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Ethnicity and body fatness in New Zealanders
Elizabeth Duncan, Grant Schofield, Scott Duncan, Gregory
Kolt, Elaine Rush
The obesity epidemicThe prevalence of obesity has
rapidly reached epidemic proportions in both developed and developing countries
around the world. The World Health Organization (WHO) estimates that there are
now more than 300 million obese people worldwide—an increase of 100
million since 1995.1
This rising level of obesity has increased the incidence of
obesity-related morbidities, such as cardiovascular disease, Type 2 diabetes,
and hypertension, thus imposing a major burden on healthcare systems and
lowering the quality of life for those affected. Furthermore, it is predicted
that rates of obesity in the next 20 years could be as high as 45–50% in
the USA, and 30–40% in Australia and
England.2
New Zealand is no exception to these trends. National survey
results show an 88% increase in the number of obese adults since
1989,3 to a stage where more than half of all
adults are overweight and obesity-related illnesses cost the health care system
an estimated $303 million each year.4
Maori and Pacific Island (PI) adults appear particularly
susceptible, with obesity rates 1.9 and 2.5 times (respectively) higher than
that of New Zealand Europeans.5 Similar
patterns have been observed in younger New Zealanders. Results from the 2002
National Children’s Nutrition Survey indicated that Maori and PI children
were 3.0 and 5.3 times (respectively) more likely to be obese than children from
other ethnicities.7 Overall, 21.3% of New
Zealand children aged 5-14 years were classified as overweight, with a further
9.8% obese.
Body Mass Index as a measure of obesityObesity is defined as a condition of
excessive fat accumulation to the extent that health and wellbeing may be
impaired.8 In population research, body fatness
(BF) is most commonly estimated using body mass index (BMI), a simple
anthropometric measurement of weight (kg) divided by squared height
(m2), which tends to correlate well with both
percent body fatness (%BF) and
health-risk.9–13 Although more accurate
techniques are available; such as four compartment models that measure bone
mineral content, body water, and body density independently; BMI remains the
most cost-effective and practical tool in studies of this type.
In 1998, the WHO provided international BMI standards for
classifying overweight and obesity in adults based on the risk of
obesity-related disease for Europeans at each BMI
category.8 Overweight was defined as BMI
≥25 kg.m-2 and obesity as BMI
≥30kg.m-2, with the latter corresponding
to approximately 25% and 35% BF in young European men and women,
respectively.14 An obvious limitation of this
measure is its inability to distinguish between fat and fat-free mass. As such,
standard BMI cut-offs for overweight and obesity may not represent the same
levels of BF in populations that differ significantly from the typical European
phenotype.
For young people, different BMI standards are required. The
US Centers for Disease Control and Prevention issued age- and sex-specific BMI
charts for defining overweight and obesity in those aged 2 to 20 years based on
the 85th and 95th percentile of an American reference
population.15 Alternative thresholds have been
provided by the International Obesity Task Force (IOTF) using the mean of the
BMI-age curves from six major countries.16 At a
given age, individuals are classified as overweight or obese if they have a BMI
greater than the mean BMI-age curve that passes through 25
kg.m-2 or 30
kg.m-2 (respectively) at age 18 years. The
intention of these IOTF cut-offs was to establish a higher degree of
international applicability, although the averaging of the six diverse datasets
could be considered arbitrary.
