For years, as fitness enthusiasts, we have been keen to keep track of our body fat measurements as part of our training progress, many people claim ridiculously low measurements and others high.
As a Strength and Conditioning consultant - to both the novice athlete and International athlete, and more so in my time working with the general fitness enthusiast - I often get asked questions like: what actually is body fat and why do we need to measure it? Below I have outlined a brief explanation about the whole subject of body fat, methods of measurement and the effect it can have on human performance.
What Is Body Fat?
The term body fat refers to the excess fat stored by the body. Basically body fat is a store of energy that has not yet been utilized by the body; body fat is created when one ingests a greater amount of calories than that which is required to carry out the day to day functions of the body. The excess calories are stored as fat throughout the body and is classified as both essential body fat and nonessential body fat.
Essential body fat is required by the body to function properly; it is responsible for the regulation of body temperature, optimal functioning and cushioning of the internal organs and also as an emergency source of energy during illness.
This essential body fat has a percentage measure of 2-4% for men and 10-12% for women, below these levels poses a serious risk to the health of the individual. Excess or nonessential body fat however can be dangerous and lead to serious health problems and is associated with type 2 diabetes, hypertension and hyperlipidemia.
The fat free mass exists primarily as the chief structural and functional component of the human body. Fat free mass consists of water (72%) protein (21%) and bone minerals (7%) typically an adult has only 3-4kg bodyweight from bone.
Fat mass varies considerably between individuals, in terms of absolute amount. Fat mass consists of 20% water and 80% adipose tissue.
Healthy ranges of body fat range from between 14-20% for men, a percentage of greater than 25% is considered to be unhealthy and puts the individual at an increased risk of obesity related illness. Healthy ranges for women range between 17-24% where a level of 30% body fat is considered unhealthy and at a level of increased risk.
Acceptable Body Fat Percentage
Methods Of Evaluating Body Fat
There are numerous methods of evaluating body fat, for both the clinical setting and in the field. The degree of accuracy associated with each procedure, increases as we move from the field to the clinical setting, such as skinfold measurements compared to DEXA.
Some methods of measurement are more suitable for one sample group than another, for example the Body Mass Index (BMI) is more suited to measuring body composition in sedentary folk than athletes, due to the fact that athletes (depending on their chosen sport) will have a higher lean body mass and may fall into the wrong category by BMI standards (rugby players, bodybuilders).
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A more suitable approach to measuring a group of athletes would be the skinfold measurement as it would provide a more accurate prediction of percentage body fat for that particular sample group (Than BMI). Below are numerous methods used for the measurement of body fat and a brief summary of how they work.
Hydrostatic (Underwater) Weighing
Archimedes' principle is the theory on which this technique for measuring body composition is based. It states that when a body is immersed in water, it is buoyed by a counterforce equal to the weight of the water displaced.
The density of body fat is less than that of water, therefore contributing to the buoyancy of the body, as does air in the lungs. Muscle and bone tissue have a greater density than that of water and can cause a body to sink, therefore an individual with greater fat free mass but the same overall body mass, will weigh more in water due to the higher body density and lower percentage body fat.
Although hydrostatic weighing is considered the "standard" when measuring body composition, there are numerous sources of error associated with the procedure.
The formulas used to convert body density to percent fat assume a density of 0.900 g/ml for the fat mass and 1.100 g/ml for the density of the fat free mass. However the density of the fat free mass varies with age, gender, and ethnicity, use of a single general formula to convert body density to percentage body fat for all populations would therefore result in systematic errors.
Even with flawless measurement technique, hydrostatic weighing results in an error estimate of ±2% body fat due to the individual differences in the density of fat free mass.
Plethysmography (Air Displacement)
This method is based on a two compartment model of body composition, fat mass and fat free mass, based on Boyle's law it uses the inverse relationship between pressure and volume to derive body volume for a subject. If both volume and bodyweight are known body density can be calculated by using the following equation:
Body Density = mass/volume, where mass = body weight (kg) and volume = liters (l)
The change in pressure and volume between the empty chamber and the chamber with the subject present can be used to calculate the volume of the subject, since one side of the equation is already known.
Boyles Law: P1 x V1 = P2 x V2 Where P = pressure and V = volume
Body Mass Index (BMI)
This method is used to assess weight relative to height, it is calculated by dividing bodyweight in kilograms by height in meters squared (kg/m2) A BMI ranging between 25.0 to 29.9kg/m2 is considered overweight whereas a BMI of 30 or greater is considered obese. However due to the large margin of error associated with estimating percentage body fat from BMI (+/- 5% fat) it is not considered a useful tool for evaluating an individual's body fat.
A skinfold thickness is two layers of skin and two layers of fat. To measure the skinfold thickness a caliper is used, this should be calibrated and have a constant pressure of 10 g/mm2 (4).
Body fat percentage determined from skinfold measurements correlates well with that determined through hydrostatic weighing (r = 0.79 - 0.90) (3).
This technique is based on the principle that the amount of subcutaneous fat is proportional to the total amount of body fat. However, age gender and ethnicity all have an effect on the proportion of subcutaneous fat to total fat, (5) which means that these variables must be considered when using the regression equations to convert sum of skinfolds to percentage body fat.
Assuming that the correct technique has been used and that the correct equations have been applied, the prediction of percentage body fat has an accuracy of ±3.5%.
Bioelectrical Impedance Analysis
Is a relatively easy to administer method of measuring body fat percentage. Bioelectrical impedance analysis works by determining the electrical impedance of the body tissues, which provides an estimate of total body water, which can be used to estimate fat free mass and percentage body fat.
It involves the passing of a small electrical current through the body and measuring the impedance to current flow. Fat free tissue is a good conductor of electrical currents, therefore body fat impedes current flow, and the impedance to current flow is inversely related to the fat free mass and total body water, both of which can be estimated by using this technique.
