Body Mass Index

BIBLIOGRAPHY

Body Mass Index, or BMI, is a common measure of weight status in adults. BMI can be calculated by multiplying weight in pounds by 703, divided by height in inches squared, and it serves as an index of weight-for-height measured in kg/m². BMI indicates overweight between 30 kg/m² and 34.9 kg/m²; obesity between 35 kg/m² and 39.9 kg/m²; and clinically severe obesity above 40 kg/m². BMI is an indirect estimate of body fat and is highly correlated with body fat at about .7 (Gray and Fujioka 1991). Although there are more accurate measures of body fat (e.g., underwater weighing and DXA), they are more expensive, inaccessible, and cumbersome compared to BMI (Blew et al. 2002). The widespread use of BMI is likely due to its cost-effectiveness and ease of calculation.

The measurement and definition of overweight and obesity has varied over time. For much of the twentieth century, physicians and researchers referenced Metropolitan Life Insurance Company (MLIC) tables, which recommended ideal weight-for-height. The MLIC tables suffered from limitations (e.g., unstandardized and inaccurate measurement protocols) that prompted the government to adjust the weight guidelines in the 1980s (Kuczmarski and Flegal 2000). In the mid-1980s BMI became the preferred measurement of weight status, and recommendations were based upon data from national epidemiological surveys such as the National Health and Nutrition Examination Survey. BMI emerged in the first annual federal report on the prevalence of obesity in the United States, and a National Institutes of Health (NIH) panel defined overweight in terms of sex-specific BMI cutoffs (Kuczmarski and Flegal 2000; National Center for Health Statistics 1984; National Institutes of Health Consensus Development Panel 1985).

The current classification system adopted by the National Heart, Lung, and Blood Institute uses BMI to determine weight category. Classification of weight status is important because numerous medical comorbidities are associated with increased BMI. The BMI cutoff for overweight has decreased over time from 30 to 27, and most recently 25. Further, BMI provides a relative index of growth stunting, a condition that may result in significant developmental delays and adverse physiological effects (Dickerson 2003).

There is empirical evidence that BMI may be more predictive of body fatness in certain subgroups (e.g., younger adults, Caucasians) than others (Baumgartner, Heymsfield, and Roche 1995; Gallagher et al. 1996). Thus, two individuals with an identical BMI may have a different percentage of body fat depending on factors such as age, gender, body shape, and ethnicity (Prentice and Jebb 2001). BMI also overestimates body fat in persons who are very muscular (e.g., athletes), does not distinguish lean mass (muscle and bone) from fat mass, and does not determine the distribution of body fat. In children, BMI must be adjusted for growth. Despite these shortcomings, BMI classifications are still valuable for research and health care.

BMI is used to diagnose and make treatment recommendations. Epidemiological studies measure BMI to identify population trends in growth retardation and obesity along with associated adverse health consequences. Mounting evidence indicates an increased risk of mortality among obese individuals. Increased BMI has been associated with medical comorbidities including cardiovascular disease, reduced fertility, sleep apnea, metabolic syndrome, hypertension, type 2 diabetes, and certain cancers. In addition to medical risks, evidence suggests that there is a powerful social stigma associated with obesity. Discrimination affects overweight individuals in numerous facets of life, including employment, education, and psychological well-being (Friedman et al. 2005; Puhl and Brownell 2003).

SEE ALSO Body Image; Obesity

BIBLIOGRAPHY

Baumgartner, Richard N., Steven B. Heymsfield, and Alex F. Roche. 1995. Human Body Composition and the Epidemiology of Chronic Disease. Obesity Research 3: 73–95.

Blew, Robert M., Luis B. Sardinha, Laura A. Milliken, et al. 2002. Assessing the Validity of Body Mass Index Standards in Early Postmenopausal Women. Obesity Research 10: 799–808.

De Onis, Mercedes. 2004. The Use of Anthropometry in the Prevention of Childhood Overweight and Obesity. International Journal of Obesity 28: 581–585.

