International Journal of Obesity (1999) 23, 272±277
ß 1999 Stockton Press All rights reserved 0307±0565/99 $12.00
http://www.stockton-press.co.uk/ijo
School performance and weight status of
children and young adolescents in a transitional
society in Thailand
L Mo-suwan*1, L Lebel2, A Puetpaiboon1 and C Junjana1
1
Department of Pediatrics, Faculty of Medicine, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand and 2Department of
Epidemiology Unit, Faculty of Medicine, Prince of Songkla University, Hat Yai, Songkhla 90110, Thailand
OBJECTIVE: To study the association between current or previous weight status and school performance among
children and young adolescents of Hat Yai municipality, southern Thailand.
DESIGN: Cross-sectional and longitudinal study
SETTING: Primary and secondary schools of Hat Yai municipality, southern Thailand.
SUBJECTS: 1207 grades 3 ± 6 and 587 grades 7 ± 9 students.
MEASUREMENTS: Body mass index (BMI, kg=m2) calculated from weight and height measurement of subjects in 1992
and 1994; parental education level and occupation, and monthly income, by questionnaire performed in 1992; gradepoint-average (GPA) and grades of mathematics and Thai language from the school records of ®nal examinations in
1994.
RESULTS: Overweight subjects (BMI value > 85th percentile of the NHANES-I data for age and gender) in grades 7 ± 9
had a mean GPA 0.20 point (95% con®dence internal (CI) ˆ 0.04, 0.37) lower than that of the normal weight children
after controlling for gender, age, school and grade. They were twice more likely to have low grades (lower than 2 on
the scales of 0 ± 4) of mathematics and Thai language than normal weight children. There were no associations
between GPA or individual subject grades and previous BMI status in 1992. Children in grades 7 ± 9 who became
overweight over the two years, had a mean GPA of 0.48 point lower than those who did not become overweight (95%
CI ˆ 0.12, 0.84). In grades 3 ± 6 subjects, however, becoming overweight had no effect on GPA and individual subject
scores.
CONCLUSIONS: Our study showed that being overweight and becoming overweight during adolescence (grades 7 ± 9)
was associated with poor school performance, whereas such an association did not exist in children (grades 3 ± 6).
Keywords: child; adolescent; overweight; school performance; transitional society
Introduction
Obesity in children is an increasingly important nutritional disorder in developing countries.1,2 Persistence
of obesity into adolescence results in increasing risk
of adult obesity.3 Apart from health risks, obesity has
been shown to be associated with social and economic
disadvantages. In one study in the US, obese adolescents had poor high school performance,4 possibly
affecting their acceptance into colleges.5 They also
had fewer years of schooling than did their non-obese
peers.6 Later in adulthood, women who had been
overweight during adolescence had lower incomes
when compared with their non-obese counterparts,
independent of their baseline social status and aptitude-test scores.6,7
Correspondence: Dr Ladda Mo-suwan, Department of Pediatrics,
Faculty of Medicine, Prince of Songkla University, Hat Yai,
Songkhla 90110, Thailand.
Received 24 March 1998; revised 21 September 1998; accepted
19 October 1998
A review of the literature revealed an inverse
relationship between obesity and socioeconomic
status among women in industrialized societies,
whereas in the developing countries the relationship
was direct among men, women and children.8 In
Thailand, although in transition from an agronomybased to an industrial-based economy,9 the obesitysocioeconomic relation is like that of developing
countries. Obesity was more prevalent in the af¯uent
urban dwellers.10 From our previous study, children
from the high income families were three times more
likely to be obese than those from the low income
group.11 However, there is little data on the school
performance of obese children from developing countries. A report from China showed that the moderately
severe obese ( > 50% overweight) school-aged children had average IQ scores 11 points lower than that
of the non-obese controls.12
In this report, we examined the association between
weight status and school performance among school
children residing in an urban area in southern Thailand. We also explored the relation of school performance with the weight status measured two years
School performance and weight status
L Mo-suwan et al
earlier, as a stronger test of whether academic
achievement is a consequence of obesity.
