed constant value for fat-free body density when converting total body density to percentage of body fat.

This is why age and gender specific equations need to be used for estimating body fat. As researchers learn more about age-related changes in bone mineral, hydrostatic weighing will eventually provide a more accurate prediction of body fat for older men and women.

Bioelectrical impedance analysis BIA is based on the fact that the body contains intracellular and extracellular fluids capable of electrical conduction. A non-detectable, safe, low-level current flows through these intracellular and extracellular fluids.

Since your fat-free body weight contains much of your body's water and electrolytes, it is a better conductor of the electrical current than the fat, which contains very little water.

So this technique is essentially an index of total body water, from which fat-free mass is estimated. The popularity of the BIA method has grown significantly over the last few years because it is painless, quick, and easy to administer the test. To take the test, you lie on a testing table or floor and electrodes are attached to your hands and feet.

You do not feel a thing as the current passes through your body. Average time for administering this test is about 10 minutes. No eating or drinking within 4 hours of the test 2.

No exercise within 12 hours of the test 3. Urinate within 30 minutes of the test 4. No alcohol consumption within 48 hours of the test 5.

There is a tendency for BIA to overestimate percent body fat in very lean clients and underestimate body fat in obese clients. All in all, if the guidelines for testing are followed, the BIA method is a satisfactory method for assessing body composition of most people.

Skinfold Method The skinfold method of measuring body fat is a practical, economical, and administratively feasible field technique for body composition analysis. It involves measuring the skinfold subcutaneous fat thickness at specific sites of the body.

Most equations use the sum of at least three skinfolds to estimate body density from which body fat may be calculated. Skinfold measurement does not require expensive equipment and it can be routinely incorporated into many health promotion settings.

Skinfold technicians can be trained rather easily, but must practice on at least clients before the skinfold technique is mastered.

When using the skinfold method, it is assumed that the distribution of subcutaneous fat and internal fat is similar for all individuals. This assumption is not fully supported. It is now known that older subjects of the same body density and gender have proportionately less subcutaneous fat than their younger counterparts.

There is considerable biological variation in the distribution of subcutaneous, intermuscular, intramuscular, and internal organ fat due to age, gender, and degree of fatness Heyward, Accuracy of Skinfold Measurements The accuracy of the skinfold method is dependent on the technician's skill as well as the type of caliper and the skinfold prediction equation used.

Reasonably priced plastic calipers have a less precise measuring scale, and often provide variable pressure and a smaller range of measurement. Despite this, a number of researchers have reported only small differences between skinfolds measured with high quality calipers and plastic calipers for highly skilled technicians refer to Guide to Skinfold Caliper for more information on where to purchase calipers.

However, plastic calipers are not recommended for use by untrained technicians. To assure accuracy, the skinfold technician must follow standardized testing procedures: 1.

Take all skinfold measurements on the right side of the body. Carefully identify and mark the skinfold sites. Place the thumb and index finger approximately 3 inches 8 cm perpendicular to the skinfold, following the natural cleavage lines of the skin.

Do not release the skinfold during the measurement. Always release the caliper jaw pressure slowly. The skinfold measurement should be taken 4 seconds after the pressure is released.

You should take a minimum of two measurements at each site. It is advisable to take measurements in a rotational order rather than consecutive readings at the same site.

If your values differ by more than 1 mm, take additional measurements. The client's skin should be dry and free of any oils and lotions. Skinfold measurements should not be done immediately after exercise due to the shift of body fluid to the skin.

Fortunately, the time of day or the phase of the menstrual cycle will have little effect on the skinfold measurements. As with many skills, the more you practice the better you will become at measuring skinfolds. It always helps if you have another trained technician to compare your results.

One alternative for obese clients would be to use fat-specific equations Segal et al. Measuring Sites for Women Triceps: Take a vertical fold on the posterior midline of the upper arm halfway between the top of the shoulder and the elbow joint.

Keep the elbow extended and relaxed. Thigh: Take a vertical fold on the front aspect of the thigh, midway between the top of the knee cap and the hip. Suprailium: Take a diagonal fold above the crest of the ilium hip- bone , at the spot where an imaginary line comes down from the anterior line of the armpit anterior axillary line.