Deurenberg, Paul, Jan A. Weststrate, and Jaap C. Seidell. 1991. Body Mass Index as a Measure of Body Fatness: Age- and Sex-specific Prediction Formulas. British Journal of Nutrition 65: 105–114.

Dickerson, John W. T. 2003. Some Aspects of the Public Health Importance of Measurement of Growth. The Journal of the Royal Society for the Promotion of Health 123: 165–168.

Forbes, Gilbert B. 1999. Body Composition: Overview. Journal of Nutrition 129 (1): 270S–272S.

Friedman, Kelli E., Simona K. Reichmann, Philip R. Costanzo, et al. 2005. Weight Stigmatization and Ideological Beliefs: Relation to Psychological Functioning in Obese Adults. Obesity Research 13: 907–916.

Gallagher, Dympna, Marjolein Visser, Dennis Sepulveda, et al. 1996. How Useful is Body Mass Index for Comparison of Body Fatness Across Age, Sex, and Ethnic Groups? American Journal of Epidemiology 143: 228–239.

Gray, David S., and Ken Fujioka. 1991. Use of Relative Weight and Body Mass Index for the Determination of Adiposity. Journal of Clinical Epidemiology 44: 545–550.

Greenberg, Isaac, Frank Perna, Marjory Kaplan, and Mary Anna Sullivan. 2005. Behavioral and Psychological Factors in the Assessment and Treatment of Obesity Surgery Patients. Obesity Research 13: 244–249.

Headley, Allison A., Cynthia L. Ogden, Clifford L. Johnson, et al. 2004. Prevalence of Overweight and Obesity Among U.S. Children, Adolescents, and Adults, 1999–2002. Journal of the American Medical Association 291: 2847–2850.

Kuczmarski, Robert J., Katherine M. Flegal. 2000. Criteria for Definition of Overweight in Transition: Background and Recommendations for the United States. American Journal of Clinical Nutrition 72: 1074–1081.

National Center for Health Statistics. 1984. Health, United States, 1984. Washington, DC: U.S. Government Printing Office.

National Center for Health Statistics Consensus Development Panel on the Health Implications of Obesity. 1985. Health Implications of Obesity. Annals of Internal Medicine 103: 1073–1077.

National Heart, Lung, and Blood Institute. 1998. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report. Rockville, MD: National Institutes of Health.

Pietrobelli, Angelo, Steven B. Heymsfield, ZiMian M. Wang, and Dympna Gallagher. 2001. Multi-component Body Composition Models: Recent Advances and Future Directions. European Journal of Clinical Nutrition 55: 69–75.

Prentice, Andrew M., and Susan A. Jebb. 2001. Beyond Body Mass Index. Obesity Reviews 2: 141–147.

Puhl, Rebecca, and Kelly D. Brownell. 2003. Psychosocial Origins of Obesity Stigma: Toward Changing a Powerful and Pervasive Bias. Obesity Reviews 4: 213–227.

Seidell, Jaap C., Henry S. Kahn, David F. Williamson, et al. 2001. Report from a Centers for Disease Control and Prevention Workshop on Use of Adult Anthropometry for Public Health and Primary Health Care. American Journal of Clinical Nutrition 73: 123–126.

U.S. Department of Agriculture and U.S. Department of Health and Human Services. 1980. Nutrition and Your Health: Dietary Guidelines for Americans. Washington, DC: U.S. Government Printing Office.

Kelli Friedman

Erin E. Martinez

Body Mass Index

Body weight is used as an indicator of an individual's health. It is usually compared to tables that list "ideal" or "desirable" weight ranges for specific heights. Some of these tables use values gathered from research studies, while some include the heights and weights of individuals who have bought life insurance (e.g., the Metropolitan Height and Weight Tables). An individual's weight can be described as a percentage of the ideal or desirable weight listed, and can also be categorized as healthy, underweight, over-weight , or obese . An additional method of comparing an individual to a population group is with the body mass index .