Subjects and methods
Subjects
A cohort of 2252 primary schoolchildren (grades 1 ±
6) was recruited in 1991 and 1992 using two-stage
sampling. Six schools (two municipality-operated and
four private) were randomly selected from 13 primary
schools in the Hat Yai metropolitan area, southern
Thailand. Then, one or two classrooms of each grade
were randomly selected from each school. Baseline
demographic and family data including parental education, occupation and monthly income were obtained
by questionnaire completed by parents. Weight and
height were measured using a Detecto1 beam balance
and stadiometer (Detecto Scales Inc., Brooklyn, NY,
USA) to the nearest 0.1 kg and 0.5 cm, respectively.
Anthropometric data were collected on an annual
basis (in January) thereafter. Parental consent was
obtained and the study was approved by the Committee for Research in Humans, Faculty of Medicine,
Prince of Songkla University.
School performance
Compulsory education in Thailand is for six years.
Children usually enter school at the age of 7 y old and
leave primary school at the age of 13 y. The academic
year begins in May and ends in March with a mid-year
break in October. A grade system is used to score
students' performance. Grades of a learning subject,
in an ordinal scale of 0 ± 4, are obtained mainly from
written examinations. Teacher's subjective assessment of students' performance accounts for approximately 10% of total scores. Grade-point-average
(GPA), a continuous value of 0 ± 4, which re¯ects
overall school performance, is calculated from grades
of all subjects learned in that academic year. We
obtained GPAs of our subjects from the school records
at the end of March 1994. Grades of individual
learning subject were available only for the secondary
school (grades 7 ± 9) children. We used grades of
mathematics and Thai language to explore different
areas of learning ability in this group of children.
Socioeconomic variables
We used the baseline socioeconomic data collected in
1992 and assumed that there were no or relatively
minimal changes of grouping in the two years. Parental education was classi®ed as illiterate, primary,
secondary and university level and occupation as
casual worker, farmer, trader, government service
and of®ce worker. Parental income variable was
categorized into four classes by using monthly
income cut at < 5000, 5 ± < 10 000, 10 000 ±
< 30 000 and 30 000 baht (1 baht ˆ 0.04 US
dollar at the time of questionnaire).
Statistical analysis
Association of school performance in 1994 with
current (1994) and previous (1992) weight status
was examined. Since primary and secondary schoolers
may have a different school performance ± weight
status relation, we did a separate analysis of grades
3 ± 6 and grades 7 ± 9 subjects. We used ANOVA and
chi-square tests to describe and compare differences
of variables. We used body mass index (BMI, the ratio
of weight in kilograms (kg) divided by height squared
in meters (m2)) to categorize weight status. Subjects
were classi®ed as underweight, normal or overweight,
if their BMIs' were < 15th percentile ( < P15), P15P85, > P85 of the US First National Health and Nutrition Examination Survey (NHANES I) data for age
and gender.13
To examine the association of weight status with
GPA, we computed multiple linear regression
adjusted for gender, age, parental and family factors.
Only factors with signi®cant partial F-test were
retained in the model. Since standardized scores are
not available for comparison across schools, computation of association was also adjusted for schools and
grades. For the grades 7 ± 9 group, to analyze the
grades for individual learning subject, dichotomous
outcomes were created with scores less than 2 classi®ed as a low grade in that subject. We used logistic
regression to examine the relationship between weight
status and examination grades of each subject, also
adjusted for the aforementioned confounders.
Changes of BMI status over the two years from
1992 ± 1994 was categorized into four groups:
stable-non-overweight, thinner (move from overweight to non-overweight), stable-overweight and
becoming overweight (move from non-overweight to
overweight). Multiple linear regression and logistic
regression analysis were then performed accordingly
to investigate whether the change in BMI status has
any association with academic achievement.
All analyses were performed using statistical software package STATA version 5.14
Results
Of 2252 subjects, follow-up anthropometric measurements were completed for 2003 children in 1994.
Schools could provide GPA of 1794 children
(89.6%), 1207 grades 3 ± 6 and 587 grades 7 ± 9
students. Since subjects were recruited over two
school years (1991=1992), the former grades 1 ± 4
became grades 3 ± 6 and the former grades 4 ± 6
became grades 7 ± 9 in 1994. A number of former
grades 4 ± 6 subjects moved to other secondary
schools, the total number of schools was ®nally 12.
