I've been in some wards where severely overweight members have been denied temple recommends on the basis of "Word of Wisdom Violation". The difficulty is that we don't know whether their weight problem is due to lifestyle or genetic issues. That said, however, I believe that our lifestyle does influence our genetically-based tendencies. It just means that some people might need to make more adjustments than others. For example, I have a genetic tendency toward depression, which means that I need to monitor my own thought habits more closely than somebody else might have to. (On a related note, studies have shown that
stress is a contributing factor in weight gain, whether directly or through its impact on one's lifestyle.)
One problem is that we tend to focus too much on the "don'ts" of the WofW while ignoring the positive recommendations found there, along with learning more about healthy living. Meridian Magazine recently ran a very good
article on how Latter-day Saints have ignored much of the counsel in the Word of Wisdom and are paying the price.
In all of this, of course, the key is to focus on ourselves and how we can improve, rather than judging each other. We don't know all of the variables that contribute to weight gain. In addition, many of us (myself included) could stand to lose a few pounds and are in no position to point fingers anyhow, especially on such a sensitive and personal issue.