Oh, Brother! Having a Male Twin Increases a Woman’s Obesity Risk

That old adage, “He ain’t heavy, he’s my brother” takes on a strange twist in in light of a new Swedish study showing that having a twin brother increases a woman’s risk of being overweight.

Researchers at the Sahlgrenska Academy, University of Gothenburg, studied middle-aged females from 17,576 dizygotic twin pairs, and found that those who have twin brothers (N=8409) were more likely to be overweight and dyslipidemic than those who have twin sisters (N=9166). This “twin brother effect” was most pronounced in the women after age 60.

“BMI was found to be moderately but significantly higher in women with opposite sex twins than in those with same sex twins. Although there was no difference in height, women with male twins displayed larger body weight than those with female twins. Self-reported birth weight did not differ between groups,” reported lead investigator Dr. Camilla Alexanderson.

The differences were fairly small—for example 40.8% of the women over age 60 with twin brothers were overweight versus 38% of those with twin sisters. Mean BMI in the “twin sister” group was 25.3 versus 24.8 in the “twin brother” group. But the effect was consistent, and not mitigated when the investigators ruled out other risk factors for overweight. Further, the population was large more than adequate to demonstrate small but meaningful differences.

The findings, published in the December edition of the International Journal of Obesity, are consistent with the “hormonal programming” theory of obesity in which intrauterine exposure to various hormones influences adult metabolic predispositions. The researchers noted that having a male twin would be expected to raise in utero androgen levels, conferring a more masculine risk profile later in life.

This type of effect has been observed in animal experiments. In humans, the effect would be modest in comparison to intra-individual variations in androgenization from factors like maternal androgen production, and genetically determined differences in fetal androgen responsiveness, hence the relatively small differences between the same-sex twin and opposite-sex twin groups.

Dr. Alexanderson said the fact that the difference became more apparent in the women over age 60 versus those younger, is compatible with the hormonal programming hypothesis. “It may, for example, be speculated that high serum levels of estradiol before menopause counteracts the influence of early hormonal programming on metabolism.”

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