When discussing life quality and quantity often go hand-in-hand, or at least they are assumed to do so. And the vast majority of the time they do. Measures taken to extend your life almost invariably also enhance it's quality. More exercise and a healthier diet often results in a clearer mind, more energy and other things that are often associated with the quality of one's life. In other words, while quality and quantity in other contexts are often presented as having an inversely proportional relationship, in the case of life they typically have a directly proportional relationship. That is to say that in other contexts sacrifice of one means some gain of the other, while with life gain in one is usually considered a gain in the other as well.
However 'quality' can only fully be judged in the mind of the individual, whereas quantity can be, well, quantified. Any life decision you make has associated risks and rewards, many of which are known. For example, one study concluded that a smoker's life expectancy is 10 years less than that of a non-smoker on average. The value of our behaviors can therefore be objectively analyzed in terms of quantity of life.
The same is not true for quality, however. The reason is that although measures taken to extend life are often considered to be generally 'positive', not everyone may evaluate them on the same terms. It's possible, for example, for an individual to be so passionate about the exquisite cuisine that is the McDonald's Big Mac that he refuses to give it up, even if he is well aware that eating it will knock a couple of years off of his life expectancy relative to a healthier diet. It very may well be a sacrifice that he is willing to make. And who am I, or anyone else, to tell him that his evaluation is 'wrong'? Only he can fully weigh the value of quality against the quantity, because only he knows what it is that he values. Certainly he values his own life, but he may not value a 75 year life without Big Macs over a 72 year life with them.
Or consider a person who so abhors physical exercise, who isn't concerned with his physical appearance and who also doesn't mind the added risks associated with a sedentary lifestyle. You can educate him on the 'value' of exercise all you want, he still is not going to exercise because the utility of not-exercising is worth more to him than the value of exercising. Neither is he necessarily 'wrong' in his evaluation, given his particular values and his particular physical state or metabolism.
So then the picture gets muddied a bit. We can't simply say that all measures that lengthen life also increase it's quality, because it turns out that is not the case for all people.
Furthermore, other individuals might not inherently value longer lifespans themselves. The Big Mac lover saw the desirability of living longer, but just apprised the taste of a Big Mac to be worth the sacrifice of some years of lifespan. For him it was a trade-off. Another individual might actually prefer a shorter lifespan, up to a point. At least in the abstract way that we are talking about. Someone might genuinely, truly feel that a life of 85 years is too long, and so might take deliberate steps to ensure that his or her lifespan isn't quite maximized.
While these are certainly exceptions to the general rule of thumb for attitudes toward life quantity/quality, they can't be dismissed. I feel that much of the advice dispersed by the media and the health industry to be dismissive of the values of individuals. Though they generally have good intentions, they often slip into self-righteous demagoguery as they assume a basis of shared values. Values that often are, in fact, shared by a great many people, but certainly not unanimously. And so they become the self-anointed do-gooders, here to save our minds and bodies, even those of us who have no interest in being 'saved'.
Health professionals often assume they have 'answers' for people even when they don't know the questions that people are asking themselves. People could be asking themselves any of the following important questions on a regular basis :
* How can I put food (healthy or otherwise) on the table for my family over the next 6 months?
* How can I fix my relationship with my spouse/parent/sibling/friend?
* How can I improve educational opportunities for my children?
* Should I change careers?
* Should I buy a house or rent?
* What is something I could do charitably for my community?
* Does God exist? / Who is God? / How is my relationship with Him?
With "How can I tack some extra years onto my lifespan?" often being assigned a very low priority, or perhaps not being asked at all. Would you necessarily blame someone if such a question never or rarely arose given the gravity of everything else they could be asking themselves?
Thus I find the common wisdom regarding the primary importance of one's health to be lacking in a certain sense. In the most immediate sense of air and water, food and sustenance, yes health is extremely important, and everyone except the suicidal can agree on that point. Ditto with the importance of minimizing the likelihood of disease, or medically treating ailments as they appear. But in the sense of maximizing life expectancy or sacrificing certain comforts in order to make what could amount to minimal improvements, if they are improvements at all, to one's 'quality of life', the issue isn't that clear and depends on what we value as individuals.