Adolescence refers to the developmental period between childhood and adulthood. The World health organization (WHO) defines adolescents as individuals between the ages of 10–19 years and they make up about 20% of the world’s population [1, 2]. There are about 1.2 billion adolescents in the developing nations, making up one fifth to one quarter of the population [3].

Adolescence is the second most critical period of physical growth after the first year [4, 5]. It is a time of enormous physiological, cognitive and psychosocial changes, largely dependent on hormonal and environmental influences [4, 6]. Twenty five percent of adult height, and up to fifty percent of adult weight are attained during adolescence [4, 7]. It is also a time of sexual development and increase in fat especially in girls and muscle mass in boys [8]. The demands of this normal physical growth and maturation lead to increased need for nutrients and micronutrients like vitamins A, B₁₂, C, folic acid, calcium and iron. This places extra nutritional demands on adolescents [8]. A combination of the energy demands of the adolescent growth spurt and inadequate diet has contributed a lot to the poor weight status of adolescents.

Adolescent weight status is a cumulative effect of the health and nutritional problems occurring during early childhood as well as those originating in adolescence. Previous studies have shown that adolescents suffer from a range of health problems especially those associated with substance abuse, sexual behaviour and poor eating habits [9]. For many of them, inadequacies of the quality and quantity of food they eat, are the prime causes of weight problems which often go unnoticed since assessment of weight status is not considered a priority in adolescence as it is in children [9, 10].

Although, studies on adolescent weight status have focused on undernutrition, some adolescents have the problem of overnutrition (obesity) [9, 11–15]. Undernutrition (stunting and wasting) in adolescents has detrimental effects, as it affects their ability to learn and work at maximal productivity. It affects their sexual maturation, increases the risk of poor obstetric outcomes for females and jeopardizes the healthy development of future children [10]. Obesity is a risk factor for a range of conditions including cardio-vascular disease, diabetes mellitus, arthritis and some cancers [16]. Concern about obesity is particularly important in adolescents because of their increased interest in physical appearance, self esteem and an associated poor self image.

The weight status of children and adolescents is often measured using Body Mass Index (BMI) and compared with reference standards. The BMI is an anthropometric index of weight and height that is defined as body weight in kilograms divided by height in meters squared [17]. BMI is the most widely used and simplest anthropometric index for assessing the weight status of children and adolescents since it can be used to identify those who may be overweight, at risk of being overweight, or underweight based on age and sex [18].

Few studies in Africa that used the WHO recommended references to assess the weight status of adolescents, documented the prevalence of undernutrition (BMI <5^th percentile of the WHO/NCHS reference) to be between 4–30% [11–13]. In South Asia, a high prevalence of about 50% was found among adolescents [14, 19, 20]. In Nigeria, there is scanty information on the weight status of adolescents and no available reference standard of their BMI exists. Port Harcourt, the oil city of Nigeria has a mixed population of residents ranging from high to low socioeconomic class. The town has experienced rapid population growth due to oil exploratory activities with overcrowding and springing up of many fast food outlets. The effects of these factors on the weight status of the adolescents have not been examined. This study therefore attempts to evaluate the weight status of adolescents in Port Harcourt using BMI, identify factors influencing it, with a view to developing and producing a reference standard of BMI among adolescents in our environment.

Over a 3 week period in April 2010, a cross-sectional descriptive study was carried out in Port Harcourt City, capital of Rivers State of Nigeria.

The mean BMI of females in this study was significantly higher than that of males from 11 years of age throughout adolescence. Similar trend was observed by Ijarotimi et al. [15], Ukegbu [24] and Mukhopadhyay et al. [14]. This is different from NHANES findings in which the BMI of males and females were almost identical. This difference in the BMI between sexes may be the result of increased fat mass in females in contrast to males who stabilize their fat mass and enlarge their fat free mass [13]. The fact that males are taller than females may also have contributed to their low BMI since height is a denominator in calculating BMI. Also decreased physical activity in females may result in storing of calories which contributes to their increased weight [25].

Eighty-five percent of the study population had normal weight. This is agreement with previous studies [24, 26]. Majority of the normal weight subjects were males.

The prevalence of underweight in this study was 6.4%, which contrasts with findings from previous studies with underweight prevalence of 33.1% [26] in Philippines and 20% [27] in Nigeria. In the Philippine study [26], although the study population was similar to that of the present study, their sample size was much larger (6079) and therefore would be more representative of the total population. This could explain the disparity between this study and that reported in their study. The low prevalence in this study may probably be due to exclusion of adolescents with chronic illnesses. Although, Ukegbu et al. [24] studied adolescents in mid and late adolescence (15–18 years) who should have a low prevalence of underweight, the high prevalence obtained in their study may be because their study population consisted of boarding school students, compared to day students who are likely to have more access to nutritious and well balanced food.