Body mass index (BMI) is an estimate of body composition that correlates an individual's weight and height to lean body mass. The BMI is thus an index of weight adjusted for stature. Body mass index is figured by dividing weight in kilograms by height in meters squared and multiplying by 100. It can also be figured by dividing weight in pounds by height in inches squared and multiplying by 705. High values can indicate excessive fat stores, while low values can indicate reduced fat stores. In this way, the BMI is a diagnostic tool for both obesity and protein-energy malnutrition . The BMI has also been associated with mortality, with lower values generally correlating with longer life.

However, when evaluating the BMI, several characteristics of an individual need to be known. An individual's age, gender, ethnicity, and level of fitness must be considered when using BMI to determine health risk. Also, the significance of the BMI is affected by disease state and hydration status. As with most assessment tools, the BMI is most effective when used in conjunction with other measurements.

Tables are available to identify the significance of the BMI. Calculations based on values for ideal body weight suggest the BMI for normal men and women should be in the range of 19 to 27 kg/m². This range corresponds to the 25th to 75th percentile values recorded from adults followed in the 1971–1974 National Health and Nutrition Examination Survey (NHANES). Tables also list levels of protein-energy malnutrition and obesity. These values were determined by research in which height, weight, and age were associated with functional measurements and health outcomes.

A BMI between 13 and 15 corresponds to 48 to 55 percent of desirable body weight for a given height and describes the lowest body weight that can sustain life. Body weight at this level consists of less than 5 percent fat. The maximum survival body weight is about 500 kg, which corresponds to a BMI of about 150.

Research with children indicates annual increases in BMI are usually due to increases in lean mass rather than fat tissue. Not until late adolescence does fat mass begin to affect the BMI—and adult values begin to be achieved.

There is a strong correlation between BMI and total fat mass, though individual variation in body type or height can cause misclassification. Unfortunately,

Body mass index equation

Significance of BMI values for adults

the same BMI value can correlate with a range of body-fat percentage. For example, athletes usually have large skeletal muscles (which weigh more than fat) and therefore a high BMI, but they are not obese. Shorter individuals can also be identified as obese, since their BMIs are usually high. An older individual may have a higher body-fat percentage than a younger individual, but have the same BMI. Adult females can have a BMI of 20, which correlates to a body-fat percentage of 13 to 32 percent, while adult males can have a BMI of 27 and a body-fat percentage of 10 to 31 percent.

Findings from the third NHANES (1988–1994) describe misidentification of the elderly when self-reported height, rather than measured height, is used in the BMI equation. Height decreases over an individual's lifetime due to vertebral compression, loss of muscle tone, and postural slump. An individual may, therefore, report a height that is no longer accurate, and the resulting value will be lower than the value that actually describes the individual, possibly leading to the wrong intervention.

Research has shown that both high BMIs and low BMIs can indicate increased morbidity and mortality. A low BMI, usually an indication of protein-energy malnutrition or the effects of wasting or a disease process, is a significant predictor of mortality among young and old hospitalized patients. A high BMI has been shown to be predictive of mortality only among young hospitalized patients, usually an effect of cardiovascular disease and obesity. Risk of mortality is only slightly elevated at the highest BMI for elderly hospitalized patients.

Because ethnicity has been shown to require adjustments to the levels of concern for the BMI, care must be taken when comparing different population groups. For example, Asian populations may require a lower BMI to describe health risk, while Pacific populations, specifically Hawaiian, may require a higher threshold to indicate that an individual is at risk. This variation can be explained by body type.

BMI and waist circumference have been used to evaluate health risks associated with overweight and obesity. Because both are easy measures to do, standardization of both are encouraged for widespread use as a reference. Additionally, the two measurements have been used in an algorithm with a cardiovascular risk index to determine which individuals would benefit most from weight loss.