273
School performance and weight status
L Mo-suwan et al
274
We included only schools with more than 10 students
in the analysis, resulting in dropping 30 subjects. The
®nal numbers of subjects were 1193 students in grades
3 ± 6 from 6 schools and 571 students in grades 7 ± 9
from 7 schools.
Table 1 shows the mean GPA of grades 3 ± 6 and
grades 7 ± 9 subjects by baseline (1992) family data.
Several of these variables were associated. Parental
occupation was closely associated with education.
Likewise, mother's education and occupation were
strongly associated with those of the father. Monthly
income was also correlated with both parental education and occupation. All of these associations among
variables were statistically signi®cant at the P < 0.001
level using chi-square tests.
BMI status and GPA are shown in Table 2. The
current prevalence of overweight using the 85th percentile cut-off of the NHANES-I BMI values for age
and gender13 of grades 3 ± 6 and 7 ± 9 was 20.9% and
15.2%, respectively, and differed signi®cantly (w2
(2) ˆ 48, P < 0.001). The percentage of grades 3 ± 6
subjects becoming overweight was twice that of the
grades 7 ± 9 subjects (w2 (3) ˆ 182, P < 0.001). For
overall school performance, there were no demonstrable difference of mean GPA among the students in
grades 3 ± 6 with different BMI status, whereas the
overweight grades 7 ± 9 group had an average GPA
0.23 point lower than that of the normal weight and
underweight groups (F(2,568) ˆ 1.13, P ˆ 0.03).
For the grades 3 ± 6 group, a regression analysis
showed that there was no signi®cant association of
Table 1
GPA with current BMI status, the adjusted regression
coef®cients and 95% con®dence intervals (95% CI)
were 7 0.03 ( 7 0.13,0.06) and 0.06 ( 7 0.05,0.16)
for underweight and overweight groups, respectively.
The association with the previous (1992) BMI status
was signi®cant for the overweight group, regression
coef®cients were 7 0.03 (95% CI ˆ 7 0.14, 0.07)
Table 2
Body mass index (BMI) statusa and grade-pointaverage (GPA)b of students in grades 3 ± 6 and 7 ± 9 groups
Gender ˆ male, n(%)
1994 Age (y), mean (s.d.)
Current (1994) BMI status, n (%)
Underweight
Normal
Overweight
Previous (1992) BMI status, n (%)
Underweight
Normal
Overweight
Change of BMI category
over two years, n (%)
Stable-non overweight
Thinner
Stable-overweight
Becoming overweight
1994 GPA of subjects by 1994
BMI category, mean (s.d.)
Underweight
Normal
Overweight
Grades 7 ^ 9
556 (46.6)
10.7 1.1
247 (43.3)
13.8 0.9
275 (23.1)
668 (56)
250 (20.9)
69 (12.1)
416 (72.7)
87 (15.2)
265 (22.2)
724 (60.7)
204 (17.1)
123 (21.5)
350 (61.2)
99 (17.3)
919
24
180
70
456
28
71
16
(77.0)
(2.0)
(15.1)
(5.9)
3.06 0.76
3.11 0.75
3.15 0.71
(79.9)
(4.9)
(12.4)
(2.8)
2.60 0.78
2.60 0.77
2.37 0.71
a
BMI status categorized by using NHANES-I data for age and
gender: < P15 ˆ underweight, P15-P85 ˆ normal, > P85 ˆ overweight; bGPA in a continuous scale of 0 ± 4.
Mean grade-point-average (GPA) by baseline (1992) family data of subjects in grades 3 ± 6 and grades 7 ± 9 groups
Grades 3 ^ 6
Variables
Fathers' occupation
No
Casual
Farmer
Trader
Government of®cial
Of®ce worker
Mothers' occupation
No
Casual
Farmer
Trader
Government of®cial
Of®ce worker
Fathers' education
No
Primary
Secondary
Higher
Mothers' education
No
Primary
Secondary
Higher
Parental monthly incomec
< 5000 baht
5 ± < 10 000 baht
1 ± < 30 000 baht
30 000 baht
a
Grades 3 ^ 6
a
n (%)
Mean GPA (s.d.)
