Americans Weigh in Over Time - Defining And Assessing Ideal Weight, Overweight, And Obesity

body fat bmi health

Historically, desirable, healthy, or ideal weights have been derived from demographic and actuarial statistics (data compiled to assess insurance risk and formulate insurance premiums). The National Center for Health Statistics compiles and analyzes demographic data—the heights and weights of a representative sample of the U.S. population to develop standards for desirable weights. In 1943 the Metropolitan Life Insurance Company (MetLife) introduced standard weight-for-height tables for men and women based on an analysis of actuarial data. The MetLife weight-for-height tables assisted adults in determining if TABLE 1.4
Number and percent of births of low birthweight, by race, Hispanic origin of mother, and state or territory, 2002
[By place of residence. Low birthweight is birthweight of less than 2,500 grams (5 lb 8 oz)]

Number Percent
White Black White Black
State All races1 Total2 Non-Hispanic Total2 Non-Hispanic Hispanic3 All races1 Total2 Non-Hispanic Total2 Non-Hispanic Hispanic3
Wyoming 553 511 457 7 7 54 8.4 8.3 8.3 * * 8.7
Puerto Rico 6,039 5,480 - - - 558 - - - - - - 11.5 11.5 - - - 11.3 - - - - - -
Virgin Islands 193 42 9 141 125 44 11.8 13.0 * 11.5 11.5 13.7
Guam 255 13 10 4 4 3 8.0 * * * * *
American Samoa 64 3 - - - - - - - - - 3.9 * - - - * - - - - - -
Northern Marianas 89 1 - - - - - - - - - 6.9 * - - - * - - - - - -
*Figure does not meet standards of reliability or precision; based on fewer than 20 births in the numerator.
- - - Data not available.
–Quantity zero.
1Includes races other than white and black and origin not stated.
2Race and Hispanic origin are reported separately on the birth certificate. Data for persons of Hispanic origin are included in the data for each race group according to the mother's reported race.
3Includes all persons of Hispanic origin of any race.
4Excludes data for the territories.
SOURCE: Joyce A. Martin, Brady E. Hamilton, Paul D. Sutton, Stephanie J. Ventura, Fay Menacker, and Martha L. Munson, "Table 46. Number and percent of births of low birthweight, by race and Hispanic origin of mother: United States, each state and territory, 2002," National Vital Statistics Reports, Births: Final Data for 2002, vol. 52, no. 10, Centers for Disease Control and Prevention, National Center for Health Statistics, Hyattsville, MD, December 2003 [Online] http://www.cdc.gov/nchs/births.htm [accessed December 30, 2003]

their weights were within an appropriate range for height and frame size. Revised in 1959 and 1983, the tables were based on actuarial data, in which desirable or ideal weight is defined as the weight for height associated with the lowest mortality rate, or longest life spans, among the client population of adults (policyholders) insured by MetLife.

Although the MetLife and other weight-for-height tables remain in use in 2004, many health professionals and medical researchers believe they have limited utility. Nearly every version of desirable weight-for-height tables shows different acceptable weight ranges for men and women, and considerable debate continues among health professionals over which table to use. The tables lack information about body composition, such as the ratio of fat to lean muscle mass; their data were derived primarily from white populations and do not represent the entire U.S. population; they generally do not take age into consideration; and it is often unclear how frame size was determined. Further, it is now known that ideal, healthy, or low-risk weights vary for different populations, and vary for the same population at different times and in relation to different causes of morbidity and mortality.

The limitations of weight-for-height tables have prompted health-care practitioners and researchers to adopt other measures that allow comparison of weights independent of height and frame across populations to define desirable or healthy weights as well as overweight and obesity. For example, the 2000 Dietary Guidelines for Americans published jointly by the U.S. Departments of Agriculture (USDA) and Health and Human Services (HHS), and weight-control information published by the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health (NIH) include updated weight-for-height tables that incorporate height, weight, and body mass index (BMI). (See Table 1.5.)

