|Year : 2013 | Volume
| Issue : 1 | Page : 5-9
Body mass index among school adolescents in Sokoto, North-Western Nigeria
Murtala M Ahmad1, Hamidu Ahmed1, Kareem Airede2
1 Department of Paediatrics,Usmanu Danfodiyo University Teaching Hospital, Sokoto, Nigeria
2 University of Abuja, Gwagwalada, FCT, Abuja, Nigeria
|Date of Web Publication||17-May-2013|
Murtala M Ahmad
Department of Paediatrics, UDUTH Sokoto
Background: Reports have shown significant increase in the prevalence of obesity in developing countries. There are limited representative data available from African countries on the trends in childhood obesity, because most public health- and nutrition-related efforts have been focused on under-nutrition. Aims: To determine the prevalence of overweight and obesity using body mass index (BMI) among school adolescents. Design: A cross sectional study involving apparently healthy school students in Sokoto. Materials and Methods: Participating schools and students were selected by a multi-stage random sampling. Anthropometric measurements were done and body mass index derived from the ratio of weight per height squared. Subjects with age and sex-adjusted BMIs of 85 th to <95 th and ≥95 th percentiles of the International Obesity Task Force were defined as overweight and obese, respectively. Statistical Analysis: Data were analyzed using SPSS version 17.0 software. GraphPad inStat version 3.05 was also used to analyze some variables. Results were expressed as means with standard deviations. P-values <5% were considered significant. Results: The mean BMI of the male subjects was 18.3 2.7kg/m 2 , and 19.3 3.1kg/m 2 for the females. The prevalence of overweight was 3.3%, and that of obesity was 1.4%. Conclusions: The prevalence of overweight and obesity appear to be low in the study area, especially when compared to some recent studies in Africa.
Keywords: Adolescent, body mass index, obesity, overweight
|How to cite this article:|
Ahmad MM, Ahmed H, Airede K. Body mass index among school adolescents in Sokoto, North-Western Nigeria. Sahel Med J 2013;16:5-9
| Introduction|| |
While obesity is one of the most common nutritional disorders in the developed countries, micronutrient deficiencies and protein-calorie malnutrition are the leading causes of malnutrition in most developing countries, particularly Africa.  However, some reviews have reported significant increase in the prevalence of overweight (OW) and obese (OB) individuals in developing countries. , Ten percent of the world's school-aged children are estimated to have excess body fat, a quarter of which is obese.  As many as 250 million people are said to be obese.  Obesity has significant impact on both physical and psychological health, hence it is no longer considered benign. ,
The prevalence of obesity in Nigerian children has not been fully documented, , and most public health- and nutrition-related efforts have been focused on under nutrition.  However, it is documented that most countries in Asia, Latin America, North Africa, and the Middle East have experienced a shift in the overall structure of its dietary and disease patterns over the last few decades.  Therefore, this study aimed to determine the prevalence of overweight and obesity, using body mass index (BMI) among school adolescents.
| Materials and Methods|| |
This study was cross sectional; conducted in March, 2010. Three hundred and sixty apparently healthy students aged 10-18years, drawn from the secondary schools within Sokoto metropolis (state capital) were enrolled. This sample size was based on the highest reported prevalence for obesity (18%) among Nigerian children and adolescents.  Equal numbers of subjects were drawn from public and private schools, with a male to female ratio of 1:1.
Participating schools and students were selected by a multi-stage random sampling. There were 32 secondary schools within the metropolis (21 public and 11 private) with a total of 42,135 enrolled students. The schools were stratified into private and public groups based on the three local government areas (LGAs) within the Sokoto metropolis. A total of six secondary schools were selected for the study, two (one public and one private) from each of the three LGAs within the metropolis. For each group (public or private) in an LGA, the names of the schools were written on pieces of paper which were folded and mixed up. One school was picked at random, from each of the two groups and was subsequently enrolled. At the school level, 60 students were selected per school (10 at each class level) by systematic random sampling.
Approval for the study was obtained from the Ethical Committee of Usmanu Danfodiyo University Teaching Hospital, Sokoto, Ministry of Education Sokoto State and the School authorities concerned. Written informed consent was obtained from parents/guardians of the participants, and verbal consent from the students.
