The higher female requirement for essential fat is directly related to fertility and childbearing. Women whose body fat drops below essential requirements, such as gymnasts, ballerinas, long­distance runners and anorexics, will probably become amenorrheic. The period of infertility will continue until weight is gained and essential fat is restored. In both genders, essential fat represents a minimal threshold or lower limit for the maintenance of health.

Hydrostatic weighing, one of the most accurate of the measurement techniques, involves weighing subjects while they are completely submerged in water. Subjects may contribute to optimcal accuracy if they can exhale the maximum amount of air possible from the lungs and can sit still for 6 to 10 seconds while completely submerged. Accuracy is further enhanced if the technician has the equip ­ ment to measure residual air (the amount of air remaining in the lungs after a maximum exhalation).

The equipment required for hydrostatic weighing includes an autopsy scale with a capacity of approximately 8 kg. The scale is suspended over a tank of water that is at least 3 feet deep. The subject sits suspended chin-deep, exhales completely, and bends forward from the waist until entirely submerged. This position is maintained for 6 to 10 seconds to allow the scale to stabilize. From 5 to 10 trials are required, and the underwater weight is attained by averaging the three heaviest readings. The subject’s net underwater weight is calculated by subtracting the weight of the seat, its supporting structure, and a weight belt (if needed) from the gross underwater weight.

Bioelectrical Impedance Analysis (BIA)

Bioelectrical impedance analysis (BIA) is a relatively new and simple method of determining body composition. The equipment is portable, computerized, and expensive, but it is safe, noninvasive, quick, and convenient to use. A harmless electrical current is sent through the supine body via electrodes attached to the right hand and foot. Water is an excellent conductor of electricity, whereas fat, which is essentially anhydrous (lacking \illater), is a nonconductor. Lean body mass includes all tissues of the body except fat, so the two components can be separated. The measurement of electrical conductance or impedance is the tissue resistance to the transmission of an electrical current. The determination of total body water is used to calculate lean weight, fat weight, and percentage of body fat. Bioelectrical impedence accurately measures total body water, but the major source of error with this method is in the conversion of this factor to the estimation of body fat percentage and fat-free mass. This technique is about as valid as skinfold measures, however, it is easier to learn and perform proficiently.

A major limitation is that this technique does not accurately estimate fat-free mass in very lean or very fat subjects. Body fatness is generally overestimated for lean subjects and underestimated in obese subjects.

Skinfold Measurenents

Skinfold measurements are one of the least expensive and most economical methods of measuring body composition. The cost of skinfold calipers ranges from $10 to as much as $450 for computerized models. The most accurate calipers maintain constant jaw pressure of 10 g/mm 2 of jaw surface area.

The thumb and index finger are used to pinch and lift the skin and the fat beneath it. The caliper is placed beneath the pinch. When performed by skilled technicians, skinfold measurements correlate quite well (0.80 or greater) with body density calculated from underwater weighing.

Determining Desirable Body Weight from Body fat

Calculating desirable body weight is a simple procedure when the percentage of body fat is known.

Methods For Measuring Body-Weight Status

Height/Weight Tables

Optimal body weight is not necessarily reflective of optimal body composition. This was illustrated by a comparison of young and middle-aged men who were within 5% of their ideal weight as determined by height, weight, and frame-size charts. Although both groups were within the ideal range, the middle­aged subjects had twice the amount of fat as the young subjects. Height/weight tables and scales are not indicators of body composition, nor are they reliable reference points to use for weight management.

