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Original Research Communication |
1 From the Department of Medicine, the University of Washington School of Medicine, Seattle; the Department of Anthropology, the University of Washington, Seattle; and the Veterans Affairs Epidemiologic Research Information Center, Seattle.
2 Supported by National Institutes of Health grants DK-31170 and HL-49293. Facility support was provided by the Clinical Nutrition Research Unit (DK-35816), the Diabetes Endocrinology Research Center (DK-17047), and the General Clinical Research Center (RR-00037) at the University of Washington. 3 Address reprint requests to MJ McNeely, Division of General Internal Medicine, Box 356429, University of Washington School of Medicine, Seattle, WA 98195-6429. E-mail: mcneely{at}u.washington.edu.
| ABSTRACT |
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Objective: Our goal was to determine whether the National Heart, Lung, and Blood Institute (NHLBI) overweight and obesity guidelines are useful for identifying diabetes risk in Japanese Americans.
Design: This was a prospective, cohort study of 466 nondiabetic Japanese Americans [age: 52.2 ± 0.6 y; BMI (in kg/m2): 24.1 ± 0.2;
± SEM]. Diabetes status at a 5-y follow-up visit was assessed with an oral-glucose-tolerance test.
Results: Among 240 subjects aged
55 y, incident diabetes was strongly associated with overweight (BMI
25) at baseline [relative risk (RR): 22.4; 95% CI: 2.7, 183; adjusted for age, sex, smoking, and family history] and weight gain of >10 kg since the age of 20 y (adjusted RR: 4.5; 95% CI: 1.4, 14.5). NHLBI definitions of central obesity (waist circumference
88 cm for women and
102 cm for men) were unsuitable for this population because only 15 of 240 subjects met these criteria. A waist circumference greater than or equal to the third tertile was associated with diabetes (adjusted RR: 5.4; 95% CI: 1.7, 17.0). Among 226 subjects aged >55 y, incident diabetes was not associated with BMI, weight gain, or waist circumference.
Conclusions: NHLBI definitions are useful for identifying overweight Japanese Americans aged <55 y who are at high risk of diabetes. Although central adiposity is an important risk factor, the guidelines for waist circumference are insensitive predictors of diabetes risk in this population.
Key Words: Type 2 diabetes Asian Americans obesity weight gain body mass index anthropometry longitudinal study central adiposity National Heart Lung Blood Institute guidelines
| INTRODUCTION |
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30 (1, 2). These cutoffs are consistent with those used by the World Health Organization (3).
The risk of health problems, such as hypertension, dyslipidemia, coronary artery disease, and diabetes, increase incrementally above a BMI of 2022 (2). The relation between increasing BMI and morbidity is observed across many racial and ethnic groups, although the absolute risk varies. Increased mortality is associated with BMIs
25 in whites and is the rationale for the current definition of overweight (2).
The clinical suitability of a single threshold definition of "normal" weight across ethnic groups remains unclear. High rates of obesity-related disorders have been noted in Asian populations, particularly in urban and westernized areas, despite the low average BMI of these populations relative to white populations (411). Variation in body fat distribution may account for some of this difference in risk, in that Asians may be more prone to central adiposity than are whites (1214). The NHLBI guidelines include waist circumference cutoffs to identify high-risk individuals with central obesity, but the cutoffs are based on white populations (2, 15, 16) and may be inappropriate for Asians (11). Adulthood weight gain is another measure of excess adiposity that is associated with morbidity. Avoiding weight gain after reaching adult height was proposed as an appropriate health goal for individuals (17), yet data on the health consequences of weight gain in Asians are sparse. To determine the applicability of current reference ranges for overweight and central obesity to a high-risk Asian population, we studied the relation between BMI, waist circumference, and weight gain since the age of 20 y and the risk of developing type 2 diabetes over the course of 5 y in Japanese Americans.
| SUBJECTS AND METHODS |
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7.0 mmol/L (126 mg/dL) after a 10-h fast or
11.1 mmol/L (200 mg/dL) 2 h after a 75-g oral-glucose-tolerance test at baseline were excluded. This study was approved by the University of Washington Institutional Review Board, and all participants provided written, informed consent.
Measurements
Subjects were evaluated in the Clinical Research Center at the University of Washington at baseline and after 5 or 6 y of follow-up. A structured interview was used to obtain information about weight at the age of 20 y, medication use, smoking, and family history of diabetes. Subjects who reported a parent or sibling with diabetes were considered to have a positive family history of diabetes. Height and weight were measured while the subjects wore light clothing and no shoes. Waist circumference was measured at the level of the umbilicus in men and at the natural waistline (minimal waist) in women. Weight change was calculated as baseline weight minus weight at the age of 20 y. Percentage weight change was calculated as weight change divided by weight at the age of 20 y. BMI was calculated as weight divided by height squared (kg/m2). Plasma glucose was assayed by an automated glucose oxidase method. Subjects were considered to have diabetes at follow-up if they were taking medication for diabetes or if their plasma glucose was
7.0 mmol/L (126 mg/dL) after a 10-h fast or
11.1 mmol/L (200 mg/dL) 2 h after a 75-g oral-glucose-tolerance test (19).
