Environmental factors in the development of obesity in identical twins

OBJECTIVE: To study environmental factors promoting obesity when genetic factors are identical. DESIGN: Monozygotic (MZ) twins discordant for overweight were examined during a 3-day stay in an inpatient setting. SUBJECTS: The subjects were selected from the Finnish Twin Cohort. The study sample consisted of 23 healthy adult MZ twin pairs (14 female, nine male) with a difference of at least 3 kg/m(2) in BMI, The mean BMI was 29.5 kg/m(2) for the overweight twins and 22.9 kg/m(2) for their lean co-twins. MEASUREMENTS: Interviews and standardized questionnaires were used to obtain information about energy and nutrient intake, eating behaviour, physical activity history, smoking and other background factors. RESULTS: The overweight co-twins had higher disinhibition scores (P=0.007) and hunger scares (P=0.005) in the 3-factor Eating Questionnaire than their lean co-twins. Among women the mean daily energy intake was higher in the overweight twins than in their lean co-twins (8.8 vs 7.4 MJ; P= 0.045). In the twins discordant for smoking the mean BMI was higher in non-smokers than in smokers (BMI 29.7 +/- 4.4 kg/m(2) vs 23.8 +/- 3.1 kg/m(2); P=; 0.031). CONCLUSIONS: Difficulty in controlling eating in both sexes and high energy intake in women were related to overweight, independent of genetic background. Smoking explained the BMI difference among smoking discordant pairs. It is probable that individual twin pairs had different reasons behind the variation in weight gain resulting in non-significant intrapair differences in single obesity-promoting factors. Difference in living conditions (e.g, family-and work-related factors) may have promoted different living habits, especially eating behaviour, and may have led to different weight gain in identical twins.