The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Ethnic Differences in the Association Between Body Mass Index

and HypertensionColin Bell, Linda Adair, Barry Popkin

Department of Nutrition, The University of North Carolina at Chapel Hill

Department of Community Health, University of Auckland

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Does a BMI of 25 kg/m2 mean the same thing in different populations? BMI is only an approximate measure of body

fatness Good evidence exists that some populations

have different levels of body fat at similar BMIs – Asian: smaller frames, higher % body fat than

Caucasians (Deurenberg et al, Int J Obesity 1998,1999)

– Polynesians: larger frames, more lean body mass, lower % body fat than Caucasians (Swinburn et al, Int J Obesity 1999)

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

If body fat differs, do obesity related co-morbidities also differ?

In Hong Kong Chinese, Ko et al observed increased prevalence of type-2 diabetes, hypertension, dislipidaemia and albuminuria at a BMI of ~ 23 kg/m2 (Ko et al, Int J Obesity 1999)

In Polynesian populations serum lipids tend to be lower than for Caucasians in spite of higher BMIs (Bell et al, NZ Med J 2001, Scragg et al, NZ Med J 1993)

However:– direct comparisons are needed in a variety of

ethnic groups

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Objective To determine whether there are ethnic differences

in the association between BMI and hypertension in men and women aged 30 - 65 years

Ethnic groups 3,423 Chinese men and women (CHNS 1997) 1,929 Filipino women (CLHNS 1998) 7,957 non-Hispanic Whites, non-Hispanic Blacks,

Mexican Americans (NHANES III 1988 - 1994)

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Methodology Pooled cross-sectional data from three surveys

Outcome = Hypertension SBP 140 mm Hg , DBP 90 mm Hg, or on

anti-hypertension medication Including those on medication biased the

result towards the null or had no effect (see following figure)

Main explanatory variable = BMIConfounders = Age. Physical activity, smoking and

alcohol consumption were not major confounders. (see following figure)

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

We included pre-diagnoased individuals to boost cell size and because their inclusion biased the results towards the null

0

10

20

30

40

50

60

18.5 - 22.9 23 - 24.9 25 - 26.9 27 - 28.9 29 - 30.9 > = 31.0

BMI (men)

HT

N p

reva

lenc

e (%

)

Chinese new HTNChinese pre HTNWhite new HTNWhite pre HTN

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Physical activity, smoking status and alcohol consumption had a minimal effect on the association between hypertension and BMI in all ethnic groups: eg NHBlack women

0

0,5

1

1,5

2

2,5

18.5-22.9 23.0-24.9 25.0-26.9 27.0-28.9 29.0-30.9 >=31.0

BMI

Odd

s ra

tio

AgeadjustedFullyadjusted

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Compared to US ethnic groups, Chinese men & women were less hypertensive; Filipino women had similar levels of hypertension to NHWhites

1916

28

22

34

2421

23

35

0

10

20

30

40

50

60

Men Women

HTN

pre

vale

nce

(%) Chinese White Black MexicanAm Filipino

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Compared to US ethnic groups, Chinese men & women & Filipino women were less overweight (BMI 25 kg/m2)

1621

66

54

7374 74

35

60

0

10

20

30

40

50

60

70

80

Men Women

Pre

vale

nce

(%)

Chinese White Black MexicanAm Filipino

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Chinese men had higher odds of prevalent hypertension, adjusted for age, than NH-Whites at every category of BMI, including 23-24.9 kg/m2

01234

5678

18.5-22.9 23-24.9 25-26.9 27-28.9 29-30.9 >=31.0

BMI

Odd

s of

Hyp

erte

nsio

n Chinese

White

Black

MexicanAm

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Including waist circumference attenuated the association for both Chinese & NHWhite men but the ethnic differences remained

012345678

18.5 - 22.9 23 - 24.9 25 - 26.9 27 - 28.9 29 - 30.9 > = 31.0

BMI

Odd

s ra

tio

White w/o waist

White with waist

Chinese w/o waist

Chinese with waist

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

The age-adjusted odds of prevalent hypertension for Chinese and Filipino women were similar to those for NH-Whites at low levels of BMI

0

2

4

6

8

10

18.5-22.9 23-24.9 25-26.9 27-28.9 29-30.9 >=31.0

BMI

Odd

s of

Hyp

erte

nsio

n

Chinese

WhiteBlack

MexicanAmFilipino

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Two problems can arise when using odds ratios in this context

Odds ratios are dependent on a reasonable number of subjects in the reference category

Interpretation can be misleading because the analysis assumes that the underlying risk (or in this case prevalence) between the ethnic groups is the same

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Subject numbers were sufficient but hypertension prevalence differed markedly by ethnic group in the referent BMI category (BMI 18.5-22.9 kg/m2) Men Women

Chinese, n(%) 936(12.3) 976(9.0)

Non-Hispanic White, n(%) 288(16.5) 466(6.5)

Non-Hispanic Black, n(%) 244(21.2) 208(21.7)

Mexican American, n(%) 129(9.3) 134(6.3)

Filipino, n(%) - 743(16.0)

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

To overcome this problem, we used prevalence difference figures. A steeper slope was observed at low levels of BMI for Chinese men (10.8% ) compared to NHWhite men (1.8% )

0

10

20

30

40

50

60

70

18.5-22.9 23-24.9 25-26.9 27-28.9 29-30.9 >=31.0

BMI

HTN

Pre

vela

nce

(%)

Chinese

White

Black

MexicanAm

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

There was also some evidence of a steeper slope at low levels of BMI for Chinese women (7.6% ) compared to NHWhite women (4.3% ). Filipino women showed a 10.3% between the categories 23.0-24.9 and 25.0-26.9 kg/m2

0

10

20

30

40

50

60

18.5-22.9 23-24.9 25-26.9 27-28.9 29-30.9 >=31.0BMI

HTN

Pre

vela

nce

(%)

ChineseWhiteBlackMexicanAmFilipino

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Asia USA

Underweight < 18.5 < 18.5

Healthy weight 18.5 - 22.9 18.5 – 24.9

Overweight 23.0 - 24.9 25.0 – 29.9

Obese 25 30

Current WHO weight status recommendations for Asia and the USA

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

Should lower definitions of overweight and obesity be used for Asian populations?

We have shown some evidence that the association between hypertension and BMI may be stronger in Chinese compared to NHWhites

There was no evidence of a stronger association for Filipino women, however, a higher baseline prevalence may justify a lower cut-off

To fully justify lower cut-offs we need longitudinal studies, data on all co-morbidities, consensus on appropriate methodology and more specific definitions of ethnicity

The Nutrition Transition ProgramThe University of North Carolina at Chapel Hill

The value of ethnic-specific BMI cut-offs?At the clinical level: In countries such as the USA, with considerable

ethnic diversity, physicians would be better able to identify individuals at risk of obesity related co-morbidities

At an international level: At this level, the utility of a weight classification

system is in the ability to compare populations and monitor changes overtime & therefore there is no advantage in having ethnic-specific cut-offs