Generally bioelectrical impedance analysis yields similar results to skinfold measurements with regards to percentage body fat estimates, but the protocol that MUST be followed by the subject make it a little difficult.
Prior to assessment the subject must do the following to achieve maximum accuracy in results:
- 4 hours fasting prior
- No exercise within 12 hours
- No alcohol within 48 hours
- Each subject must void completely before assessment
- Abstain from diuretic agents including caffeine, unless medically prescribed
Dual Energy X-Ray Absorptiometry (DEXA)
Is the latest technology used in the assessment of body composition. This method measures bone mineral density, lean mass and fat mass, through the use of two x-ray beams which traverse the body.
The energy is then collected by a detector after it has been attenuated by the bone and soft tissue through which it has passed. The number of photons per unit of area is corrected for soft tissue by linear, two dimensional interpolation. The values are then summed to estimate total bone mineral content.
Soft tissue is resolved by using mass attenuation coefficients from tissue equivalent standards for fat and fat free mass. DEXA is considered the NEW standard in the measurement of body composition.
Chemical ingestion and the subsequent analyses of the tissues is the only direct method of evaluating body composition, which is a totally impractical approach in humans. DEXA, hydrostatic weighing and plethysmography are highly accurate but expensive methods requiring clinic settings to perform the test, making them unsuitable field methods of measurement.
Bioelectrical Impedance Analysis and skinfold measurement however are inexpensive, easy to perform and can be completed in relatively little time. The protocols which have to be in place when using the BIA method also pose a problem for an accurate estimation of percentage body fat, leaving skinfold measurement as the current forerunner for accurate field testing.
Human Function And Body Fat
In modern society obesity has become a growing concern for the health care practitioner. The most recent data from the American National Center for Health Statistics show an increasing trend, that 30% of U.S adults from 20 years of age and older suffer from obesity. That's over 60 million people!
What's even more alarming is that this increasing trend is not limited to adults, the percentage of children who are deemed as overweight has tripled since 1980, where now 16% of children between the ages of 6 and 19 are classed as overweight. This trend in the increase of the number of people who are overweight and/or obese raises great concern for the health and well being of modern society and is on the verge of becoming an epidemic.
As stated previously a high percentage body fat can increase the risk of serious health problems such as hypertension, type 2 diabetes, depression, hyperlipidemia & heart disease, osteoarthritis, respiratory problems, and even some cancers. The evaluation of percentage body fat on a habitual basis (as part of an annual medical) may be a vital tool to help minimize the risk of such complications.
The evaluation of percentage body fat can be used as a guide as to what may lie ahead for those on the higher percentage body fat scales.
A recent study entitled "The Influence of Excess Adiposity on Exercise Fitness and Performance in Overweight Children and Adolescents" hypothesized that the increased metabolic cost of having to move a greater mass, was the primary factor limiting overweight adolescents when they undertake sustained exercise, and also that the effect of diminished cardio respiratory reserve would be smaller but still important and warranted further study.
The study shows that even as a child or an adolescent increased body fat percentage is having a direct effect on human function which can only diminish with time if not rectified.
Through body fat evaluation the health care practitioner can more readily prescribe suitable measures for the individual to obtain an optimum level percentage body fat, whether it be to counter act an elevated body fat percentage or merely to maintain ones current levels. However the evaluation of percentage body fat is not limited to those who are overweight or suffering from obesity, it is an essential part of the process when treating patients with Anorexia Nervosa.
In a recent study on the refeeding of adolescent girls, bioelectrical impedance analyses was used to monitor body composition throughout the study. The initial BMI for the girls suffering from Anorexia Nervosa was 15.5 ± 1.1kg/m² and had risen to 17.4±0.7 kg/m² by the end of the 15 week trial period. A significant finding was that despite a 70.7% gain in fat mass, the patients were still categorized as underweight.
In most athletic events the athletes must move their own body mass through a series of movements as quickly and efficiently as possible, excessive body fat generally has an adverse effect on performance of a variety of motors skills and cardiovascular endurance. From a mechanical perspective, excess body fat is an inert load that must be moved, a dead weight that provides no function.
William "The Refrigerator" Perry, NFL legend, who played for the Chicago Bears during the 1980's & 90's, turned up to the 1988 summer training camp weighing in at 375lbs (170.5kg) some 55lbs (25kg) over his mandated playing weight. Not only is this of great concern regarding the ability to perform on the field at such an excessive weight, but also of even greater concern is the possible health risks associated with such extreme levels of obesity.
Another famous American athlete, Chris Taylor, a U.S Olympic wrestler, competed at a bodyweight of 400 - 450lbs (182 - 204.5kg) He died in his sleep at age 29.
Effects of excess body fat are especially apparent when comparing obese to non-obese individuals. In a recent study on the comparison of physical fitness between obese and non-obese Flemish youths, a strong correlation was found between obesity and inferior performance results on all tests which required propulsion or lifting of the body mass.
Tests included, standing broad jump, sit-ups, bent-arm hang, speed shuttle runs, and endurance shuttle runs. The sample group was divided into those who were obese and non-obese using both Body Mass Index and Skinfold measurement.
A similar study comparing the performance of obese and non-obese Belgian girls on most measures of physical performance; including strength, flexibility, and motor performance, found similar results. The obese girls under performed in comparison to non-obese in muscular endurance, trunk strength, lower back flexibility, and relative arm strength (based on % bodyweight).
From the information above we are lead to conclude that not only does excess body fat negatively influences the ability to perform physical tasks; it also poses a serious threat to health and place the obese individual at a high risk for serious health complications. The evaluation of body fat therefore plays a vital role in the evaluation of human function and performance.