Obesity and ethnicityThe WHO BMI thresholds for
overweight and obesity are widely used in field research; however, their
relevance to all populations is questionable. It is generally accepted that
associations between BMI and BF are dependent on age and gender. More recently,
these associations have been shown to vary with ethnicity. For example, Pacific
Islanders tend to have lower levels of BF than Europeans at a given
BMI.17–19 Conversely, many Asian ethnic
groups have higher levels of BF than Europeans at specific BMI, thus putting
them at greater risk of obesity-related disease at relatively low BMI
scores.20–22
Even at the same level of BF, risk profiles may differ
between ethnic groups.23 This may be explained
by ethnic-specific variation in the patterns of fat distribution. Indeed,
central fat accumulation (ie, an android fat pattern) appears to be a greater
predictor of obesity-related health risks than overall
fatness.24,25
Research indicates that, in general, Asian adults are more
prone to visceral and central obesity than
Europeans.26,27 In particular, Hughes et
al28 found that Asian Indians had a greater
predisposition for central obesity than Malay and Chinese Asians. Likewise,
there is evidence that Asian children29 and
adolescents30 have a greater central fat mass
when compared with Europeans and other ethnic groups. In accordance with a
higher %BF at a given body size, and a more centralised pattern of fat
distribution, elevated disease risks have been observed in Asian populations at
BMI scores well below the WHO thresholds defining overweight and
obesity.
In response, the WHO released provisional recommendations
that overweight and obese BMI cut-off points for Asian populations in the
Asia-Pacific region be reduced to ≥23, and ≥25
kg.m-2,
respectively.31 Although a good starting point,
these guidelines do not take into consideration variance among different Asian
populations.
More recently, a WHO expert consultation on BMI in Asian
populations concluded that there is no single cut-off point appropriate for
defining overweight or obesity in all Asian
groups.22 Recommendations from the consultation
include: (1) retaining current WHO BMI cut-off points for international
classification; (2) adding ‘action points’ of ≥23 and
≥27.5 kg.m-2 (representing
‘increased’ and ‘high’ risk) as a trigger for public
health action; (3) developing ethnic- and country-specific BMI action points;
and (4) refining BMI action points with waist circumference in populations
predisposed to central obesity.
Obesity in young peopleCompared with adults, less is known
about the body composition of children and adolescents. There is, however,
evidence that BMI is not an equivalent measure of BF among young people from
different ethnic groups. At a given BMI,
Chinese32 and
Hispanic33,34 youth have a higher level of BF
than Europeans, who in turn have more BF than
African-Americans,35 and Maori and Pacific
Islanders.36 These disparities may evolve, or
at least increase, during puberty. For example, Ellis et
al37 observed that ethnic differences in body
composition between Hispanics, African-Americans, and Europeans were much less
pronounced in children younger than 8 years of age (pre-puberty).
Sexual maturation processes (that occur during puberty)
affect body composition, and can alter the associations between BMI and fat
mass.35,38–40 Thus, differences in body
composition observed during childhood and adolescence may, in part, reflect
ethnic-specific growth and development
patterns.41 In a 6-year follow-up study of
Chinese children, Wang et al42 noted that
overweight prevalence (defined according to IOTF age- and sex-specific BMI
cut-off points) decreased as children became adolescents. This apparent
reduction in overweight may be due to different BMI-age relationships between
the study and the IOTF reference populations.
Although the authors did not determine pubertal stage, they
suggested that Chinese adolescents tend to mature later than the IOTF reference
populations, thereby causing them to be misclassified. Consequently, the IOTF
cut-offs may not be appropriate for these populations. For future studies,
consideration of sexual maturation may be beneficial.
Explaining ethnic-specific relations between Body Mass Index and percentage of body fatnessSeveral factors have been proposed
to help explain the dependency of the BMI/%BF relation on ethnicity. First, body
build/frame size (as measured by wrist and knee girths) tends to vary among
different ethnic groups. A number of studies have noted that ethnic populations
with relatively high levels of BF at a given BMI also have a more slender
build.43,44 Furthermore, Deurenberg et
al45 found that correcting for body build
eliminated most of the ethnic-specific differences associated with %BF
prediction equations for bioelectrical impedance analysis (BIA) in Chinese,
Malay, and Indian Singaporeans.