Overweight is generally defined as excess body weight in relation to height, when compared to a predetermined standard of acceptable, desirable, or ideal weight. One definition characterizes individuals as overweight if they are between 10 and 30 pounds heavier than the desirable weight for height. Overweight does not necessarily result from excessive body fat; persons may become overweight as the result of an increase in lean muscle. For example, while muscular bodybuilders with minimal body fat frequently weigh more than non-athletes of the same height they are "overweight" because of their increased muscle mass rather than increased fat.

Rather than viewing overweight and obesity as distinct conditions, many researchers prefer to consider weight as a curve or continuum with obesity at the far end of the curve. Persons who are obese constitute a subset of the overweight population—using this definition it becomes clear that while only some overweight people are obese, all obese people are overweight.

Similarly, there is still no uniform definition of obesity. Some health professionals describe anyone who is more than 30 pounds above his or her desirable weight for height as obese. Others assert that body weight 20 percent or more above desirable or ideal body weight constitutes obesity. Extreme obesity is often defined as weight twice the desirable weight or 100 pounds (45 kg) in excess of the desirable weight. Obesity also is defined as an excessively

TABLE 1.5
Body mass index (BMI) table
To use the table, find the appropriate height in the left-hand column labeled Height. Move across to a given weight. The number at the top of the column is the BMI at that height and weight. Pounds have been rounded off.

BMI
Height (inches) 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Weight (pounds)
58 91 96 100 105 110 115 119 124 129 134 138 143 148 153 158 162 167 172 177 181 186 191
59 94 99 104 109 114 119 124 128 133 138 143 148 153 158 163 168 173 178 183 188 193 198
60 97 102 107 112 118 123 128 133 138 143 148 153 158 163 168 174 179 184 189 194 199 204
61 100 106 111 116 122 127 132 137 143 148 153 158 164 169 174 180 185 190 195 201 206 211
62 104 109 115 120 126 131 136 142 147 153 158 164 169 175 180 186 191 196 202 207 213 218
63 107 113 118 124 130 135 141 146 152 158 163 169 175 180 186 191 197 203 208 214 220 225
64 110 116 122 128 134 140 145 151 157 163 169 174 180 186 192 197 204 209 215 221 227 232
65 114 120 126 132 138 144 150 156 162 168 174 180 186 192 198 204 210 216 222 228 234 240
66 118 124 130 136 142 148 155 161 167 173 179 186 192 198 204 210 216 223 229 235 241 247
67 121 127 134 140 146 153 159 166 172 178 185 191 198 204 211 217 223 230 236 242 249 255
68 125 131 138 144 151 158 164 171 177 184 190 197 204 210 216 223 230 236 243 249 256 262
69 128 135 142 149 155 162 169 176 182 189 196 203 210 216 223 230 236 243 250 257 263 270
70 132 139 146 153 160 167 174 181 188 195 202 209 216 222 229 236 243 250 257 264 271 278
71 136 143 150 157 165 172 179 186 193 200 208 215 222 229 236 243 250 257 265 272 279 286
72 140 147 154 162 169 177 184 191 199 206 213 221 228 235 242 250 258 265 272 279 287 294
73 144 151 159 166 174 182 189 197 204 212 219 227 235 242 250 257 265 272 280 288 295 302
74 148 155 163 171 179 186 194 202 210 218 225 233 241 249 256 264 272 280 287 295 303 311
75 152 160 168 176 184 192 200 208 216 224 232 240 248 256 264 272 279 287 295 303 311 319
76 156 164 172 180 189 197 205 213 221 230 238 246 254 263 271 279 287 295 304 312 320 328
SOURCE: "Body Mass Index Table," Do You Know the Health Risks of Being Overweight?, U.S. Department of Health and Human Services, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, MD, 2003 [Online] http://www.niddk.nih.gov/health/nutrit/pubs/health.htm#table [accessed December 30, 2003]

high amount of adipose tissue (body fat) in relation to lean body mass such as muscle and bone. The amount of body fat (also known as adiposity), the distribution of fat throughout the body, and the size of the adipose tissue deposits also are used to assess obesity because the location and distribution of body fat are important predictors of the health risks associated with obesity. The location and distribution of body fat may be measured by the ratio of waist-to-hip circumference. High ratios are associated with higher risks of morbidity and mortality.