BMI is an easy measurement to make—only requiring a tape measure, scale, and, perhaps, a calculator. However, for individuals who have trouble standing up straight for an accurate height measurement—either from disease process, weakness, or kyphosis (abnormal backward curvature of the spine)—BMI may not be an easy or accurate assessment tool to use. Comparisons between BMI and mid-upper arm circumference (MUAC) measurements show that they identify the same level of malnutrition in individuals. MUAC is also easily measured (it requires only a tape measure), and it is a good indicator of change in body weight and muscle mass. Standardization of these two assessment tools for reference would benefit the science of nutrition assessment.

see also Aging and Nutrition; Body Fat Distribution; Diet; Malnutrition; Nutrition Assessment; Obesity; Overweight; Underweight; Waist-to-Hip Ratio.

Carole S. Mackey

Bibliography

Collins, Steve (1996). "Using Middle Upper Arm Circumference to Assess Severe Adult Malnutrition During Famine." Journal of the American Medical Association 276(5):391–395.

Kiernan, M. (2000). "Identifying Patients for Weight-Loss Treatment: An Empirical Evaluation of the NHLBI Obesity Education Initiative Expert Panel Treatment Recommendations." Archives of Internal Medicine 160:2169–2176.

Kuczmarski, Marie Fanelli (2001). "Effects of Age on Validity of Self-Reported Height, Weight, and Body Mass Index: Findings from the Third National Health and Nutrition Survey, 1988–1994." Journal of the American Dietetic Association 101(1):28–34.

Landi, F. (2000). "Body Mass Index and Mortality Among Hospitalized Patients." Archives of Internal Medicine 160:2641–2644.

Maskarinec, G. (2000). "Dietary Patterns Are Associated with Body Mass Index in Multiethnic Women." Journal of Nutrition 130:3068–3072.

Maynard, L. M. (2001). "Childhood Body Composition in Relation to Body Mass Index." Pediatrics 107:344–350.

Pike, Ruth, and Brown, Myrtle L. (1984). Nutrition, An Integrated Approach. New York: John Wiley.

Seidel, J. C. (2001). "Report from a CDC Prevention Workshop on Use of Adult Anthropometry for Public Health and Primary Health Care." American Journal of Clinical Nutrition 73:123–126.

Shills, Maurice E.; Olson, James A.; and Shike, Moshe. (1994). Modern Nutrition in Health and Disease, 8th edition. Philadelphia: Lea & Febiger.

White, Jane V. (1999). "The Utility of Body Mass Index in Predicting Health Risk." Consultant Dietitian 24(2).

Body mass index

Definition

Purpose

Description

Precautions

Parental concerns

Definition

Body mass index (BMI), also called the Quetelet Index, is a calculation used to determine an individual’s amount of body fat.

Purpose

The BMI gives healthcare professionals a consistent way of assessing their patients’ weight and an objective way of discussing it with them. It is also useful in suggesting the degree to which the patient may be at risk for obesity-related diseases.

Description

BMI is a statistical calculation intended as an assessment tool. It can be applied to groups of people to determine trends or it can be applied to individuals. When applied to individuals, it is only one of several assessments used to determine health risks related to being underweight, overweight, or obese.

The history of BMI

The formula used to calculate BMI was developed more than one hundred years ago by Belgian mathematician and scientist Lambert Adolphe Quetelet (1796-1874). Quetelet, who called his calculation the Quetelet Index of Obesity, was one of the first statisticians to apply the concept of a regular bell-shaped statistical distribution to physical and behavioral features of humans. He believed that by careful measurement and statistical analysis, the general characteristics of populations could be mathematically determined. Mathematically describing the traits of a population led him to the concept of the hypothetical “average man” against which other individuals could be measured. In his quest to describe the weight to height relationship in the average man, he developed the formula for calculating the body mass index.

Calculating BMI requires two measurements: weight and height. To calculate BMI using metric units, weight in kilograms (kg) is divided by the height squared measured in meters (m). To calculate BMI in imperial units, weight in pounds (lb) is divided by height squared in inches (in) and then multiplied by 703. This calculation produces a number that is the individual’s BMI This number, when compared to the statistical distribution of BMIs for adults ages 20–29, indicates whether the individual is underweight, average weight, overweight, or obese. The 20–29 age group was chosen as the standard because it represents fully developed adults at the point in their lives when they statistically have the least amount of body fat. The formula for calculating the BMI of children is the same as for adults, but the resulting number is interpreted differently.

