Wednesday, April 7, 2010

Pull up or pull out?

Well, here's another stunner. After all this time, we still don't know what amount or what kind of exercise is best. With all the money, marketeering, and other scientific interest involved in understanding what makes us sick, what we know is that some exercise is good for you...but not too much. And older people don't like vigorous exercise.

How could it be that after billions of dollars in research (and promotion) over many decades, we don't know? After all, you can find routine comments about the virtue of exercise in, say, Victorian novels (and, though we haven't looked, probably in Hippocrates 2500 years ago).

This reflects the serious if not fundamental problem with epidemiology, a problem at least as serious as making causal inference in genetics, where at least DNA segments can be objectively identified. In our posts last week on reductionism, we discussed the way that science 'tunnels' through complex reality by reducing studies to isolated variables to identify their causal effects on some measured outcome. We noted that in genetics this has provided a phenomenally successful way to discover many things that genes do. But it leaves the genetic contribution to most interesting traits, known by a catchword as 'complex', poorly accounted for in any specific way.

In environmental and lifestyle epidemiology, that deals with the common causes of disease and death in the industrial world, even tunneling (with all its limitations) is exceedingly problematic. That's at least in part due to the fact that exposure variables, unlike genetic sequences, are poorly defined and measured. 'Exercise' or 'diet' are examples, but so are many other aspects of modern life. This is especially true when the fact as well as the amount and duration of exposure are needed. Who can remember what they did, or ate, decades ago--all the way, perhaps, back to early childhood? At least, we're exposed to our genes from conception, and infectious diseases often are clear-cut point-causes where a single exposure can do the trick: one sneeze from someone with the flu is enough.

Besides the fact that it's hard to isolate individual variables, such as diet or exercise, in a cogent experiment, the variables of interest are heavily confounded, that is, diet is correlated with many other aspects of our lives, measured (e.g., education, income) and unmeasured. The same is true in genetics (e.g., what's called 'linkage disequilibrium' or 'epistasis', for those who know genetics), but easier to at least think about if not measure (though by no means trivial). But if eating chocolate rather than, say Supersize sodas, is correlated with exercise, and because these are both correlated with social status, jobs, education, neighborhood, race, and so on, then it is nearly impossible to isolate one of those variables. Confounding is a very serious, if not lethal, problem for application of 'tunneling' approaches in epidemiology, even if they would work. But because of complex causation, it probably wouldn't.

We've previously posted about the problem that preventive screening as in mammography to detect breast cancer may over-diagnose and over-treat lesions that would go away on their own or turn out not to be cancer after all. Studies showed that those not screened had fewer deaths from cancer than those who were screened. This has both monetary and psychological costs. It challenges various vested interests, too, so it's not surprising that we need yet another study (with its costs, too). So the latest U-turn in this saga is getting headlines, threatening policy changes (or no-changes), because it defends screening. And a BBC report interviewed a Scandinavian expert who bluntly ridiculed the current study as patently wrong. We can't judge without attempting to read the latest studies, and their skeptics, which we haven't done. The statistics are subtle and one can probably find in them what one wants to find. But the point here is the problem of uncertainty, even in heavily studied issues, is what concerns us here.

There's no easy answer, but the fact that there's a problem is clear. Candid epidemiologists who recognize this (privately, of course, because doing so publicly will jeopardize their funding changes, as we have been told many times) often turn in frustration to genetics. They believe that we have The Truth because genes, being molecules and the blueprint for life, have to be more tractable. Well, sorry, but it's not so simple on the genetic side of the fence either.

Dealing with complexity is a challenge. Somehow, the latest finding still makes it into the news, even though we really know that next week or next year the reverse will be announced. When and how we'll start making better progress is anyone's guess.

Meanwhile, do your pull ups, but don't pull out of your health club membership.


Texbrit said...

One thing your essay brings to mind - as well as the recent studies showing conflicting or even counter-intuitive results vis-a-vis older studies, on subjects such as vegetables and exercise - is the obsession we have with extending our own lives through modified behaviour. Does any other animal do this? We try to whiddle the stick ever-finer, hoping to outfox death by a few more years if only we ate more peas or ran the extra mile. Whilst it is true that performing self-harm (smoking, for example) will not do you any favours, maybe people (and science?) should spend more time focusing on quality, not quantity, of life!

Texbrit said...

Also, if we look at common animals -- we may have all seen or known a massively obese cat or dog who died prematurely, presumably due to a life of over-indulgence (I had one!). But have any of us ever observed informally that an unremarkable animal had a longer or shorter life based on how often its owner walked it or what brand of pet food it ate? Which has more anti-oxidants anyway, Alpo or Purina? The fact is, the animal - if not hit by a car - will meet its demise between, say, 12-15 years, and we just accept anything within that range without quarrel or second-guessing the quality of our custodianship. People too, I might suggest, should abide by this philosophy?

Anne Buchanan said...

Yes, and it's rather an elusive pursuit anyway, as though we have a maximum lifespan potential that is attainable. Conflicting results are due, we think, to the fact that we may be circling around some truth, but we don't (yet?) know how to actually find it. That is, causation is too complex and heterogeneous for current methods. Until we do, everything in moderation -- and enjoy!

Ken Weiss said...

I don't think we're basically much different from other species in this regard. We do what we can to survive--we avoid danger and seek food.

But because of our symbol-based culture, an ability we have developed far beyond other species, we have abstract ways of viewing survival, danger,and so on.

Religion allows us to imagine ultimate perpetuity and safety, even to the point of being willing to go to war or commit suicide.

But clearly our culture has, within living memory, moved towards ever-more materialistic, short-term self-satisfaction. These things seem always to have been around, but our degree of short-term crassness is not the way everybody in the world, or even Americans at other times, have been obsessed.

Ken Weiss said...

I'm sure these things are very well known both by those who do experimental studies with model organisms, like the mouse but also other mammals including primates. Zoo personnel and researchers clearly must also realize these things.

Pet and agricultural food companies like Monsanto, Purina, and so on clearly do research on these areas because there are important economic implications.

So I think it's well known that we aren't the only ones who become fatted cows.