Overweight and obese body types may be characterized as "pear-or apple-shaped," depending on the anatomical site where fat is more prominent. In the apple or android type of obesity, fat is mainly located in the trunk (upper body, nape of the neck, shoulder, and abdomen.) Gynoid obesity or the pear-shape, features rounded hips, and more fat located in the buttocks, thighs, and lower abdomen). Fat cells around the waist, flank, and in the abdomen are more active metabolically than those in the thighs, hips, and buttocks. This increased metabolic activity is thought to produce the increased health risks associated with android obesity. In general, women are more likely to have gynoid obesity. However, those with the android type of obesity are subject to similar health risks as males with android overweight.

There are many ways to measure body fat. Weighing an individual underwater in a laboratory with specialized equipment provides a highly accurate assessment of body fat. By performing hydrostatic or underwater weighing, an examiner obtains an estimate of whole-body density and uses this to calculate the percentage of the body that is fat. First, the subject is weighed on a land scale. The subject puts on a diver's belt with weights to prevent floating during the weighing procedure, sits on a chair suspended from a precision scale, and is completely submerged. When maximum expiration of breath is achieved, the subject remains in this submerged position for about ten seconds while the investigator reads the scale. This procedure is repeated as many as ten times to obtain reliable, consistent values. The weight of the diver's belt and chair are subtracted from this weight to obtain the true value of the subject's mass in water.

Simpler, but potentially less accurate assessments of body fat, include skinfold thickness measurements, which involve measuring subcutaneous (immediately below the skin) fat deposits using an instrument called a caliper in locations such as the upper arm. Skinfold thickness measurements rely on the fact that a certain fraction of total body fat is subcutaneous and using a representative sample of that fat, overall body fatness (density) may be predicted. Several skin-fold measurements are obtained, and the values are used in equations to calculate body density. Using a caliper, the examiner grasps a fold of skin and subcutaneous fat firmly, pulling it away from the underlying muscle tissue following the natural contour of the skin. The caliper jaws exert a relatively constant tension at the point of contact and measure skinfold thickness in millimeters. Most obesity researchers believe there is an acceptable correlation between skinfold thickness and body fat—that it is possible to estimate body fatness from the use of skinfold calipers. Skinfold thickness measurements are considered more subjective than underwater weights because the accuracy of measurements of skin-fold thickness depends on the technique and skill of the examiner, and there may be variations in readings from one examiner to another.

Another technique used to evaluate body fat is bio-electric impedance analysis (BIA). BIA offers an indirect estimate of body fat and lean body mass. It entails passing an electrical current through the body and assessing the body's ability to conduct the current. It is based on the principle that resistance is inversely proportional to total body water when an electrical current (75 MHz) is applied through several electrodes placed on body extremities. Since greater conductivity occurs when there is a higher percent of body water, and fat cells contain less water than muscle cells, and a higher percent of body water indicates larger amounts of muscle and other lean tissue, persons with less fat are better able to conduct electrical current. BIA has been shown to correlate very well with total body water assessed by other methods.

Other means of estimating the location and distribution of body fat include waist-to-hip circumference ratios, or such imaging techniques as ultrasound, computed tomography, or magnetic resonance imaging.

Waist Circumference and Waist-to-Hip Ratio

Along with height and weight, waist circumference is a common measure used to assess abdominal fat content. An excess of body fat in the abdomen or upper body is considered to increase the risk of developing heart disease, high blood pressure, diabetes, stroke, and certain cancers. Like body fat, health risks increase as waist circumference increases. For men, waist circumference greater than 40 inches (102 cm) is considered to confer increased health risks. Women are considered at increased risk when their waist measurements are 35 inches (88 cm) or greater. Figure 1.3 shows how waist circumference is measured to obtain accurate measurements of abdominal girth. Waist circumference measures lose their incremental predictive value in persons with a BMI greater or equal to 35 because these individuals generally exceed the cutoff points for increased risk. Table 1.6 shows the relationship between BMI, waist circumference, and disease risk for persons who are underweight, normal weight, overweight, obese, and extremely obese.

