Body Mass Index

BMI is the measure commonly used in clinical settings for the identification of individuals at increased health risk. BMI is calculated using an individual's weight in kilograms divided by their height in meters squared. Height and weight are relatively simple measures, but in large studies they are often collected by self-report. Both men and women tend to overestimate their height and underestimate their weight, wherein the overestimation of height is increased with decreasing height, and the extent of the underestimation of weight is increased with increasing weight (13). Accordingly, reports suggest that as many as 41% of men and 27% of women self-report as being overweight or normal weight (13), when in fact they are obese.

BMI is positively associated with morbidity (1,2) and has a U- or J-shaped relationship with mortality (1,7-9). Based on the associations among BMI, morbidity, and mortality, BMI categories for normal weight (18.5-24.9 kg/m2), overweight (25.0-29.9 kg/m2), and obese (>30.0 kg/m2) have been established for the Caucasian population (14).

It is commonly held that a high BMI is associated with increased health risk or mortality because of its association with adiposity. Indeed, within a given population, BMI is positively associated with adiposity (Fig. 1). It is noteworthy, however, that this relationship is altered by numerous factors such as age (15), gender (15), race (16), and physical activity patterns (17). For example, for a given age and BMI, women tend to have 12% more body fat than men (15). Similarly, for a given BMI, white men aged 60 to 79 yr tend to have 4 to 5% more body fat than white men aged 20 to 39 years (15).

It is important to appreciate that although there is a strong association between BMI and adiposity within a given population, BMI is a poor indicator of adiposity and/or lean mass on an individual basis. Indeed, despite presenting with a common BMI, adiposity levels between individuals may vary substantially (Fig. 1). Interestingly, it is reported

an 60

an 60

30 40

50 n

"3

'—'

40

<l>

O

tf) 3

30

2

"5

20

a!

<l>

10

CO

30 40

2 80

40 20 0

30 40

30 40

30 40

50

40

ai

30

20

<yf O C

30 40

100

g

80

il)

V)

60

U)

c

40

10

0)

20

0

30 40

Fig. 1. Association between BMI and total body fat, skeletal muscle mass, and lean tissue mass in middle-aged men and women. Men (closed diamonds, N = 110), Women (open diamonds,N = 111). Data taken from refs. 42-45.

that factors that influence the association between BMI and adiposity may also influence the association between BMI and health risk (8,16,18). For example, Stevens et al. (18) report that mortality risk at a given BMI decreases with age. In other words, having a BMI of 29 is associated with a greater relative risk of death in individuals 30 to 44 yr of age than individuals greater than 65 yr of age. Similarly, it has been proposed that the mortality rate associated with a given BMI is higher in men than in women (8).

Race also influences the level of adiposity for a given BMI (16) and may explain why observational studies report very high incidences of obesity-related metabolic disorders despite a very low obesity prevalence when using the Caucasian BMI cut-points in some Asian populations (16). Consequently, in 2004 the World Health Organization released revised BMI cut-points for certain Asian populations that were derived to reflect body fat values similar to Caucasian populations (19). However, mortality studies in Asian populations (20,21) show a similar J-shaped pattern with increasing BMI and mortality, wherein the nadir of the curve lies around 22 to 26 kg/m2, a finding that is quite comparable to those in Caucasian populations (8,9). Thus, although it is clear that adiposity and health risk tend to increase with increasing BMI independent of age, gender, and race, the magnitude of the increment in adiposity and health risk with increasing BMI is influenced by these factors, and are therefore important to consider when examining these relationships.

There is also substantial evidence that a low body mass or BMI may also be associated with increased health risk and mortality (9). It is thought that a low BMI may be indicative of low lean mass representing a separate pathway linking BMI with disease (9). In fact, BMI is positively related to lean and skeletal muscle mass in both men and women, although there is a clear gender difference in this association (Fig. 1). However, the relationship between skeletal muscle mass and BMI is also influenced by age (22) and physical activity patterns (23). Further, as with total adiposity, substantial interindividual variation in lean and skeletal muscle mass exists for a given BMI. These observations confirm that determination of body composition using BMI on an individual basis is limited.

Weight Loss Resolutions

Weight Loss Resolutions

Are You Tired Of Failed New Year's Weight Loss Resolutions That Leave You Even More Overweight Than Ever Before? Not Anymore Finally Succeed With Your New Years Weight Loss Resolution Once And For All And Get The Body Of Your Dreams.

Get My Free Ebook


Post a comment