Although the formula for calculating BMI was developed in the mid-1800s, it was not commonly used in the United States before the mid-1980s. Until then, fatness or thinness was determined by tables that set an ideal weight or weight range for each height. Heights were measured in one-inch intervals, and the ideal weight range was calculated separately for men and women. The information used to develop these ideal weight-for-height tables came from several decades of data compiled by life insurance companies. These tables determined the probability of death as it related to height and weight and were used by the companies to set life insurance rates. The data excluded anyone with a chronic disease or anyone who, for whatever health reason, could not obtain life insurance.

Interest in using the BMI in the United States increased in the early 1980s when researchers became concerned that Americans were rapidly becoming

KEY TERMS

Anorexia nervosa— A psychiatric disorder signified by obsession with weight loss and voluntary self-starvation accompanied by serious, potentially fatal health problems.

Morbid obesity— A term used to describe individuals 100 lb (45 kg) or more than 50% overweight and/or who have a body mass index above 40.

Triglycerides— A type of fat found in the blood. High levels of triglycerides can increase the risk of coronary artery disease.

obese. In 1984 the national percentage of overweight individuals was reported in a major assessment of the nation’s health. Men having a BMI of 28 or greater were considered overweight. This BMInumber was chosen to define overweight because 85% of American men ages 20-29 fell below it. A different calculation, not BMI, was used for women in the report.

In 1985, the term overweight was redefined as a BMI equal to or greater than 27.8 for men and equal to or greater than 27.3 for women. No BMI was selected to define underweight individuals. This definition of overweight was used in reports on obesity until 1998. In 1998, the United States National Institutes of Health revised its weight definitions to bring them in line with the definitions used by the World Health Organization. Overnight 30 million Americans went from being classified as normal weight to being classified as overweight. Overweight is now defined for both men and women as a BMI of 25 or less. At the same time, an underweight classification was added, as was the classification of obese for individuals with a BMI greater than or equal to 30.

Interpreting BMI calculations for adults

All adults age 20 and older are evaluated on the same BMI scale as follows:

• BMI below 18.5: Underweight
• BMI 18.5-24.9: Normal weight
• BMI 25.0-29.9: Overweight
• BMI 30 and above: Obese

Some researchers consider a BMI of 17 or below an indication of serious, health-threatening malnourishment. In developed countries, a BMI this low in the absence of disease is often an indication anorexia nervosa At the other end of the scale, a BMI of 40 or greater indicates morbid obesity that carries a very high risk of developing obesity-related diseases such as stroke, heart attack, and type 2 diabetes.

Interpreting BMI calculations for children and teens

The formula for calculating the BMI of children ages 2-20 is the same as the formula used in calculating adult BMIs, but the results are interpreted differently. Interpretation of BMI for children takes into consideration that the amount of body fat changes as children grow and that the amount of body fat is different in boys and girls of the same age and weight.

Instead of assigning a child to a specific weight category based on their BMI, a child’s BMI is compared to other children of the same age and sex. Children are then assigned a percentile based on their BMI The percentile provides a comparison between their weight and that of other children the same age and gender. For example, if a girl is in the 75th percentile for her age group, 75 of every 100 children who are her age weigh less than she does and 25 of every 100 weigh more than she does. The weight categories for children are:

• Below the 5th percentile: Underweight
• 5th percentile to less than the 85th percentile: Healthy weight
• 85th percentile to less than the 95th percentile: At risk of overweight
• 95th percentile and above: Overweight

Application of BMI information

The BMI was originally designed to observe groups of people. It is still used to spot trends, such as increasing weight in a particular age group over time. It is also a valuable tool for comparing body mass among different ethnic or cultural groups, and can indicate to what degree populations are undernourished or overnourished.