Waist-to-hip ratio is the ratio of waist circumference to hip circumference, calculated by dividing waist circumference by hip circumference. For men and women, a waist-to-hip ratio of 1.0 or more is considered to place them at greater risk. Most people store body fat at the waist and abdomen (android body fat distribution) or at the hips (gynoid body fat distribution). Interestingly, while overweight and obesity both increase health risks, body fat that is concentrated in the lower body, such as fat deposits at the hips and thighs, is thought to be less harmful in terms of morbidity and mortality than abdominal fat—excess fat in the upper body.

Body Mass Index (BMI)

BMI is a single number that evaluates an individual's weight status in relation to height. It does not directly measure the percent of body fat; however, it offers a more accurate assessment of overweight and obesity than weight alone. It is a direct calculation based on height and weight, and it is not gender specific. BMI is the preferred measurement of health-care professionals and obesity researchers to assess body fat and is the most common method of tracking overweight and obesity among adults. BMI, calculated by dividing weight in kilograms by the square of height in meters (BMI=kg/m2) classifies persons as underweight, normal weight, overweight, or obese. Table 1.7 shows the formula used to calculate BMI when height is measured in either inches or centimeters and weight is measured in either pounds or kilograms.

The World Health Organization and National Institutes of Health consider individuals overweight when their BMI is between 25 and 29.9, and they are classified as obese when their BMI exceeds 30. Table 1.5 shows the relationship between height, weight, and BMI. Table 1.8 shows the classification of overweight and obesity by BMI and distinguishes between three levels of obesity.

Although BMI is a simple, inexpensive tool for assessing weight, it has several limitations. BMI calculations may deem a muscular athlete overweight, when he or she is extremely fit, and excess weight is the result of a larger amount of lean muscle. It may similarly misrepresent the health of older adults who as the result of muscle wasting—loss of muscle mass—may be considered normal or healthy weights when they may actually be nutritionally depleted or overweight in terms of body fat composition. While it is an imperfect method for assessing individuals, BMI is extremely useful for tracking weight trends in the population.

Definitions and Estimates of Prevalence Vary

Historically, varying definitions of, and criteria for, overweight and obesity have affected prevalence statistics and made it difficult to compare data. Some overweight- and obesity-related prevalence rates are crude or unadjusted estimates; others are age-adjusted estimates that offer different values. Early efforts to track overweight and obesity in the U.S. population relied on the 1959 or 1983 Metropolitan Life Insurance tables of desirable weight-for-height as the reference standard for overweight. During the last three decades, most government agencies and public health organizations FIGURE 1.3
Waist circumference measurement
have estimated overweight using data from a series of surveys conducted by the CDC's National Center for Health Statistics. These surveys include the National Health Examination Surveys, National Health and Nutrition Examination Surveys (NHANES), and the Behavioral Risk Factor Surveillance System.

Despite changing definitions of overweight and obesity and various methods to track changes in the U.S. population, there is irrefutable evidence that the prevalence of overweight and obesity have steadily increased among persons of both genders, all ages, all racial and ethnic groups, all educational levels, and all smoking levels. The prevalence of obesity in the United States was first reported in the National Health Examination Survey in 1960, and subsequent reports have come from three National Health and Nutrition Examination Surveys (NHANES I, 1971, NHANES II, 1976–1980, and NHANES III, 1988–1994). Most obesity data referenced in the medical literature in 2004 are the results of the most recent NHANES study in 1999, along with several other national studies. Data from the National Health Examination Survey, TABLE 1.6
Classification of overweight and obesity by body mass index (BMI), waist circumference, and associated disease risk

