A study of fat/skinny twins was only able to recruit 14 such twins from a registry of 2,453 twins. It doesn't say what fraction of those 2453 were fat/skinny ("obesity discordant"). The BMI ratio between those 14 was only 6/5.
They suggested that the obesity difference might be accounted for by mitochondrial DNA copy number. mtDNA copy number is regulated, not simply "genetically innate"; so I don't see that as a "genetic" explanation.
(I put "genetic" in quotes because the things we say have genetic causes are only a small subset of the things which have genes as causes.)
The researchers recruited 14 pairs of genetically identical Finnish twins born between 1975 and 1979 who were “obesity discordant”—that is, one twin of each pair had a BMI of about 25 (not obese); the other had a BMI of about 30 (obese). The researchers took fat and blood samples from each twin, determined the insulin sensitivity of each, and measured the body composition and various fat stores of each. They found that the obese twins had more subcutaneous, intra-abdominal, and liver fat and were less insulin sensitive than the non-obese twins. Insulin sensitivity correlated with the amount of liver fat. Analysis of gene expression in the fat samples showed that 19 gene pathways (mainly inflammatory pathways) were expressed more strongly (up-regulated) in the obese twins than the non-obese twins, whereas seven pathways were down-regulated. The most highly down-regulated pathway was a mitochondrial pathway involved in amino acid breakdown, but mitochondrial energy metabolism pathways were also down-regulated. Finally, mitochondrial DNA copy number in fat was reduced in the obese twins by nearly half, a novel observation that could partly account for the obesity-induced metabolic defects of these individuals.
I think our intuition might be miscalibrated when it comes to evaluating how much a person’s genes impact how they turn out physically (which isn’t surprising). What’s a bit strange is that we seem to be closer to the truth when it comes to twins.
Nobody’s surprised when identical twins turn out to have very similar bodies (weight, muscle mass, etc), even into adulthood.
But when it comes to non-twins, people seem to think that “making the right choices” and “willpower” are primary factors in how human bodies turn out, and that we can assign a good amount of personal credit or blame to individuals for good and bad outcomes.
There is a disconnect between these two visions, and I think that it’s the latter that needs to be updated.
After all, even if we put aside the direct ways in which our genes build our bodies (encoding how our tissues grow) and instead look at our abilities to “make the right choices” and exert “willpower”, we find that those are also greatly determined by genetic factors. Identical twins probably turn out very similar in good part because they have almost identical amounts of those qualities of mind.
This doesn’t mean that all is pre-determined and that if we all stop trying we’ll turn out the same we would have otherwise, but rather that we are playing within certain parameters, and that the part we control is probably smaller than most people think (not non-existent — we still deserve some credit — just more modest).
To be clear, I’m not saying the situation was white and we thought it was black, or even that it’s a black & white thing, but rather that most people’s intuition might be the wrong shade of gray. Otherwise, I would think there would be a bigger variation between identical twins, but they spend their lives making different choices yet most stay very similar to each other (as far as I know — if you know of a study on this, please send it my way).