Statistical analysis
All means are presented as means ± SEMs. Baseline variables were compared between groups by using Student's t test or the chi-square test, except for age, which was analyzed by using Wilcoxon's rank-sum test because of its bimodal distribution. Sex-specific tertiles for waist circumference were calculated by using data pooled across age groups. Logistic regression models were used to estimate the relative odds of developing diabetes associated with BMI, waist circumference, or weight change, with adjustment for age, sex, smoking (ever versus never), and family history of diabetes. Given the infrequent occurrence of diabetes, the relative odds approximates the relative risk (RR). The likelihood ratio test was used to determine the statistical significance of variables and interaction terms in the logistic regression models. An age x BMI interaction with diabetes risk was observed (P = 0.004), so the results were stratified by age to simplify their interpretation. Statistics were calculated by using INTERCOOLED STATA software (version 5.0 for WINDOWS 95; Stata Corporation, College Station, TX).
| RESULTS |
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10 y before baseline. Of those who developed diabetes, 57.1% reported a positive family history, compared with 34.5% of those who remained nondiabetic.
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55 y who developed diabetes had higher baseline BMIs, weight gains, and waist circumferences than did subjects who remained nondiabetic, although the statistical power for detecting differences among younger women was poor (Table 2
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55 y and 147 of 226 (65%) of those aged >55 y]. Only 16 of 240 younger (6.7%) and 7 of 226 older (3.1%) subjects were obese. More than one-half of subjects who gained 510 kg since the age of 20 y were within the normal range of BMI at baseline, whereas only 14 of 68 (20.6%) younger and 15 of 58 (25.9%) older subjects who gained >10 kg since the age of 20 y had a normal BMI. Diabetes incidence increased incrementally with both weight gain since the age of 20 y and BMI at baseline among younger, but not older, subjects.
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55 y who developed diabetes, 3 gained 515% and 9 gained >15% of their body weight since the age of 20 y. Among the 226 older subjects, diabetes incidence was 11.8% (9/76) among those with a weight gain of <5% of body weight, 15.6% (10/64) among those with a weight gain of 515%, and 16.3% (14/86) among those with a weight gain of >15%.
The proportions of subjects who developed diabetes by waist circumference tertile and BMI at baseline are shown in Table 4
. Waist circumference data were missing for 9 subjects. Diabetes incidence increased incrementally with increasing waist circumference among younger, but not older, subjects. Only 15 of 237 younger and 13 of 220 older subjects met the NHLBI guidelines for increased waist circumference.
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55 y at baseline, the RR for developing diabetes associated with a BMI
25 was 26.5 (95% CI: 3.4, 204) compared with normal-weight subjects (Table 5
25 and 22.0 ± 0.1 for the normal-weight group. Because only 16 of 96 subjects with a BMI
25 were obese, results were similar for overweight subjects (BMI: 2529.9; obese subjects excluded) compared with normal-weight subjects (adjusted RR: 23.5; 95% CI: 2.8, 196.2; P = 0.004; data not presented in table).
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55 y (RR: 4.3; 95% CI: 1.4, 12.8; P = 0.0009), although results were of borderline significance after adjustment for covariates (adjusted RR: 3.1; 95% CI: 1.0, 9.9; P = 0.054; data not presented in table). Younger subjects in the top sex-specific tertile for waist circumference were also at increased risk of diabetes (Table 5
In the older age group, BMI, weight gain, and waist circumference did not predict diabetes risk, although age and family history of diabetes were associated with increased risk (Table 5
). Even when analyzed as continuous variables, however, BMI, weight change, and waist circumference were not significantly associated with incident diabetes (data not shown).
| DISCUSSION |
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55 y, which is consistent with prior studies (21). Among younger Japanese Americans, a BMI
25 was strongly associated with diabetes incidence, with 15.6% of overweight compared with 0.7% of normal-weight (BMI < 25) subjects meeting the criteria for diabetes at follow-up. This was particularly striking because the mean BMI of the overweight group was only 27.6. Even when the 16.7% of overweight, younger subjects who met the criteria for obesity were excluded from this group, diabetes incidence remained significantly higher than in those with normal weights. This suggests that moderate increases above normal weight may substantially raise diabetes risk in younger Japanese Americans.
