Overweight and obesity

The prevalence of overweight was similar in females and in males at age 16 years, but after age 18, males were significantly more overweight than females (Table 1). At 25 years, 28% of males and 15% of females were overweight, and 4% of both genders obese.

Table 1 Background characteristics of participants Full size table

Intentional weight loss

Thirty-eight percent of females and 24% of males reported an episode of IWL of 5 kg or more at least once during the lifetime (Table 1). Fifteen percent of females and 10% of males had a history of two or more IWLs.

In a multivariate analysis using BMI, physical activity, smoking, breakfast eating, parent's BMI and father's socioeconomic status at baseline, growth in height from 16 to 18 years, age of menarche (in females) and number of children at 25 years as confounding factors, the odds for having at least one IWL was higher in males and females who had higher BMI at 16 years, smoked daily, skipped breakfast and were in the blue collar (in females also self-employed) socioeconomic group (Table 2).

Table 2 The risk of having IWL of 5 kg or more by baseline characteristics in a multivariate model Full size table

IWL and weight development

Baseline BMI at age 16 was a highly significant determinant of future IWL episodes. Males (mean±s.d. BMI 20.1±1.8 kg m−2) and females (19.5±2.0 kg m−2) who had no IWL history, weighed significantly (P<0.001) less than either males (21.3±2.5 kg m−2) and females (21.0±2.2 kg m−2) with one IWL episode, or males (22.2±2.8 kg m−2) and females (22.2±3.0 kg m−2) who had at least two IWLs. The rate of weight gain differed significantly by IWL groups in both males and females for all three baseline BMI categories (low, intermediate and high) (P<0.02) (Figure 1). Subjects with no IWL gained the least, whereas those with five times or more IWLs had the largest weight gain in all BMI groups. The effects were especially large in those with the greatest BMI at baseline.

Figure 1 (a) BMI development from 16 to 25 years by lifetime IWL frequency and baseline BMI percentile at 16 years in males. For each group defined by sex and BMI baseline percentile, the regression slopes of BMI on age were significantly different by IWL frequency. *P<0.05, ***P<0.001 for interaction tests. (b) BMI development from 16 to 25 years by lifetime IWL frequency and baseline BMI percentile at 16 years in females. For each group defined by sex and BMI baseline percentile, the regression slopes of BMI on age were significantly different by IWL frequency. *P<0.05, ***P<0.001 for interaction tests. Full size image

Figure 2 shows BMI change between 16 and 25 years by baseline BMI and the number of IWLs. Males gained much more weight than females, especially when the baseline BMI was low. Those who had the most IWL episodes had gained the most in all baseline BMI groups, in males particularly in the lowest BMI group (analysis of variance, P<0.001).

Figure 2 BMI change from 16 to 25 years by lifetime IWL frequency and baseline BMI percentile at 16 years. *P<0.05, ***P<0.001 (analysis of variance). Full size image

IWL and risk of becoming overweight

The risk of becoming overweight by 25 years in the initially non-overweight participants was proportional to the IWL frequency. Males with one (OR 1.8, 95% CI 1.3–2.6) or at least two (OR 2.0, 95% CI 1.3–3.3) IWLs were significantly more likely to become overweight than males with no IWLs (Table 3). Among females, the gradient by IWL was even steeper: OR 2.7 (95% CI 1.7–4.3) for those with one IWL and OR 5.2 (95% CI 3.2–8.6) for those with at least two IWLs. Adjusting for the possible confounders and baseline BMI did not change the result. Of the confounders, BMI at age 16 years highly significantly and independently predicted the risk of overweight at 25 years in the models in both genders. Further, in females, low physical activity at 25 years, low socio-economic status and father's overweight also predicted the risk of overweight (Table 3).

Table 3 The risk of becoming overweight from 16 to 25 years by IWL of 5 kg or more and by other baseline characteristics in the same multivariate model Full size table

IWL and weight development in MZ and DZ twins

Next, we used the co-twin control approach to examine the effects of weight loss on later weight development adjusting fully (in MZ; Figure 3a) or partially (DZ twins; Figure 3b) for genetic effects. Further, in opposite-sex twins, we studied the effects of gender on weight loss and BMI development (Figure 3c).