Disease risk1 relative to normal weight and waist circumference
BMI (kg/m2) Obesity class Men ≤ 102 cm (≤40 in) > 102 cm (>40 in)
Women ≤ 88cm (≤35 in) > 88cm (>35 in)
Underweight <18.5
Normal2 18.5–24.9
Overweight 25.0–29.9 Increased High
Obesity 30.0–34.9 I High Very high
35.0–39.9 II Very high Very high
Extreme obesity ≥40 III Extremely high Extremely high
1Disease risk for type 2 diabetes, hypertension, and cardiovascular disease.
2Increased waist circumference can also be a marker for increased risk even in persons of normal weight.
SOURCE: "Classification of Overweight and Obesity by BMI, Waist Circumference, and Associated Disease Risk," Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, the Evidence Report, National Heart, Lung, and Blood Institute in cooperation with the National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, NIH Publication No. 98-4083, September 1998 [Online] http://www.nhlbi.nih.gov/guidelines/obesity/ob_gdlns.htm [accessed December 30, 2003]

NHANES I, and NHANES II indicated that the prevalence of obesity was relatively constant from 1960 to 1980; however, the results of the NHANES III indicated a sharp increase in the prevalence of obesity.

Overweight and obesity have steadily progressed at an alarming rate over the course of the past two decades. From 1960 to 2000 the prevalence of overweight (defined as BMI greater than 25 but equal to or less than 30) increased from 31.5 to 33.6 percent in U.S. adults aged twenty to seventy-four. The prevalence of obesity (BMI greater than 30) during the same years more than doubled from 13.3 to 30.9 percent, with most of the rise occurring in the past twenty years. From 1988 to 2000 the prevalence of extreme obesity (BMI greater than 40) increased from 2.9 to 4.7 percent, up from 0.8 percent in 1960.

The annual prevalence of obesity among U.S. adults age twenty and older increased from 19.4 percent in 1997 to 20.6 percent in 1998, 21.5 percent in 1999, 21.8 percent in 2000, 23 percent in 2001, and 23.9 percent in 2002. During the first half of 2003, from January through June, 23.8 percent of adults were obese, comparable to the 2002 estimate of 23.9 percent. (See Figure 1.4.)

The prevalence of overweight and obesity generally increases with advancing age, then starts to decline among people over sixty. From January to June 2003, for men and women combined, the prevalence of obesity was highest among adults aged forty to fifty-nine (28 percent) and lowest among adults aged twenty to thirty-nine (20.4 percent). There was no significant difference in the prevalence of obesity between men and women in all three age groups. (See Figure 1.5.)

TABLE 1.7
How to calculate body mass index (BMI)

You can calculate BMI as follows
If pounds and inches are used
Calculation directions and sample
Here is a shortcut method for calculating BMI. (Example: for a person who is 5 feet 5 inches tall weighing 180 lbs.)
1. Multiply weight (in pounds) by 703
180 × 703 = 126,540
2. Multiply height (in inches) by height (in inches)
65 × 65 = 4,225
3. Divide the answer in step 1 by the answer in step 2 to get the BMI.
126,540/4,225 × 29.9
BMI = 29.9
High-risk waist circumference
Men: > 40 in (> 102 cm)
Women: > 35 in (> 88 cm)
SOURCE: "You can calculate BMI as follows," in "Assessment and Classification of Overweight and Obesity," The Practical Guide Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, National Institutes of Health, National Heart, Lung, and Blood Institute, North American Association for the Study of Obesity, Silver Spring, MD, June 1998 [Online] http://www.nhlbi.nih.gov [accessed December 30, 2003]

The age-adjusted prevalence of combined overweight and obesity (BMI greater than 25) in racial and ethnic minorities, especially minority women, is generally higher than in whites in the United States. In 2003 for both genders, non-Hispanic black persons were more likely than Hispanic and non-Hispanic white persons to be obese. (See Figure 1.6.) The age-adjusted prevalence of obesity was highest among non-Hispanic black women (38.7 percent) and lowest among non-Hispanic white women (21.1 percent). Earlier studies, including the NHANES, reported a high prevalence of overweight and obesity among Hispanics and Native Americans and lower prevalence of overweight (BMI greater than 25) and obesity (BMI greater than 30) in Asian Americans than in the U.S. population as a whole.

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