Exercise And The Limitations Of Observational Studies ? CardioBrief

Last week I wrote twice about exercise.?Strictly speaking, both stories were complete lies.

The first story was about?a?study published in the?Lancet?which analyzed data from more than 10,000 patients at 2 VA Medical Centers and found that patients with high fitness levels were less likely to die than patients with low fitness levels. The pattern held true whether patients were taking statins or not taking statins. The researchers concluded:

?Statin treatment and increased fitness are independently associated with low mortality among dyslipidaemic individuals. The combination of statin treatment and increased fitness resulted in substantially lower mortality risk than either alone, reinforcing the importance of physical activity for individuals with dyslipidaemia.?

Look carefully at that. The?association of fitness and low mortality leads to the recommendation about ?the importance of physical activity.? An?accompanying editorial?went further, recommending that??prescription of physical activity should be placed on a par with drug prescription.? Widespread media coverage of the study followed suit, with nearly all reports emphasizing the positive effects of exercise.

So what?s wrong here? It almost seems churlish to insist on the point, but of course the study (like all other observational studies) didn?t? couldn?t? actually say anything about the real effect of exercise on health. It seems reasonable to assume that more exercise leads to increased fitness leads to improved health. That?s what we all probably think and believe. It?s common wisdom. But it?s not entirely unreasonable to suppose that healthy people are much more likely to exercise, in effect reversing the cause and effect. And of course there may be other confounding factors that cloud the simple equation of exercise and health.

There?s more: even if you could prove that more exercise leads to better health that wouldn?t lead to an automatic conclusion that doctors should recommend exercise as much as drugs. First you would need to prove that an exercise prescription is just as effective as a drug prescription. It?s hard enough to get people to take inexpensive, life-saving drugs once a day. Is there any reason to think we can get any kind of effective level of compliance with an exercise prescription?

The other exercise story from last week had a much different conclusion. Editorialists in Heart?concluded that although most exercise is good for you,?too much?exercise can actually be harmful.?They may well be right, but there?s never been? and there almost certainly never will be? a good, properly designed randomized trial that could prove this theory. It?s entirely possible (in fact, almost certain) that people attracted to extreme endurance sports are different in many ways from the rest of us, and it?s quite plausible that some of those differences may have significant effects on health. Confounded again.

The last thing I want to do is trash exercise. I?m a big supporter of it. But these standards shouldn?t be compromised. So what is the proper way to report this kind of study? Here?s a brief, completely responsible version of the story (written by Kelly Young for?Physician?s First Watch):

Exercise Plus Statin Therapy Associated with Reduced Mortality

Physical activity and statin use, both independently and combined, are associated with reduced mortality risk among patients with elevated cholesterol, according to a?Lancet?study.

Researchers prospectively collected data on roughly 10,000 veterans (mostly male) with dyslipidemia who underwent exercise tolerance tests. Over a median follow-up of 10 years, statin users had a mortality risk of 18.5%, while nonusers had a mortality risk of 27.7%. The mortality risk was reduced among the fittest participants, both among nonusers and users of statins.

The authors conclude: ?Improved fitness is an attractive adjunct treatment to statins or an alternative when statins cannot be prescribed. The low exercise capacity (roughly 7 MET) associated with the aforementioned health benefits is clinically significant and reinforces the importance of physical activity for individuals with dyslipidemia.?

This works well for a professional readership, but it?s hard to imagine how this could be reported to consumers in a responsible way. Every story about an observational study would need to turn into a lesson in the limits of such studies, though this would no doubt please Gary Schwitzer and his great reviewers at HealthNewsReview.

I discussed this issue with a distinguished cardiologist known for his scientific rigor (in fact, he pointed me to the?First Watch story as a model for coverage of the Lancet study). He agreed that the?Lancet study supported ?the idea that fitness is a good thing for longevity? and agreed that ?it seems reasonable to encourage fitness.? But, he observed, ?prescribing exercise is not without direct and indirect costs for doctors (who use the short time with patients to focus on that issue rather than something else) and patients (who may then allocate time for exercise, which costs them time and maybe dollars). The question in the end is how confident can you be that it matters for life expectancy.?

I don?t have a solution to this problem, just a few observations. A reasonable recommendation to people to exercise more need not require the same level and depth of evidence as a recommendation for a drug or a device. Although it?s certainly true that exercise has its own risks and dangers, these are risks that most people can intuitively understand. The same can not be said for drugs and devices.

I?m also not convinced that prolonging life is the best reason to exercise. Fitness may be its own best reward. By definition, fit people are well suited to their environment. That strikes me as a reasonable goal. Is there anyone who would argue that being unfit is desireable?

Source: http://cardiobrief.org/2012/12/03/exercise-and-the-limitations-of-observational-studies/

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