Our study does not explain why overweight is such a strong diabetes risk factor for younger Japanese Americans. There is some evidence that the relation between percentage body fat and BMI varies by ethnicity (22, 23). In these studies, Asians had a higher degree of adiposity for a given BMI than did whites. Another likely possibility is that Japanese Americans may be genetically predisposed to an unfavorable fat distribution pattern, namely, increased intraabdominal fat (24, 25). In our study, central adiposity measured as waist circumference was associated with diabetes risk in younger Japanese Americans, but only 6.3% of subjects in this group exceeded NHLBI sex-specific cutoffs. Recently, experts from several Asian and Pacific countries recommended lower thresholds for BMI and waist circumference for Asians than for whites (overweight, BMI
23; obese, BMI
25; high-risk waist circumference,
90 cm for men and
80 cm for women) (26). These proposed cutoffs for waist circumference are similar to the upper tertile cutoffs associated with increased diabetes risk in our study.
Among older Japanese Americans, we found no association between increased BMI or waist circumference and diabetes risk, possibly because subjects susceptible on the basis of increased overall adiposity already had diabetes at baseline. We previously reported that among older (Nisei) Japanese American men, intraabdominal fat was a significant predictor of diabetes risk at the 30-mo follow-up, whereas BMI did not predict diabetes (24). Thus, it appears that waist circumference and BMI are poor markers of intraabdominal fat in this older population. One possible explanation for these findings is that among older persons, increased intraabdominal fat is primarily associated with aging, whereas among younger persons, increased intraabdominal fat is due to excess adiposity (as measured by BMI and waist circumference). Thus, it may be that individuals with a propensity toward intraabdominal fat deposition who become overweight are those at highest risk of developing diabetes at a younger age. If an individual has not developed diabetes by the time of maximum adulthood weight gain (typically by the fifth decade of life) (27), then his or her major risk factor becomes age-related fat redistribution.
Our study confirms prior reports that adulthood weight gain is associated with an increased risk of diabetes (2830). In these studies of white and African American populations, weight gain of >5 kg was associated with significant risk. We found no incident cases among younger adults with weight gains of <5 kg (probably because of the small sample size), but we found a significantly increased diabetes risk with a weight gain >10 kg. There are several advantages to using weight change as an adjunct to BMI for assessing disease risk in adults. From a clinical standpoint, weight on reaching adult height can be easily monitored and requires no calculations or tables to interpret. Focusing on adulthood weight change is a reasonable approach to both preventing and managing obesity. Because substantial weight loss is difficult to achieve, minimizing weight gain can also be expected to result in greater absolute risk reduction than can delaying interventions until after individuals become overweight. Larger studies may help to clarify whether adulthood weight gain cutoffs provide a practical alternative to population-specific BMI or waist circumference reference ranges for risk assessment of individuals in the clinical setting.
We found that a family history of diabetes predicted diabetes risk in older, but not in younger, Japanese Americans. Previously reported cross-sectional data from this population showed that diabetes was associated with increased BMI, waist circumference, and intraabdominal fat only among those without a family history of diabetes (31). The current study suggests that a family history of diabetes is associated with age-related derangements of glucose metabolism in older Japanese Americans, possibly as a result of a genetic predisposition to impaired islet ß cell function with age. However, it appears that persons predisposed to adiposity-related diabetes, presumably via insulin resistance (32), cannot be identified on the basis of a family history of diabetes in this population.
There are several limitations to our study. We used self-reported weight at the age of 20 y to determine adulthood weight gain at baseline. Recalled weight is undoubtedly less precise than measured weight, and error may be accentuated in older subjects. However, BMI derived from adolescent height and weight as recalled by elderly subjects was shown to correlate well with measured BMI (r = 0.75) (33). Additionally, self-reported weight change was associated with morbidity in several other studies (28, 29, 34, 35). Another potential limitation of our study is the use of volunteers rather than a population-based sample. However, it is unlikely that our conclusions were substantially influenced by selection bias because all subjects were determined to be nondiabetic at baseline and adjustment for family history of diabetes did not significantly alter the results. Finally, our sample size was fairly small, particularly after stratification by age. It remains to be seen whether these results are confirmed by larger prospective studies and in other Asian populations.
In summary, Japanese Americans aged
55 y who are only moderately overweight (mean BMI of 27.6) are at high risk of developing diabetes. Although Japanese Americans may be especially prone to central obesity, the NHLBI guidelines for waist circumference were insensitive markers of diabetes risk in this population. These findings illustrate the potential limitations of applying uniform BMI and waist circumference cutoffs to assess the health risks of individuals in a diverse patient population. Additional research is needed to examine the feasibility of incorporating ethnic-specific cutoffs or dynamic measures (such as weight gain and waist circumference increase) into clinical guidelines.
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