Height/weight tables do not actually measure body composition. They simply act as a standard for total body weight based on height, body-frame site, and gender without regard to the composition of weight. These tables are therefore poor criteria for the establishment of weight-loss recommendations. The height/weight tables’ other limitations include the following:

• Body-frame sizes were never measured for the thousands of insurance policy holders on whom the charts were developed.
• The current tables are more liberal than the former tables with regard to the range of weight to height.
• The tables do not reflect the general population. The subjects were predominantly white, middle class adults aged 25 to 59 who were able to afford private insurance.
• The major limitation of the height/weight tables was that no allowances were made for cigarette smoking. Cigarette smokers are lighter in body weight than nonsmokers but have a shorter life expectancy. When some leaner than average people in this analysis died sooner than expected, it was not because of leanness but because many of them were cigarette smokers. The developers of the tables erroneously concluded that leanness was a detriment to longevity. As a result they increased the desirable range of weight to height, unwittingly contributing to the perception that mild to moderate overweight is not harmful to health or longevity.
• Another serious limitation concerns muscularity. Those who are muscular may be heavier than the recommendations for their height. People in this category should not follow the height/weight tables because they are not at risk. Muscularity is not a risk for premature death but obesity is. Conversely, sedentary people may be in the desirable weight range for height but carrying a higher than average percentage of fat. The height/ weight tables provide a false sense of security for these people because overfat rather than overweight is the risk.

The case against using height/weight tables to determine one’s weight status or as a basis for making judgments about the need to lose weight is so compelling that your authors have decided not to present any of these tables in this text. Instead you are encouraged to select from one or more of the techniques that follow.

Body Mass Index

Another method for measuring body-weight status is by body mass index (BMI). Body mass index is the ratio of body weight in kilograms (kg) to height in meters squared. There are several body mass index protocols, all of which emanate from height/weight measurements. These protocols represent an attempt to adjust body weight to derive a height-free measure of obesity. Although BMI does not provide an estimate of percent body fat, it is more useful than the height/weight tables. BMI uses height/weight data, but it is more relevant and can be used to compare population groups. It also correlates fairly well with percent fat derived from hydrostatic (underwater) weighing. Also several investigations have indicated that the risks to health associated with obesity begin at BMI’s of 25 to 30 kg/m2 . There is a consistently high relationship between a high BMI and hypertension, elevated total serum cholesterol, depressed high density lipoprotein cholesterol, high serum triglycerides, and glucose intolerance.

There are two major limitations to using BMI measurements -

(1) The technique is misleading for individuals with greater than average muscle mass because it measures overweight rather than overfat.

(2) The results are difficult for the general public to interpret. Also the average person does not know how to apply BMI values to weight loss.

The first limitation is easily surmounted. People with large amounts of muscle tissue should be directed to use a technique such as skin fold measurements or underwater weighing to measure their body composition.

BMI is easily calculated. However, care should be taken in assessing weight and height. The following are minimal guidelines for determining body weight -

• Weigh yourself on a beam scale (physician’s scale) that has been calibrated to zero.
• Weigh in the morning, after voiding and before eating, while wearing light clothing and no shoes.
• Make sure you are not dehydrated.
• The following guidelines apply to the assessment of height
• Create a ruler by marking a flat wall in 1/4-inch increments starting at 4’6″ up from the floor. The wall should have no baseboard and the floor should have no carpeting.
• Subjects should stand erect, without shoes, and with their heels, buttocks, shoulders, and head against the wall.
• Place a right-angle object, such as a framing square, a short piece of 2 X 4, or a clipboard, on edge against the wall and on top of the subject’s head. This should provide a straight edge from head to wall.
• Read the rule for height to the nearest 1/4 inch.

To calculate the BMI find your height in the left column move across to your weight in the same row. The number at the top of this column is your BMI. For example, a man who is 71 inches tall and weighs 200 pounds has a BMI of 28 kg/m 2 . How much weight would he need to lose if he wanted to achieve a BMI of 24 kg/m 2 ? He can each calculate this from the table. Find the column his desired BMI of 24 kg/m 2 and drop down in table to the row with his height (71 inches). He should weigh 172 pounds to achieve a BMI of 24 kg/m 2. Then subtract his desired weight (172 pound from his current weight (200 pounds). He needs to lose 28 pounds to reach his goal. Now you should. turn to to calculate your BMI and your desired body weight.