Anthropometric measurements of weight and height were taken. The height was measured (to the nearest 0.1cm) using stadiometer scale (Seca 213, UK), whereas, the weight was measured (to the nearest 0.01kg), with the subject lightly dressed in their school uniforms without shoes, stockings, caps, sweater or cardigan, and all pockets were emptied out. This was to maintain uniformity and respect the sensitivity of these adolescent students. Body mass index (BMI) was derived from the ratio of weight (kg) per height squared (m 2 ) for each subject. Because there are no risk-based fixed BMI values to determine overweight in children, percentile values are commonly used.  Therefore, the International Obesity Task Force (IOTF) BMI cut off values  were used. Subjects with age and sex-adjusted BMI-z scores at 85 th to <95 th and ≥95 th percentiles were defined as overweight and obese, respectively.
| Results|| |
Ninety-six (26.7%) of the subjects were aged 10 to 13 years (early adolescence), 174 (48.3%) were between 14 to 16years (middle adolescence) and 90 (25%) were aged 17 to 18years (late adolescence). The mean ages of the male and female subjects were 15 (±2.5) years and 14.8 (±2.6) years, respectively (X 2 = 0.8095, df = 2,
P= 0.667). Middle socioeconomic class students formed the predominant group in the study population (47.5%), with a fair distribution between the public and private schools. Only 24.2% of the studied subjects belonged to the upper SEC stratum, out of which 65 (74.7%) were from private schools. Seventy nine (77.5%) of the lower SEC students came from public schools. (X 2 = 52.986; df = 2; P < 0.0001).
The mean weight of the study subjects were 46.4 ± 11.5kg for males, and 47.1 ± 9.2kg for the females. In both gender, the mean weight generally increased with age as depicted in [Figure 1]. There was a noticeable reversal in the mean weight pattern between the males and females from age 16 years, when the males became heavier than their age-matched females as shown in [Figure 1] (t = −0.817; df = 180; P = 0.415).
The mean values were 158.2 ± 11.2cm for males, and 155.7 ± 6.5cm for the females. Mean height generally increased with increase in age and was initially higher in the females until age 14. From 15years through 18, the males became taller than their age-matched female counterparts (t= 3.166; df= 180; P 0= 0.002). None of the females had a height of up to 160cm as shown in [Figure 2].
Body mass index
The mean BMI values were 18.3 ± 2.7kg/m 2 for the males, and 19.3 ± 3.1kg/m 2 for the females. Mean BMI values ranged from 15.7 to 20.8kg/m 2 in males and 15.4 to 21.7kg/m 2 in females as shown in [Table 1]. Mean BMI generally increased with age in both gender, with females having higher mean BMI values compared to their age male counter parts.
The mean BMI of the study subjects were generally lower, but comparable to that of the 50 th percentile values of the CDC-2000 BMI charts as shown in [Figure 3].
|Figure 3: Comparison of the 50th percentile of the CDC-BMI curves with the mean BMI of the study subjects. Key: CDC = Center for Disease Control; CDCM= CDC 50th percentile BMI for males; CDCF= CDC 50th percentile BMI for females; STUDM= mean BMI for males in present study; STUDF= mean BMI for females in present study.|
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[Table 2] shows analysis of variance for the weight, height and BMI in relation to age and gender. All the three parameters (body weight, height and BMI) varied significantly with age (P ≤ 0.004), but not with gender (P > 0.05).
|Table 2: Two-way analysis of variance (ANOVA) for the assessed parameters in relation to age and gender|
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Only 17.6% of the overweight/obese subjects came from the upper SEC, 47.1% from the middle class while 35.3% were from the lower SEC as shown in [Table 3] (X 2 = 6, df = 6, P = 0.423)
|Table 3: Distribution of overweight/obese subjects according to type of school and socio-economic class|
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| Discussion|| |
The mean weight of the studied subjects increased with age in both males and females. The females were generally heavier than their age-matched male counterparts, though the difference was not statistically significant. This finding of females being heavier than males was similar to that of Akesode and Ajibode,  among school children aged 6-19 years in Abeokuta, South-Western Nigeria. Female subjects were also initially noticed to be taller than the males up to the age of 14 years, with the difference being statistically significant. Thereafter, the males were taller. Similar pattern in mean height between the genders was reported by Ansa et al.  who also showed initial higher mean height values among their female subjects. By 15 years and beyond, the males in the latter study  demonstrated higher mean height values as compared to the females. This trend in height pattern could be as a result of pubertal growth spurt in the males around this age (14-15 years), which usually occur later in males than it does occur in the females. However, Owa and Adejuyigbe  did not demonstrate any difference in mean height values between the males and the females they studied, who were in the range of 5-15 years.