In contrast, earlier research concluded that the prediction
of %BF from BMI was only slightly improved by the inclusion of body build
parameters.46,47 It is possible that the
effects of body build were not observed in these studies due to low inter-group
and/or high intra-group
variability.44
A second factor that may contribute to ethnic-specific
relationships between BMI and %BF is variation in sitting height relative to
total height. Individuals with long legs (low sitting height) generally have a
lower BMI and, as such, %BF may be underestimated from
BMI.48 Relative sitting height tends to be
higher in Asian ethnic groups, although the effects on BMI are
inconclusive—most likely due to the large intra-group variation in this
parameter.43,44
Given that differences in body build may explain a large
proportion of the ethnic variation in relationships between BMI and %BF, frame
size represents an alternative criterion to ethnicity on which to base BMI
cut-offs. As ethnicity is self-identified, individuals may affiliate with an
ethnic group with which they have no genetic relation. As such, classification
according to body build (rather than ethnic group) may help control for
inaccuracies when defining ethnicity. However, it is unlikely that collecting
data on frame size will be practical in population studies.
Finally, there may be differences in physical activity level
among ethnic groups. More active individuals are likely to have a higher
proportion of muscle mass, and therefore the potential for overestimation of %BF
from BMI.44 Such a tendency may only be
observable in athletes performing high levels of activity. Nevertheless, future
studies should include anthropometric measures of body build and physical
activity levels in order to increase our understanding of differences in the
BMI/%BF relationship among ethnic groups.
New Zealand’s issues regarding body fatness and ethnicityNew Zealand has an ethnically
diverse population comprising mainly New Zealand Europeans (80.0%), Maori
(14.7%), Asians (6.6%), and Pacific Islanders
(6.5%).49 Despite this diversity, ethnic
variation in BF and other body composition variables has yet to be investigated
in all major ethnic groups.
However, researchers have compared ethnic differences in BMI
and %BF among Maori, PI, and European populations. Several studies have found
that Maori and PI adults tend to be leaner (ie, have a lower %BF, and higher
fat-free mass) than New Zealand Europeans of the same body
size.17-19
Similar results have been observed in children. Rush et
al36 noted that Maori and PI girls have (on
average) 3.7% less BF than New Zealand European girls of the same body size.
Furthermore, a related study by Tyrell et al6
found a small, but statistically significant, difference in the relationship
between BMI and %BF in New Zealand European, Maori, and PI schoolchildren aged
5–10 years; although they suggested that the effects of ethnicity were not
clinically relevant.
Even though Maori and Pacific Islanders tend to have a
higher proportion of lean mass to fat mass than New Zealand Europeans at a given
BMI, as a population they maintain a greater absolute fat mass. Indeed, when
higher BMI thresholds are applied to Maori and PI peoples to counteract the high
lean-to-fat mass ratio (26 kg.m-2 and 32
kg.m-2 for ‘overweight’ and
‘obesity’, respectively), these two groups remain twice as likely to
be obese than the ‘European and Other’
group.5 Not surprisingly, Maori and PI
populations also have a much higher prevalence of type 2 diabetes when compared
to Europeans.50 However, it is noteworthy that
the prevalence of type 2 diabetes among New Zealand Indians exceeds that seen in
Maori and Pacific Islanders.
The high prevalence of diabetes among Indians is in line
with the elevated levels of BF at a given body size seen among Asian populations
overseas. This is an issue of increasing importance to New Zealand given that
Asian people make up the fastest growing ethnic group, more than doubling in
number between 1991 and 2001.49 Furthermore,
Asians are projected to account for 13% of New Zealand’s population by
2021.
In spite of their population growth, Asian ethnic groups
have been largely neglected by New Zealand health and research policies. For
example, only Maori and PI children were over-sampled in the 2002 National
Children’s Nutrition Survey. In addition, Maori and PI children were
analysed separately, whereas children of Asian decent were grouped with New
Zealand Europeans. This is a common theme in national surveys by government
organisations; such as the Ministry of Health, and Sport and Recreation New
Zealand. In order to understand the public health needs of Asian populations in
New Zealand, and to tailor preventative health strategies, it is vital that
future surveys distinguish between these ethnic groups.
At present, standard BMI thresholds for
‘overweight’ and ‘obesity’ are applied to Asian
populations as there are no robust New Zealand data available on the
relationship between BMI and body composition variables in this ethnic group.
