Activity Determines Discordant Obesity in One-Egg Twins

So for Easter I wanted a posting with “Egg” in the title. What better paper to choose than the recent publication by Kirsi Pietiläinen and colleagues from Helsinki University published in last month’s OBESITY?

In this remarkable study, Pietiläinen and colleagues went to the considerable effort of screening 1,870 young adult twin pairs to find 658 monozygotic (MZ) pairs, of whom only 14 (!) pairs reported a BMI difference of at least 4 kg/m2, with one twin being non-obese (BMI approximately 25 kg/m2) and the other obese (BMI approximately 30 kg/m2).

This effort alone shows how rare it is to find MZ twins that are discordant for weight, clear evidence for the well-known fact that body weight is one of the most heritable complex traits found in man, in fact, only marginally less heritable than height.

Nevertheless, the investigators were able to further characterize at least 10 of the 14 BMI-discordant twin pairs and readily identified the difference in physical activity as the key determinant of the discordance. Virtually all of the co-twins who ultimately became obese reported having been less physically active in adolescence than their non-obese co-twins. Furthermore, as they grew heavier, the obese co-twins’ physical activity levels declined even further as did their self-perceived physical fitness. Based on accelerometer recordings, the obese co-twins had less than half the daily activity of their non-obese co-twins

In contrast, in the weight-concordant reference MZ twin pairs, physical activity patterns and fitness changed little during adolescence and were similar in the co-twins.

Thus, this paper appears to suggest that even in genetically identical individuals, large amounts of physical activity (or lack thereof) can “override” the genetic determinants of BMI.

What the paper, however, fails to tell us is why in these rare instances (only 14 out of 658 or 2% of MZ pairs) the obese co-twins were less active than their non-obese counterparts.

Why, as discussed in a previous posting on this blog, if the disposition to be physically active is such an ‘innate” trait, did these 14 co-twins behave so differently from their siblings? Was it lack of interest, competing hobbies, sibling rivalry, injury?

Or was it epigenetics – i.e. post-conceptional modification of DNA that may involve paramutations, bookmarking, imprinting, gene silencing, X chromosome inactivation, position effects, reprogramming, transvection or maternal (intrauterine) effects – all of which could impact character traits or behaviour in later life?

So, while the paper shows that even in genetically identical individuals the heritability of BMI can be overridden by marked differences in physical activity, it does not provide the answer to whether or not this level of activity can be cognitively induced or, rather, happens as a result of rare quirks of nature.

Clearly, there are a few more eggs to crack before we fully understand why some people “chose” to be physically active and others don’t.

Happy Easter!