When applied to individuals, the BMI is not a diagnostic tool. Although there is an established link between BMI and the prevalence of certain diseases such as type 2 diabetes, some cancers, and cardiovascular disease, BMI alone is not intended to predict the likelihood of an individual developing these diseases. The National Heart, Lung, and Blood Institute recommends that the following measures be used to assess the impact of weight on health:

• BMI
• Waist circumference (an alternate measure of body fat)
• GALE ENCYCLOPEDIA OF DIETS
• Risk factors for disease associated with obesity. These include high blood pressure, high LDL or “bad” cholesterol
• Low HDL or “good” cholesterol
• High blood glucose (sugar)
• High triglycerides
• Family history of cardiovascular disease
• Low physical activity level
• Cigarette smoking

Precautions

BMI is very accurate when defining characteristics of populations, but less accurate when applied to individuals. However, because it is inexpensive and easy to determine BMI is widely used. Calculating BMI requires a scale, a measuring rod, and the ability to do simple arithmetic or use a calculator. Potential limitations of BMI when applied to individuals are:

• BMI does not distinguish between fat and muscle. BMI tends to overestimate the degree of “fatness” among elite athletes in sports such as football, weightlifting, and bodybuilding. Since muscle weighs more than fat, many athletes who develop heavily muscled bodies are classified as overweight, even though they have a low percentage of body fat and are in top physical condition.
• BMI tends to underestimate the degree of fatness in the elderly as muscle and bone mass is lost and replaced by fat for the same reason it overestimates fatness in athletes.
• BMI makes no distinction between body types. People with large frames (big boned) are held to the same standards as people with small frames.
• BMI weight classes have absolute cut-offs, while in many cases health risks change gradually along with changing BMIs. A person with a BMI of 24.9 is classified as normal weight, while one with a BMI of 25.1 is overweight. In reality, their health risks may be quite similar.
• BMI does not take into consideration diseases or drugs that may cause significant water retention.
• BMI makes no distinction between genders, races, or ethnicities. Two people with the same BMI may have different health risks because of their gender or genetic heritage.

BMI is a comparative index and does not measure the amount of body fat directly. Other methods do give a direct measure of body fat, but these methods generally are expensive and require specialized equipment and training to be performed accurately. Among them are measurement of skin fold thickness, underwater (hydrostatic) weighing, bioelectrical impedance, and dual-energy x-ray absorptiometry (DXA). Combining BMI, waist circumference, family health history, and lifestyle analysis gives healthcare providers enough information to analyze health risks related to weight at minimal cost to the patient.

Parental concerns

Childhood obesity is an increasing concern. Research shows that overweight children are more likely to become obese adults than normal weight children. Excess weight in childhood is also linked to early development of type 2 diabetes, cardiovascular disease, and early onset of certain cancers. In addition, overweight or severely underweight children often pay a heavy social and emotional price as objects of scorn or teasing.

Both the American Academy of Pediatrics (AAP) and the United States Centers for Disease Control and Prevention (CDC) recommend that the BMI of children over age two be reviewed at regular intervals during pediatric visits. Parents of children whose BMI falls above the 85th percentile (at risk of being overweight and overweight categories) should seek information from their healthcare provider about health risks related to a high BMI and guidance on how to moderate their child’s weight. Strenuous dieting is rarely advised for growing children, but healthcare providers can give guidance on improving the chid’s diet, eliminating empty calories (such as those found in soda and candy) and increasing the child’s activity level in order to burn more calories and improve fitness.

Tish Davidson, A.M.

Body mass index

Definition

Body mass index (BMI), also called the Quetelet Index, is a calculation used to determine an individual's amount of body fat.

Purpose

The BMI gives healthcare professionals a consistent way of assessing their patients' weight and an objective way of discussing it with them. It is also useful in suggesting the degree to which the patient may be at risk for obesity-related diseases.

