The Seychelles is an economically rapidly developing small island state in the African region, where a high prevalence of overweight/obesity has been documented in adults and children [18–22]. Under-nutrition is no longer considered a major public health problem in the Seychelles, in contrast to several other countries in the region.
This study used the pooled data set of the annual school-based surveillance program conducted in the Seychelles between 1998 and 2004. All children attend obligatory school up to the 10th grade in the Seychelles. The majority of the population is of African descent with minorities of European, Indian, and Chinese origins. The sampling frame, measurement methods and results for overweight/obesity have been published previously [19, 23].
Briefly, data were collected every year by approximately 20 school nurses in all children in four selected grades of obligatory school: kindergarten, 4th, 7th and 10th grades, in children aged [mean (± SD)] 5.4 (±0.4), 9.1 (±0.4), 12.5 (±0.4), and 15.6 (±0.5) years, respectively (~6000 children were screened every year). Eligible children consist of the entire population of the country. A written consent was sought from the parents before screening as part of the routine procedure. Virtually no parents or children declined participation. Because nurses performing the screening in schools also have regular duties in health centres, screening in schools may not always be as completed as expected. This underlies that a main reason for non participation of children to the screening program is likely due to random factors and not to students' personal characteristics.
Weight was measured to the nearest 0.1 kg with subjects dressed in light garments and without shoes using precision electronic scales (Seca 879, Seca, Hamburg, Germany). Height was measured to the nearest 0.1 cm against a wall using fixed stadiometers (Seca 870, Seca, Hamburg, Germany). The equipment was regularly checked for accuracy and school nurses were trained every year on the measurement procedures. BMI was calculated as the ratio of weight to height squared (kg/m2). Thinness was determined using the age and sex specific IS and the WHO cut-offs, as provided in tables in the original papers referring to these standards [7, 8]. The IS cut-offs define three grades of thinness based on centiles passing at age 18 years through BMI 18.5, 17.0 and 16.0 kg/m2, respectively . The WHO cut-offs are provided for -1, -2 and -3 standard deviations (SD) for BMI-for-age, of which <-2 SD defines "thinness" and <-3 SD "severe thinness" .
We generated, separately for boys and girls, third degree polynomial functions of BMI cut-offs according to age for each thinness category based on the BMI cut-off values tabulated at age intervals of 0.5 year for the IS  and age intervals of 1 month for WHO (http://www.who.int/growthref). Thinness (yes or no) for the different IS and WHO grades was defined for each child (i.e. at individual level) by comparing the actual age and sex specific BMI to the BMI predicted by the polynomial age and sex specific function. Analyses were performed with Stata 10 (Stata Corp LP, College Station, USA). Results are expressed as mean ± SD for continuous variables and as percentage and (95% confidence interval) for prevalence. Because results refer to the entire population of children and adolescents of the country, confidence intervals may not be informative in this situation.