Consequently, Asian groups at risk for health complications (accompanying their
overweight and obesity conditions) may not be targeted in interventions to
prevent/treat obesity. The only evidence available is from a study by Tyrell et
al6 that included two small groups of Asian
children in their investigation of the relationship between BMI and body
composition in Maori, Pacific Islanders, and New Zealand Europeans. Although
results were not presented, the authors commented that Asian Indian children
tended to have a higher %BF at a given BMI compared with New Zealand Europeans.
However, caution must be taken when interpreting this statement given the small
sample size and the fact that %BF was estimated from bioelectrical impedance
analysis using a prediction equation that was not specifically developed for
Asian Indian children.
The recommendations put forward by the recent WHO expert
consultation22 offer promise for the
classification of overweight and obesity in New Zealand’s multiethnic
society. For clinicians assessing the health status of individuals, BMI
thresholds should be refined by consideration of ethnicity and other risk
factors such as waist circumference. At a population level, implementation of
additional BMI action points will better reflect the continuum of BF and
associated health risk. However, valid comparisons with overseas statistics will
only be possible if the criteria used to define overweight and obesity are
consistent. In these instances, the retention of the standard WHO cut-offs (25
and 30 kg.m-2) is advisable. Ultimately,
national BMI action points should be developed for all major ethnic groups in
New Zealand based on large-scale population studies of BMI, BF, and health
risk.
ConclusionsAccurate assessment of overweight
and obesity is vital to assist public health organisations in identifying
at-risk groups and to facilitate development of appropriate preventative
strategies. At a population level, BF is most commonly assessed using BMI.
Although WHO established universal BMI standards for defining overweight and
obesity, studies have shown that these BMI thresholds do not provide an
equivalent measure of BF and associated health risk across different ethnic
groups. Consequently, WHO recently recommended the use of additional BMI
cut-offs as public health action points, such as ethnic- and country-specific
BMI cut-offs for overweight and obesity.
In New Zealand, knowledge of the ethnic variation in BF and
other body composition variables is restricted to New Zealand European, Maori,
and PI ethnic groups. New Zealand Asians are of particular interest because of
their rapid population growth, and the lack of published data on their BMI/%BF
relationships. Furthermore, compared with Europeans, Asians from other countries
show elevated levels of BF and greater morbidity and mortality at a given
BMI.
In conclusion, large-scale studies are needed to determine
the relations between BMI, %BF, BF distribution, and health risk across all
major ethnic groups in New Zealand. For young people, these studies should also
consider maturational stage. Resulting data will enable development of
ethnic-specific BMI thresholds for overweight and obesity—however this is
only a starting point.
There is also a clear lack of knowledge concerning ethnic
variation in other areas, such as physical activity and diet. An understanding
of these issues is imperative for tailoring preventative interventions that will
counteract the burgeoning epidemic of obesity in New Zealand.
Author information:
Elizabeth K. Duncan, Doctoral Student, Division of Sport and Recreation, Faculty
of Health, Auckland University of Technology, Auckland; Grant Schofield,
Lecturer, Division of Sport and Recreation, Faculty of Health, Auckland
University of Technology, Auckland; Scott Duncan, Doctoral Student, Division of
Sport and Recreation, Faculty of Health, Auckland University of Technology,
Auckland; Gregory Kolt, Professor, Faculty of Health, Auckland University of
Technology, Auckland; Elaine Rush, Associate Professor, Division of Sport and
Recreation, Faculty of Health, Auckland University of Technology,
Auckland.
Acknowledgement:
Elizabeth K Duncan acknowledges the support of the Foundation for Research,
Science and Technology through the Top Achiever Doctoral Scholarship.
Correspondence:
Elizabeth K Duncan, Division of Sport and Recreation, Faculty of Health,
Auckland University of Technology, Private Bag 92006, Auckland. Fax: (09) 917
9746; email: elizabeth.duncan@aut.ac.nz
References:
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