Figure 3 (a) BMI development from 16 to 25 years in MZ twin pairs concordant and discordant for lifetime IWL. (b) BMI development from 16 to 25 years in same-sex DZ concordant and discordant for lifetime IWL. (c) BMI development from 16 to 25 years in opposite-sex DZ concordant and discordant for lifetime IWL. *P<0.05, ***P<0.001. Full size image

The development of BMI from age 16 to 25 years was similar in co-twins who were concordant for never or ever IWL among both MZ and DZ twin pairs of the same sex (Figures 3a and b). Twins with a history of IWLs were heavier at all ages (for example, 2.7 and 3.0 kg m−2 for MZ and same-sex DZ pairs, respectively, at 16 years) and gained more weight (1.9 kg m−2 in MZ and 1.7 kg m−2 in same-sex DZ) than those with no such history (P<0.001 for all comparisons between pairs concordant for ever versus never weight loss).

A total of 145 MZ twin pairs were discordant for never versus ever IWL. At the age of 16 years, the weights of the co-twins of these pairs were similar. By age 25 years, co-twins with IWLs were slightly but significantly (0.4 kg m−2, P=0.041) heavier than co-twins with no IWLs. In 179 DZ pairs discordant for IWL, co-twins with IWLs were constantly heavier (BMI difference at age 16 years 1.7 kg m−2, P<0.001, and at 25 years 2.2 kg m−2, P<0.001) than co-twins without IWLs.

In the opposite-sex DZ twins, males gained more weight than females in pairs where co-twins were concordant for IWL (Figure 3c). In IWL-discordant pairs, we examined the results separately by the gender of the dieting twin. Compared with their male co-twins, female twins with IWLs were 1.0 kg m−2 (P<0.001) heavier at 16, and as heavy at 25 years. In discordant pairs where the male had a history of IWLs, male twins were consistently much heavier than their twin sisters: 2.2 kg m−2 at 16 years and 4.3 kg m−2 at 25 years (P<0.001 for both).

The results presented above show that twin pairs where both co-twins have IWL episodes are heavier throughout their adolescence and young adulthood than pairs where neither of the co-twins had IWLs. Such group differences may depend on an underlying genetic background, where the group, which later ends up experiencing IWLs, is in fact heavier and more susceptible to obesity in the first place. Further, the fact that in the DZ twins co-twins who had IWLs were clearly heavier already at age 16 than their non-IWL twin pair members would suggest that the need to diet only arises in co-twins who are more genetically predisposed to obesity.

Parental BMI in the IWL groups

To further elucidate the possible role of inherited predisposition, we analysed parental BMIs by the twin pair IWL groups (concordant for never IWL, concordant for IWL, discordant for IWL). In twin pairs concordant for never IWL, mothers’ and father's mean±s.d. BMIs (24.6±3.9 kg m−2 and 25.8±3.2 kg m−2, respectively) were significantly (P<0.001) lower than in twin pairs concordant for IWL (mother's BMI 25.8±4.1 kg m−2 and father's BMI 27.0±3.6 kg m−2). In pairs discordant for IWL, mother's (25.5±4.3 kg m−2) and father's BMIs (26.3±3.6 kg m−2) were significantly higher than in pairs concordant for never IWL (P<0.001 for mother's and P=0.0065 for father's BMI). Father's BMI was also significantly lower than in pairs concordant for IWL (P=0.021). Such findings in concert would support the findings that subjects with a history of IWL are genetically predisposed to higher BMIs than subjects who do not have IWL.

Combined effects of IWL and exercise

The physical activity of subjects with and without IWLs differed only slightly at age16 and no differences were found at 25 years (data not shown). Among females with IWLs, those who were physically passive at age 16 gained more weight (3.8 kg m−2) than those who at baseline were active (1.9 kg m−2) or intermediately active (2.2 kg m−2) (analysis of variance, P=0.0032). Passive females also had an increased risk of becoming overweight (Table 3). This was not observed in males. Within twin pairs, physical activity did not explain the detected differences between dieting and non-dieting twins’ weight development (data not shown).