4 (0.4)
227 (23.4)
22 (2.3)
387 (39.9)
258 (26.6)
71 (7.3)
2.85 1.11
3.05 0.72
2.70 0.67
3.24 0.69
3.16 0.77
3.23 0.75
185
160
20
412
188
41
(18.4)
(15.9)
(1.9)
(40.9)
(18.7)
(4.1)
3.15 0.73
2.95 0.76
2.81 0.85
3.16 0.73
3.32 0.68
3.32 0.68
19
218
296
438
(1.9)
(22.4)
(30.5)
(45.1)
2.09 0.74
3.04 0.71
3.13 0.73
3.24 0.74
35
378
197
396
(3.5)
(37.6)
(19.6)
(39.4)
3.03 0.76
3.01 0.76
3.17 0.69
3.29 0.69
187
373
352
112
(18.3)
(36.4)
(34.4)
(10.9)
2.97 0.72
3.06 0.74
3.30 0.69
3.29 0.67
Grades 7 ^ 9
b
P-value
0.0019
0.0000
0.0113
0.0000
0.0000
n (%)
Mean GPA (s.d.)
2
90
11
224
92
38
(0.4)
(19.7)
(2.4)
(49.6)
(20.1)
(8.3)
2.31 0.49
2.39 0.69
2.48 0.64
2.70 0.73
2.61 0.73
2.56 0.80
86
58
10
213
86
15
(18.4)
(12.4)
(2.1)
(45.5)
(18.4)
(3.2)
2.58 0.74
2.29 0.72
2.02 0.47
2.63 0.76
2.70 0.72
2.81 0.83
13
120
158
167
(2.8)
(26.2)
(34.5)
(36.5)
2.78 0.63
2.46 0.73
2.58 0.75
2.70 0.76
24
182
101
163
(5.1)
(38.7)
(21.5)
(34.7)
2.59 0.66
2.45 0.73
2.66 0.78
2.71 0.77
63
157
188
64
(13.3)
(33.3)
(39.8)
(13.6)
2.31 0.68
2.54 0.77
2.64 0.79
2.89 0.69
P-valueb
0.0201
0.0003
0.0431
0.0130
0.0006
Numbers of children in each variable with non missing values; bChi-square test; c1 baht ˆ 0.04 US dollar at the time of study.
School performance and weight status
L Mo-suwan et al
and 0.12 (95% CI ˆ 0.005, 0.24) for underweight and
overweight groups, respectively. However, the marginal association of GPA with the overweight was no
longer signi®cant after adjustment, regression coef®cient ˆ 0.09 (95% CI ˆ 7 0.01,0.19).
Changes in BMI status had no signi®cant effect on
GPA for the grades 3 ± 6 group, adjusted regression
coef®cient with the non-overweight as the reference
level ˆ 0.03 with 95% CI between 7 0.14 and 0.19
for the becoming overweight group, 0.09 (95%
CI ˆ 7 0.02, 0.19) for the stable overweight, and
0.15 (95% CI ˆ 7 0.13, 0.43) for the thinner group.
Table 3 shows the results of analysis for the
association of GPA with BMI status and change of
BMI category over the past two years of the grades
7 ± 9 group. It demonstrated that the overweight children had on average a GPA 0.24 (95% CI ˆ 0.06,
0.41) point lower than that of the normal weight
children when no other possible confounding factors
were taken into account. After adjustment for gender,
age group, school and grade, the relationship with
obesity was still signi®cant and only slightly changed,
suggesting there was little confounding by these other
variables. The adjusted mean GPA of overweight
children was 0.20 point lower than normal weight
children (95% CI ˆ 0.04, 0.37). However, all of these
factors in the adjusted model explained approximately
one ®fth of the variability of GPA (r2 of the adjusted
model ˆ 0.18). Boys had a mean GPA 0.30 point
lower than girls (95% CI ˆ 0.15,0.46). In particular,
we could not detect any association between GPA and
the previous (1992) BMI status, the adjusted regression coef®cients with the normal weight group as the
reference were 7 0.02 (95% CI ˆ 7 0.17,0.14) and
7 0.09 (95% CI ˆ 7 0.26,0.08) for the underweight
and overweight groups, respectively.
