Our study confirmed a substantial increase in the upper BMI-percentiles over time in the adolescent population. However, no significant increases in mean BMI were found for girls 14–17 years, and the study showed an increased dispersion of the BMI-distribution with a consistent pattern of a decrease in the lowest BMI percentiles, a trend for both sexes and all ages. Previous studies have focused primarily on changes in the upper percentiles, missing these changes in the lower percentiles indicating that the increase in BMI does not involve all adolescents.
Strengths and limitations
The strength of our study is that both NHS and Young-HUNT sampled data from the same county, including more than 80% of the adolescent population, and with the same standardized procedures for measurements of height and weight. Mean ages in all age- and sex groups were similar. We have no data on secular changes in the study periods, but in both studies data were collected over a restricted time period (2–4 years), reducing the probability of major secular changes within each survey period. The population of Nord-Trondelag County has been stable with small sociodemographic and ethnic differences. The large number of participants and high participation rates support a high external validity. Also, Nord-Trondelag County has a sex and age distribution similar to Norway as a whole. The same is true for geography, industry, sources of income and economy.
When comparing results from two cohorts studied 30 years apart, some potential biases need to be addressed. Despite using the same protocols for measurements, differences in measurement-accuracy, time since meal and clothing including seasonal variations could influence the results. No systematic biases are likely in these aspects. We have no reasons to believe that the weight measurements were performed differently in the two studies in respect to weight of light clothes and time since meal. Both studies were conducted during the school-day (08.30 – 14.00), and school-year, with no measurements in June, July and August.
Another issue is the effect of rounding off height and weight to the nearest cm and kg. However, testing this, the resulting uncertainties in BMI percentile differences, given in the footnote for Table
3, is too small to explain the observed differences in BMI-values. The consistent pattern across age-and gender groups, together with the lack of increase of mean BMI in girls 14–17 year, also support the findings of a decrease in the lowest BMI-percentiles.
Potential bias due to age truncation is practically negligible, as stated in the footnote for Table
3.
Although a marked increase in the upper BMI-percentiles is well established, not all studies support the notion of an overall increase in BMI. This may support our findings of a decrease in the lowest BMI-percentiles and no increase in mean BMI in girls 14–17 years. While the prevalence of overweight and obesity in British children increased from 1984 – 1994 [
17], a later short report found no significant changes in mean BMI 1986–1996 for girls aged 12–16 [
10]. Also an Australian study among primary school children showed no increase in mean BMI from 1985–1997 among girls 12 years old and boys 7, 8 and 10 years [
18]. Two Swedish studies have different conclusions. One article comparing the values of the 50-percentile between two cohort studies in the 1950s and in the 1970s summarised that the Swedish child population had remained at a very similar BMI-level since the Second World War [
11], another study concluded that the median BMI was significantly higher in 2001 than in 1987 for boys aged 10, 13 and 16 years, and in girls 16 years old [
2]. Also in this latest study, the greatest change in BMI was found in the highest percentiles.
Comparing different studies of changes in BMI- percentile is complicated due to use of different methodology, including measurements, sampling procedures, the decade the study started, and time span between measurements. The fact that other studies conducted within different time frames also reports some decrease in the lower percentiles, can indicate that this not only is a Norwegian phenomenon. Studies from both Europe [
2,
4‐
7] and the United States [
8,
9] have all focused on the changes in upper percentiles. Few studies have evaluated changes in the entire BMI-distribution. A Finnish [
5] study in age groups 12, 14, 16 and 18 years found, according to the tables, little or no change in the lower (5
th and 15
th) and middle (50
th) percentile. A Swedish study [
3] evaluating the differences in BMI-distribution in children 2–15 years of age born in 1973/75 and 1985/87 found an increase only above the 25
th percentile, and more pronounced in the upper parts of the BMI-distribution. Girls 13 and 14 years old had lower values of the 5th percentile in 1985/87 compared to 1973/75. Different other studies [
2,
7‐
9] have also presented tables with decrease in the lowest percentile in a few age- and sex groups, but without any comment on these findings. Although statistically significant only in boys 16 and18 years and girls 14 years, the consistency of the pattern in all ages and both sexes in our study, indicates a clear trend of decrease in the lower percentile that should not be ignored.
Existing Norwegian standards for height and weight [
19] in children and adolescents were constructed from data sampled in Bergen from 1971–74. No recent official measures of height and weight have been published, making it difficult to predict when and how fast the changes have occurred. British studies in adolescents have shown little change in the prevalence of overweight and obesity between 1974–1984, but a significant increase between 1984 and 1994 [
10,
17]. A follow-up study of Young- HUNT in 2000–01 with 1613 girls and boys aged 17 and 18 years found no significant changes in height and weight from the Young- HUNT studies 1995–97 (data not shown) indicating that changes mainly occurred before 1995. This assumption is supported by the adult part of the HUNT-study, showing a marked increases in BMI among young adults between HUNT-I (1984–86) and HUNT-II (1995–97) [
20].
Changes in the BMI-distribution over 30 years, as shown in this study from Nord- Trondelag Norway, may have multiple different explanations, both biological and socio-cultural, and their clinical relevance should be explored. Better socioeconomic conditions may have led to two different problems among adolescents, on one side obesity on the other side unhealthy weight control behaviour including eating problems. Especially in girls dieting and eating problems was reported quite frequently in the Young-HUNT population [
12]. The fact that mean BMI in our study showed no increase in girls 14–17 years supports the need to follow the changes in BMI -distribution, not only the highest percentiles,