Institute of Preventive Medicine, Copenhagen University Hospital, Copenhagen, Denmark.
Int J Obes (Lond). 2013 Feb;37(2):211-5. doi: 10.1038/ijo.2012.151. Epub 2012 Sep 4.
Twin and adoption studies suggest that family environment has little, if any, influence on body mass index (BMI) in adulthood. We investigated the hypothesis that the differences in the years of birth between siblings influence their similarity in BMI at comparable ages, which would give evidence for a possibly modifiable influence of the environment shared by family members.
Swedish full-brother pairs (N=261 712) born between 1951 and 1983 were measured for BMI in conscription examination at 16-26 years (median: 18.2 years) of age and were divided into quartiles by the difference between their birth-years (< 2.25 years, 2.25-3.33 years, 3.34-5.08 years and >5.08 years). Furthermore, 1961 dizygotic twin brother pairs from the same population representing brothers born at the same time were included. In addition, the log BMI of the younger brother was modeled as a linear function of the log BMI of the older brother. Subsequently, the significance of the interaction between birth-year difference and the BMI of the older brother was tested.
Intraclass correlation for BMI in dizygotic twin pairs was higher (0.431, 95% confidence interval (CI) 0.394-0.466) than the correlation for full-brothers in the first quartile of birth-year difference (0.376, CI 0.342-0.408). Among full-brothers, the BMI correlation decreased from 0.376 (CI 0.342-0.408) [corrected] in the first quartile to 0.338 (CI 0.331-0.345) in the last quartile. The regression analysis showed a statistically significant decrease in correlation with increasing birth-year difference (P<0.001).
The influence on BMI in young men of the environment shared by dizygotic twin brothers is greater than between non-twin full-brothers, indicating important influences of concomitant exposure to the same early life environment before and/or after birth. Among non-twin siblings there is a slight possibly modifiable influence as evidenced by declining correlations by increasing distance in years of birth.
双胞胎和收养研究表明，家庭环境对成年人体重指数（BMI）的影响很小，如果有的话。我们研究了这样一种假设，即兄弟姐妹之间出生年份的差异会影响他们在可比年龄时的 BMI 相似性，这将为家庭成员共享的环境可能存在可改变的影响提供证据。
在瑞典，1951 年至 1983 年间出生的全同胞兄弟对（N=261712）在 16-26 岁（中位数：18.2 岁）的兵役体检时测量 BMI，并根据他们的出生年份差异将其分为四分位数（<2.25 年、2.25-3.33 年、3.34-5.08 年和>5.08 年）。此外，还包括了来自同一人群的 1961 对同卵双胞胎兄弟，代表同时出生的兄弟。此外，还将弟弟的 BMI 对数建模为哥哥的 BMI 对数的线性函数。随后，检验了出生年份差异与哥哥 BMI 之间的交互作用的显著性。
同卵双胞胎兄弟对 BMI 的组内相关系数（0.431，95%置信区间（CI）0.394-0.466）高于出生年份差异第一四分位的全同胞兄弟对（0.376，CI 0.342-0.408）。在全同胞兄弟中，BMI 相关性从第一四分位的 0.376（CI 0.342-0.408）下降到最后四分位的 0.338（CI 0.331-0.345）。回归分析显示，随着出生年份差异的增加，相关性呈统计学显著下降（P<0.001）。
同卵双胞胎兄弟之间共享的环境对年轻男性 BMI 的影响大于非双胞胎全同胞兄弟，这表明在出生前后同时暴露于相同的早期环境会产生重要影响。在非双胞胎兄弟姐妹中，存在着轻微的、可能可以改变的影响，这可以从出生年份差异增加导致的相关性下降中得到证明。