The overall overweight prevalence of 6.3% was similar to that in the Philippine study of 7.3% [26]. Females were more overweight than their male counterparts. This however differed from Ukegbu’s [24] study where the prevalence of overweight was lower in females than in males. This difference cannot be readily explained as females are generally known to have higher subcutaneous fat and BMI than their male counterparts. Al-Saheed et al. [28], in Saudi Arabia reported a higher overweight prevalence of 20%. This may be due to their study group which included children 6–17 years of age. The preadolescents are known to have higher overweight rates [4] and coupled with higher socio-economic standards in their country [29], one is not surprised at the high prevalence of overweight in their study.

The prevalence of obesity (BMI equal or greater than the 95^th percentile for age and sex) in this study was 1.8%. This is in agreement with other studies by Alabi [27] in Port Harcourt and Izuora [30] in Lagos. Females were significantly more obese than males. The lower prevalence of obesity in males may be the result of higher degree of physical activity [25]. In Ukegbu’s study [24] no obese adolescents were found, this was probably due to the small sample size. This prevalence rate of 1.8% is much lower than results obtained in developed and more affluent industrialized countries like USA (15%), UK (20%), France (14%), Russia (6.7%), and China (3.6%) [31].

The extent of height deficit in relation to age may be regarded as a measure of the duration of malnutrition. Height is affected not only by chronic malnutrition but also by genetic and hormonal factors [32]. The low prevalence of stunting (5.4%) reported in this study was similar to that reported by Brabin et al. [33] in Port Harcourt in 1997 in a study of South-eastern Nigerian adolescent girls and de-Onis et al. [34] in Calcutta, India. Although they used less or equal to second centile for height-for-age of British Reference standard and –2SD Z-scores of NCHS reference as cut-off point, it however differed from other studies by Ukegbu et al. [24] who reported stunting rates in males and females with an overall prevalence of 62.5% in both sexes. The reason for this enormous difference is not clear. The essential differences in the study population in both studies are the age ranges of the subjects and their tribes being predominately Igbos and Ijaws in Ukegbu’s study [24] and the present study respectively. Further investigations need to be done to determine if the differences in age and tribe significantly affected the heights of the subjects. In the present study, subjects with chronic illnesses were excluded, as chronic illnesses usually account for most cases of stunting in our environment. Therefore the exclusion of subjects with chronic illnesses may be responsible for the low prevalence found in this study. Generally, males are more stunted than their female counterparts [24, 26], this was similar to findings in this study.

Socioeconomic status (SES) affects access to culturally appropriate and affordable food, thus affecting the quality of diet. The prevalence of underweight in this study was higher in the lower SES as compared to the upper SES, where overweight and obesity was found to be higher in the higher SES. This is not surprising as there is growing evidence that perhaps due to decreased physical activities, sedentary lifestyle, altered eating patterns and increased fat content of the diet the prevalence of overweight/obesity is higher in the upper SES. The relationship between obesity and socioeconomic status varies across countries. The higher the socioeconomic class (SEC) the more the risk of obesity in Russia and China, while in USA, the lower the SEC the higher the risk of obesity [35]. In the USA, obesity is more prevalent among the lower SEC and this has been attributed to the theory that food deprivation or the fear of deprivation can lead to overeating when food is available [36]. Consumption of junk food has been associated with incidence of obesity.

This study revealed that subjects who ingested beverages everyday tended to be overweight. Although the authors did not come across similar studies, Dehghan et al. [31], reported that excessive beverage intake has been associated with epidemic of obesity and type II diabetes mellitus in children and adolescents. This may be explained by the fact that adolescents who ingest beverages daily tend to fill their stomachs with these carbonated drinks with high sugar and caloric content which promote weight gain.

The study also revealed that the adolescents who ingested snacks everyday were more underweight than those who took only once a week. This may also be explained by the fact that daily ingestion of these highly processed snacks with poor nutritional value will consequently cause reduction in appetite and consumption of nutritious foods provided at home.

Television viewing greater than 3hours per day was associated with increased prevalence of overweight and more than 5hours a day with more obese subjects. The risk associated with this behavior may operate through several mechanisms, including reduction of time spent in higher intensity activities, a lowering of metabolic rate and more frequent snacking [37]. It is known that fast foods are frequently advertised on television. Children and adolescents are often the targeted market, therefore it is not surprising that there is increased snacking while viewing television [31].

Another finding in this study was worsening of the nutritional status of subjects with increasing family size. Although this finding was not statistically significant, it is similar to findings from another study in children less than five years which showed an association between nutritional status and family size [38]. This is in line with the findings of other authors who found that subjects from small family sizes, higher socioeconomic class, with highly educated mothers tended to be more obese [31].

The overall mean BMI for females was significantly higher than that of males while the males were significantly more stunted than females. Adolescents from upper social class, whose mother’s had higher education and spent more than 3hours watching television tended to be more overweight than the counterparts whose parents were from lower social class and spent less hours watching television.

Periodic weight assessment of adolescents should be carried out in schools and communities as part of school health programme. This is essential for early detection, planning and implementation of intervention programmes to reduce morbidity and mortality associated with under and overnutrition. Also, health education to promote healthy eating behaviors, regular physical exercise and regulated television viewing should be inculcated into the school curriculum. This would contribute to controlling overweight and obesity.