Twin Studies and Genetics of Obesity

Much research effort and expense is expended trying to find the causes of obesity, both genetic and environmental. Researchers have used one of nature's own experiments, the phenomenon of twin births, to derive valuable evidence that has informed our understanding of the causes of obesity.

The study of twins has played a vital part in quantitating the heritable influences on obesity and explains how this naturally occurring human model has helped researchers detect and quantify the effect of environmental influences independent of genetic factors. Twin studies play a part in understanding causation of obesity and in improving efficiency in the hunt for gene loci (sites).

When many believed that obesity was a matter of willpower, influenced only by environmental effects, the adoption studies of Dr. A. J. Stunkard were a historic turning point in scientific thinking. His landmark study showed strong genetic influence on the weight of Danish adults adopted between 1924 and 1947. He demonstrated that the offsprings' adult weight related strongly to the weight of the biological parents, rather than the adoptive parents who had provided their environment. For further evidence, he performed twin studies that confirmed the relatively stronger importance of inheritance than environment on adiposity.

One study compared the heritability of adiposity in adult twin men who had been reared together, compared to twin men reared apart. Stunkard's studies showed that there was no difference in the estimated heritability of fatness (which was high, at about 70 percent) between adult twins, whether reared together or apart. This also confirmed that genetic influences on fatness were greater than the environmental ones. The method for calculating heritability is described below.

Identical or monozygotic (MZ) twins occur about 4 in every 1,000 confinements and nonidentical or dizygotic (DZ) twins about 8 per 1,000 in Caucasians, with increasing age a factor in producing DZ twins. MZ twins share all their genes because they come from the same egg, and DZ twins, like ordinary siblings, come from two separately fertilized eggs and share half their genes. Unlike siblings, DZ twins are, of course, identical in age. Twins share very closely their pre- and postnatal gestational influences and rearing.

It is accepted that well-conducted twin studies have been able to estimate the relative importance of genes and environment by comparing the concordance (similarity) of the adiposity trait in MZ versus DZ twins. Heritability, a statistic which indicates the size of the heritable effect, refers to the amount of observed variation in the trait (e.g., fatness) that can be attributed to genetic variation. Heritability approximates to the doubling of the difference between MZ and DZ correlations for the trait, that is, 2x(r2 MZ – r2 DZ) where r is the correlation between twin pairs for that trait. The remainder of the variation is made up of environmental influences, which are either shared or unique to the individual. In most twin studies, this estimate for heritable fatness is in the range of 40 to 70 percent, indicating that 40 to 70 percent of the population variance in fatness across the study group is attributable to genetic factors. Some of the difference in heritability estimates is due to the estimate of fatness used, although most results are within this range whether measuring body mass index or more accurate, direct measures of total body fat, waist, or the more accurate and direct measures of central body fat.

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