For the mean BMI, the values were consistently and significantly higher in the females, compared to those of their male counterparts. This finding is similar to what was reported previously among Nigerian adolescents. ,,
However, Cole et al. showed BMI to increase faster in males after age 18 years. Because this study did not involve subjects above 18years old, this fact could not be substantiated by our study. The mean BMI values in our study were generally lower but comparable to those of the 50 th percentile values from the CDC-2000 BMI charts.
When the BMI for the age were converted to z-scores, the BMI for age z-scores generally increased with the age in both gender. This means that the subjects tend to increase in weight relative to a standard (SD/z score) over time. This connotes more tendencies for obesity with age amongst this adolescent group. This finding is consistent with that of Kimani-Murage et al., among adolescents in rural South Africa.
The BMI-based prevalence rate for obesity in this study is within the range reported by Janssen et al.,  among school-aged children aged 10-16 years, from 34 countries in Europe (2000-2002). This study  found large variations in the prevalence figures for obesity (ranging from 0.4 to 7.9%) using BMI as diagnostic criteria.
The gender-specific prevalence of obesity in this study is comparable between the males and females. However, for the gender-specific prevalence for overweight, the prevalence rate was found to be more in the females. This female preponderance in overweight/obesity is in conformity with the other earlier Nigerian studies by Ansa et al.,  Owa, and Adejuyigbe  as well as Akesode and Ajibode.  Furthermore, obesity studies in South Africa have all reported obesity to be more prevalent among South African female adolescents compared with their male counterparts. ,, Similarly, Mokhtar et al.,  in North Africa stated that overweight increases with age and seemed to take hold in adolescence, particularly among the girls.
Highest frequencies of overweight/obesity in our female subjects were recorded among the age group 18years, followed by those aged 15 and 16 years. This finding is consistent with that of Akesode et al., who demonstrated that highest frequencies of obesity and overweight in females occurred in age groups 18 and 17 years, respectively. However, the highest prevalence of obesity/overweight among male subjects in this study was found among the age groups 13 and 15 years. Fat gain has been shown to occur in both boys and girls early in adolescence, but it ceases and may even temporarily reverse in boys, while it continues throughout adolescence in girls.  The females lay down fat as a natural part of the ontogeny of their sexual and reproductive physiology, whereas the males gain proportionately more muscle mass rather than fat. ,, Since BMI is a screening tool, that does not measure adiposity directly, , additional clinical assessment is required to further ascertain the overall health of all children with high BMI-for age values.
Furthermore, assessment of body fat distribution by measurement of waist circumference, waist-to-hip ratio, or skin fold thickness at different sites (trunk skin folds relative to the limb skin folds) are additional markers of the health risks associated with overweight and obesity.  Truncal obesity is considered to be more dangerous because the adipose tissues in this area are more lipolytic, thus, they generate more free fatty acids when metabolized with resultant increase in the generation of oxidant stress molecules.  However, this aspect of body fat distribution is outside the scope of our study. There is therefore the need for further studies on this important aspect of obesity, which like the BMI, can be readily determined even in resource poor settings.
When both obesity and overweight were combined in terms of socioeconomic classification of the subjects, only 17.6% of the overweight/obese subjects belonged to the upper SEC, 47.1% belonged to the middle socioeconomic class, whereas, the remaining 35.3% came from the lower SEC. However, this distribution was not statistically significant. Majority of these overweight/obese subjects (59%) came from private schools. Therefore, our finding did not favor the higher SEC as a significant risk factor for overweight and obesity as reported by other studies. , The negative association between lower SEC and obesity may be related to lack of awareness of the problems of obesity as well as excessive eating of cheaper, high calorie diet by the lower SEC groups, who are usually poorer and less informed section of the community.
| Conclusion|| |
The prevalence of overweight and obesity appear to be low in the study area, especially when compared to some recent studies in Africa. However, there is the need to monitor the trends in the African sub region where under nutrition is still a problem, so that early detection and prompt measures are taken to prevent its multi-systemic complications.
National surveys on childhood obesity are required in order to establish an overall prevalence rate for overweight and obesity among the Nigerian children and adolescents.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]