Twin Studies are often more feasible (and sometimes more ethical) than a random control study. A Twin Studies can have results that qualify as "interesting" even with just a dozen or two groups of twins; a random control study on the same sample size would be discounted for its small p value. And with a Twin Study, you can sometimes do post-hoc analysis on years of effect, without waiting the years for those to occur.
This study may even be one of the ones that's unethical to do in a random fashion. We know that lack of exercise is bad for your health; telling people not to exercise for a year could be seen as doing active harm to study participants.
Of course, Twin Studies suffer from some issues on their own. I would prefer random-control tests where they're possible. But sometimes Twin Studies are all that you're going to get. And sometimes the Twin Study of an effect is the economic small-scale pilot that generates interest and funding for the "real" study, and there's nothing wrong with that.
This study may even be one of the ones that's unethical to do in a random fashion. We know that lack of exercise is bad for your health; telling people not to exercise for a year could be seen as doing active harm to study participants.
Of course, Twin Studies suffer from some issues on their own. I would prefer random-control tests where they're possible. But sometimes Twin Studies are all that you're going to get. And sometimes the Twin Study of an effect is the economic small-scale pilot that generates interest and funding for the "real" study, and there's nothing wrong with that.