Body weight is one of the most highly regulated genetic traits.
This is perhaps best demonstrated by the fact that it is almost impossible to find genetically identical individuals (or monozygotic twins) with marked differences in body weight.
In contrast, it is much easier to find non-identical twins (who only share some of their genes but the same environment) with great differences in body weight.
Despite this strong influence of genes on body weight, lifestyle can very much make a difference.
This was now demonstrated by Tuija Leskinen and colleagues from the University of Jyväskylä, Finland, who after combing through thousands of twin pairs from the Finnish Twin Cohort, identified seven genetically identical (monozygotic) and nine non-identical (dizygotic) middle-aged (50-74 years) same-sex twin pairs who reported a long-term discordance for physical activity (International Journal of Obesity).
Irrespective of the genetic make up, the physically inactive co-twins had a 50% greater visceral fat area, a 170% higher liver fat score, and 54% more intramuscular fat.
This study clearly demonstrates that even in individuals who share the same genes and/or similar childhood environments, regular physical exercise can prevent the accumulation of high-risk fat over time.
Thus, whatever your genetic background or early childhood environment, it is better to be regularly physically active than sedentary (who would have guessed?).
Long-time readers of my blog have probably figured out by now that I'm a big believer that obesity is largely physiological in nature and that the behavioral issues follow or contribute to rather then cause the problem for most. The evidence that obesity is largely genetic in nature is fairly compelling as is the evidence that the factors that have lead to the obesity epidemic have survival advantage in other environments and so are probably evolutionary adaptations that are no longer to our advantage.
But I don't think that resolves individuals of the responsibility to do something about their own weight issues. After all, if you know you are genetically pre-disposed to heart disease, wouldn't you do more than the average person to keep your heart healthy and not less? You don't have to blame yourself (and put yourself down) in order to be motivated to change.
The question is: what to do? We know diets don't work. We know that exercise alone (without calorie restriction) doesn't work for a lot of people either. We know our bodies fight to maintain a certain weight and that it's very hard to convince them to let us maintain a different weight.
This article gives some hope though. If genetically identical twins can maintain different weights due to exercise differences then exercise should help us non-twins overcome our genetics as well.
But what about the studies that show that exercise alone doesn't correlate to weight loss? As usual when it comes to obesity, it's complicated. As far as I can tell, the real key to treating obesity is to prevent it. It doesn't seem, from what was reported, that the exercising twin took up exercise to lose weight. Likewise, they didn't take two obese twins and make one of them exercise. Instead exercising is something the thinner twin decided to do and the end result was not gaining as much weight as the non-exercising twin.
What if it's too late to prevent obesity? While exercising (without calorie restriction) doesn't correlate with weight loss, it does correlate with weight maintenance. And, the first step in combating obesity is to stop gaining at whatever weight you are at.
Plus, people who workout are healthier than people otherwise like them (same BMI, age, gender) who don't workout. We also know that any movement is better than nothing. You don't have to do an Ironman to be healthy. You can walk your dogs instead of letting them out in the backyard by themselves or take up salsa dancing or whatever floats your boat.
Therefore, I think the key is to chose to be a person who moves. Chose to be the exercising twin and not the sedentary twin. If it results in weight loss, great. If not, well, you're still better off than if you didn't chose to move.