Description

BMI is a statistical calculation intended as an assessment tool. It can be applied to groups of people to determine trends or it can be applied to individuals. When applied to individuals, it is only one of several assessments used to determine health risks related to being underweight, overweight, or obese.

The history of BMI

The formula used to calculate BMI was developed more than one hundred years ago by Belgian mathematician and scientist Lambert Adolphe Quetelet (1796-1874). Quetelet, who called his calculation the Quetelet Index of Obesity, was one of the first statisticians to apply the concept of a regular bell-shaped statistical distribution to physical and behavioral features of humans. He believed that by careful measurement and statistical analysis, the general characteristics of populations could be mathematically determined. Mathematically describing the traits of a population led him to the concept of the hypothetical “average man” against which other individuals could be measured. In his quest to describe the weight to height relationship in the average man, he developed the formula for calculating the body mass index.

Calculating BMI requires two measurements: weight and height. To calculate BMI using metric units, weight in kilograms (kg) is divided by the height squared measured in meters (m). To calculate BMI in imperial units, weight in pounds (lb) is divided by height squared in inches (in) and then multiplied by 703. This calculation produces a number that is the individual's BMI. This number, when compared to the statistical distribution of BMIs for adults ages 20–29, indicates whether the individual is underweight, average weight, overweight, or obese. The 20–29 age group was chosen as the standard because it represents fully developed adults at the point in their lives when they statistically have the least amount of body fat. The formula for calculating the BMI of children is the same as for adults, but the resulting number is interpreted differently.

Although the formula for calculating BMI was developed in the mid-1800s, it was not commonly used in the United States before the mid-1980s. Until then, fatness or thinness was determined by tables that set an ideal weight or weight range for each height. Heights were measured in one-inch intervals, and the ideal weight range was calculated separately for men and women. The information used to develop these ideal weight-for-height tables came from several decades of data compiled by life insurance companies. These tables determined the probability of death as it related to height and weight and were used by the companies to set life insurance rates. The data excluded anyone with a chronic disease or anyone who, for whatever health reason, could not obtain life insurance.

Interest in using the BMI in the United States increased in the early 1980s when researchers became concerned that Americans were rapidly becoming obese. In 1984 the national percentage of overweight individuals was reported in a major assessment of the nation's health. Men having a BMI of 28 or greater were considered overweight. This BMI number was chosen to define overweight because 85% of American men ages 20–29 fell below it. A different calculation, not BMI, was used for women in the report.

KEY TERMS

Morbid obesity —A term used to describe individuals 100 lb (45 kg) or more than 50% overweight and/or who have a body mass index above 40.

Triglycerides —A type of fat found in the blood. High levels of triglycerides can increase the risk of coronary artery disease.

In 1985, the term overweight was redefined as a BMI equal to or greater than 27.8 for men and equal to or greater than 27.3 for women. No BMI was selected to define underweight individuals. This definition of overweight was used in reports on obesity until 1998. In 1998, the United States National Institutes of Health revised its weight definitions to bring them in line with the definitions used by the World Health Organization. Overnight 30 million Americans went from being classified as normal weight to being classified as overweight. Overweight is now defined for both men and women as a BMI of 25 or less. At the same time, an underweight classification was added, as was the classification of obese for individuals with a BMI greater than or equal to 30.

Interpreting BMI calculations for adults

All adults age 20 and older are evaluated on the same BMI scale as follows:

• BMI below 18.5: Underweight
• BMI 18.5–24.9: Normal weight
• BMI 25.0–29.9: Overweight
• BMI 30 and above: Obese

Some researchers consider a BMI of 17 or below an indication of serious, health-threatening malnourishment. In developed countries, a BMI this low in the absence of disease is often an indication anorexia nervosa. At the other end of the scale, a BMI of 40 or greater indicates morbid obesity that carries a very high risk of developing obesity-related diseases such as stroke , heart attack , and type 2 diabetes.