The mean GPA of children in grades 7 ± 9 shows a
signi®cant association with the changes of weight
status over the past two years. After adjustment for
confounders, children who became overweight had a
mean GPA of 0.48 point lower than those who
remained non-overweight.
Overweight students in grades 7 ± 9 had a greater
percentage with low grades of Thai language and
mathematics than the non-overweight subjects
(Table 4). A crude analysis shows that overweight
children were twice more likely to have low mathematics and Thai grades than normal weight children.
Using logistic regression analysis, the associations
still remain signi®cant after adjustment with variables
similar to the GPA analysis. Males were about six
times more likely than females to have low Thai
grades (odds ratio (OR) ˆ 5.84, 95% CI ˆ 2.84,
11.99), but performance on mathematics was not
different between genders (OR ˆ 0.85, 95%
CI ˆ 0.49, 1.45). No association between individual
subject grades and previous BMI status in 1992 could
be detected. Likewise, there was no association
between individual subject scores and changes of
BMI categories over the two years.
Discussion
Our study shows that in young adolescence (grades
7 ± 9), school performance as measured by GPA was
associated with the current BMI status. Overweight in
this group was related to a low mean GPA and
signi®cant risks of having low Thai language and
mathematics scores. An upward change in BMI
status was associated with a greater risk of having a
low mean GPA. Such effects of overweight on GPA
scores were not seen in the younger children (grades
3 ± 6).
The inverse relationship of obesity and being overweight, with socioeconomic indices, including academic achievement, observed in several studies,4 ± 8,15
could be viewed as either causal or consequent. To
count low socioeconomic status (SES) as a cost of
obesity, requires that low status be a consequence of
obesity that would not occur if obesity were prevented.16 Follow-up studies of adolescent and young
adult cohorts for 7 ± 12.5 y6,7,15 suggest that this may
be the case. In these studies the association between
baseline obesity and subsequent socioeconomic attainment was controlled for other possible determinants of
Table 3 Association of grade-point-average (GPA) with 1994 body mass index (BMI) status and changes of
BMI status over the two years of the grades 7 ± 9 group by multiple linear regression analysis
1994 Mean GPA
1994 BMI statusa
Underweight
Normal
Overweight
Change of BMI status over two yearsb
Thinner
Stable-non overweight
Stable-overweight
Becoming overweight
2.60 0.78
2.60 0.77
2.37 0.71
2.68 0.74
2.59 0.77
2.43 0.69
2.11 0.73
Regression coefficient
(95% CI)
P
0.06 ( 7 0.12, 0.25)
NS
7 0.20 ( 7 0.04, 7 0.37)
0.017
0.14 ( 7 0.12, 0.41)
NS
7 0.14 ( 7 0.32, 0.04)
7 0.48 ( 7 0.12, 7 0.84)
NS
0.008
95% CI ˆ 95% con®dence interval; NS ˆ non-signi®cant; anormal weight group ˆ reference level, regression
analysis adjusted for age, gender, school, and grade; bstable-non overweight group ˆ reference level,
regression analysis adjusted for age, gender, school, and grade.
275
School performance and weight status
L Mo-suwan et al
276
Table 4 Association of low Thai language scores and low mathematics scores with 1994 body mass index
(BMI) status and changes of BMI category over the two years, for the grades 7 ± 9 subjects by logistic
regression analysis
Subjects with low grades
n/total(%)
Odds ratio
(95% CI)
10/58 (17.2)
46/284 (16.2)
19/68 (27.9)
1.09 (0.48, 2.45)
1
1.98 (1.02, 3.85)
15/45 (33.3)
73/241 (30.3)
28/61 (45.9)
1.11 (0.53, 2.32)
1
2.08 (1.13, 3.83)
3/17
53/269
14/38
5/11
(15.0)
(16.5)
(26.9)
(31.3)
0.69 (0.18,2.64)
1
1.67 (0.81,3.45)
3.07 (0.93,10.15)
NS
3/17
85/269
20/45
8/16
(17.7)
(31.6)
(44.4)
(50.0)
0.37 (0.09,1.36)
1
1.84 (0.94,3.62)
2.35 (0.82,6.75)
NS
Low Thai language grades
BMI statusa
Underweight
Normal
Overweight
Low mathematics grades
BMI statusb
Underweight
Normal
Overweight
Low Thai language grades
Change of BMI category over two yearsc
Thinner
Stable- non overweight
Stable- overweight
Becoming overweight
Low mathematics grades
Change of BMI category over two yearsd
Thinner
Stable-non overweight
Stable-overweight
Becoming overweight
P-value
NS
0.043
NS
0.018
NS
NS
NS
NS
NS ˆ non signi®cant; aNormal weight group ˆ reference level, regression analysis adjusted for age, gender,
school, and grade; bnormal weight group ˆ reference level, regression analysis adjusted for age, gender,
school, grade and father's education; cstable-non overweight group ˆ reference level, regression analysis
adjusted for age, gender, school, and grade; dstable-non overweight group ˆ reference level, regression
analysis was adjusted for age, gender, school, and grade.