Interpreting BMI calculations for children and teens

The formula for calculating the BMI of children ages 2–20 is the same as the formula used in calculating adult BMIs, but the results are interpreted differently. Interpretation of BMI for children takes into consideration that the amount of body fat changes as children grow and that the amount of body fat is different in boys and girls of the same age and weight.

Application of BMI information

The BMI was originally designed to observe groups of people. It is still used to spot trends, such as increasing weight in a particular age group over time. It is also a valuable tool for comparing body mass among different ethnic or cultural groups, and can indicate to what degree populations are undernourished or overnourished.

When applied to individuals, the BMI is not a diagnostic tool. Although there is an established link between BMI and the prevalence of certain diseases such as type 2 diabetes, some cancers, and cardiovascular disease, BMI alone is not intended to predict the likelihood of an individual developing these diseases. The National Heart, Lung, and Blood Institute recommends that the following measures be used to assess the impact of weight on health:

• BMI
• waist circumference (an alternate measure of body fat)
• fisk factors for disease associated with obesity. (These include high blood pressure, high LDL or “bad” cholesterol.)
• low HDL or “good” cholesterol
• high blood glucose (sugar)
• high triglycerides
• family history of cardiovascular disease
• low physical activity level
• cigarette smoking

Precautions

BMI is very accurate when defining characteristics of populations, but less accurate when applied to individuals. However, because it is inexpensive and easy to determine BMI is widely used. Calculating BMI requires a scale, a measuring rod, and the ability to do simple arithmetic or use a calculator. Potential limitations of BMI when applied to individuals are:

• BMI does not distinguish between fat and muscle. BMI tends to overestimate the degree of “fatness” among elite athletes in sports such as football, weightlifting, and bodybuilding. Since muscle weighs more than fat, many athletes who develop heavily muscled bodies are classified as overweight, even though they have a low percentage of body fat and are in top physical condition.
• BMI tends to underestimate the degree of fatness in the elderly as muscle and bone mass is lost and replaced by fat for the same reason it overestimates fatness in athletes.
• BMI makes no distinction between body types. People with large frames (big boned) are held to the same standards as people with small frames.
• BMI weight classes have absolute cut-offs, while in many cases health risks change gradually along with changing BMIs. A person with a BMI of 24.9 is classified as normal weight, while one with a BMI of 25.1 is overweight. In reality, their health risks may be quite similar.
• BMI does not take into consideration diseases or drugs that may cause significant water retention.
• BMI makes no distinction between genders, races, or ethnicities. Two people with the same BMI may have different health risks because of their gender or genetic heritage.

BMI is a comparative index and does not measure the amount of body fat directly. Other methods do give a direct measure of body fat, but these methods generally are expensive and require specialized equipment and training to be performed accurately. Among them are measurement of skin fold thickness, underwater (hydrostatic) weighing, bioelectrical impedance, and dual-energy x-ray absorptiometry (DXA). Combining BMI, waist circumference, family health history , and lifestyle analysis gives healthcare providers enough information to analyze health risks related to weight at minimal cost to the patient.

Tish Davidson A.M.

body mass index (BMI) n. the weight of a person (in kilograms) divided by the square of the height of that person (in metres): used as an indicator of whether or not a person is over- or underweight. A BMI of between 20 and 25 is considered normal, between 25 and 30 is overweight, and greater than 30 indicates clinical obesity.

Calculation of body mass index

BMI can be calculated using metric units (kilograms and metres) or imperial units (pounds and inches). The metric system is more widely used.

Metric calculation

Weight in kilograms is divided by height in metres squared, i.e. weight (kg)/height2 (m)

For example:

height=1.65 m, weight=67 kg

Calculation:

67ɇ(1.65)2=24.6

Imperial calculation

Weight in pounds is divided by height in inches squared and multiplied by 703, i.e. [weight (kg)/height² (in)]×703

For example:

height=144 lb, height=60kg

Calculation:

[144ɇ(60)²]×703=28.12

Interpretation of body mass index

The range within which a person's BMI falls will help determine whether they are of a healthy weight for their height. The ranges are as follows:

BMI and children