SES such as, parental SES and test scores for intelligence. Our ®ndings of no association for poor academic performance with the previous BMI status,
however, do not support these studies. If obesity
results in stigmatization and social handicap17,18 causing poor school performance, our subjects who were
obese in 1992 should have been more likely to have
lower school grades than those who were not and the
formerly overweight subjects who did not change over
the two years of follow-up, not the newly overweight,
should have performed worst. It is possible that our
follow-up period of only two years is too short for the
adverse effect of obesity on the academic achievement
to be expressed. Or it re¯ects the different weight ±
school performance relation of the transitional society
of the developing countries.
In our study, BMI status and parental SES account
for 18% of the variability in GPA scores, suggesting
the existence of other factors. It is possible that
obesity and low school performance share some
causes. Probable common factors are heredity and
social environment. In¯uences of genetic factors on
obesity are well established.19,20,21,22 Genetic and
familial environmental in¯uences on educational
attainment has also been demonstrated through the
study of adoptees and their adoptive and biological
fathers.23 This study of Danish adoptees revealed that
SES of biological father in¯uenced the SES and years
of schooling of offspring with whom they had no
contact, probably through the inheritance of some
personal characteristics. A report from the Colorado
Adoption Project showed that cognitive ability and
academic achievement at the age of 7 y were correlated and genetic in¯uences accounted for 33 ± 64% of
these observed correlations.24 These genetic and
social environmental factors may be the main predictors of the association between overall school
performance and BMI status in our study.
Whether negative attitude and social discrimination
against fat persons affect grade assignment is also a
concern. Since subjective assessment accounted for
only 10% of the ®nal score, it is less likely that
teachers' biases would contribute to poor performance
of the overweight subjects observed in our study. If
discrimination and stigmatization did exist, children
who remained overweight over the two years should
have been worst affected and obesity associated poor
school performance should have been demonstrated
for both grades 3 ± 6 and grades 7 ± 9 groups. On the
contrary, we only found an inverse relationship
between academic performance and overweight in
the grades 7 ± 9 students (aged 12 ± 18 y). This ®nding
con®rms the late expression, more strongly in adulthood, of this relationship found in other studies.6,7
Adolescents have been shown to use eating as a means
of coping with stress.25 Putting on weight, as well as
the poor academic performance, in our grades 7 ± 9
group could possibly be the consequence of social
environmental factors, for example, stress in school.
The grades 3 ± 6 students, being younger, probably
have other ways of dealing with the stress. Our
previous study (unpublished observations) has shown
that children younger than 10 y were less concerned
about obesity than older children.
School performance and weight status
L Mo-suwan et al
Conclusion
In a transitional society setting, we found that the
socioeconomic-obesity relation was typical of developing countries,11 whereas the school performanceobesity relation was similar to that of industrialized
countries. We have shown that being overweight
during adolescence (grades 7 ± 9, 12 ± 18 y) is associated with poor school performance. This unwanted
association may be persistent if our society adopts the
social value of slimness as it is now being highly
publicized in most commercial advertisements. Long
term follow-up of these subjects will clarify whether
obesity during this period, in a transition society like
Thailand, will have an unwanted social and economic
consequence or not.
Acknowledgements
We are grateful to the teachers of the studied schools
for their cooperation in this study. We would like to
thank Mrs Pranom Boonpan for her assistance in data
collection.
This study was supported by a grant from the
Songklanagarind Hospital Foundation.
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