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Comment by nidieunimaitre | Hacker News Reader

I always wonder to what degree this kind of study might invert causality: that is, people who are subclinically "ill" may be less likely to exercise in the first place because it is harder or less pleasant for them. Someone with, for example, undiagnosed but significant cardiovascular disease (unfortunately very common: in many cases the first sign anyone recognizes as concerning is a heart attack) is likely to have some degree of decreased exercise tolerance and discomfort at longer durations and intensities -- and thus will be less likely to meet the recommendations.

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You're not the first to wonder about this. http://www.ncbi.nlm.nih.gov/books/NBK62370/ "It should be noted that while these results may reflect the effect of exercise patterns on mortality, reverse causation is also possible—that the results reflect instead the effect of health on physical activity."

But what if you studied people who made a decision to change their exercise habits, and checked if it made a difference in general health?

That would seem to be subject to the same basic problem; whether people make a decision to add exercise may well be strongly affected by their experience with activity eighth may be affected by the same kind of subclinical condition that itself is a mortality predictor discussed up thread.

Of course the potential for such inverted causality is a cause for developing and testing hypotheses about the alternative explanations, not a reason to casually dismiss the study or the more obvious conclusion on the causal relationship.

Note that the biggest reduction in mortality was found in people who walked for an hour a day. Yet that group probably contained the most of what you call "subclinically ill". After all that's one of the easiest forms of exercise and there is a higher likelihood it will be chosen by people with health problems, diagnosed or not.

Or indeed other causes which could create an accidental correlation. The more you eat the fatter you're likely to be, the fatter you are the harder exercise is, the less of it you're likely to do. The more you smoke, the more out of breath you'll get when exercising, the less of it you're likely to do.

These are two examples I personally feel contribute to my not getting as much exercise as I did 10 years ago. If I die early is it that I haven't exercised enough, or that I had an unhealthy diet and smoked a lot of cigarettes? (I'm not saying lack of exercise doesn't contribute, just that picking it out on its own doesn't necessarily show the full story.)

Additionally, given how well-known the basic concept of "exercise is healthy" is, I would imagine there would likely be some correlation between people who exercise specifically for that purpose (rather than because they enjoy it) and who also take other steps to stay healthy (whether that's having a good diet, or taking vitamins, or.. whatever).

A well-run study should control for weight and smoking. But the undiagnosed cardiovascular disease can't be controlled for. Exercise is much harder for me than for others, for example, and I don't know why.

You can only control for weight and smoking if you have good data, which is to say that at least a significant portion of the weight and smoking behaviours are not causally linked to the exercise behaviour. If the data is not sufficiently diverse, no amount of statistical power will overcome the garbage-in garbage-out nature of regression.

"Exercise is much harder for me than for others, for example, and I don't know why."

genetics can partially account for the lack of improvement after training, one specific gene "alpha-actinin 3 (speed gene)". The other component is environment.

    Why is it that some people respond better 
    to training than others? Are you a sprinter 
    or a marathon runner?

http://www.abc.net.au/catalyst/stories/4132778.htm

Do your lungs/throat clog up or shut down? It could be asthma, or, more interestingly, subclinical cystic fibrosis. (CF requires a mutation on two genes, and it's very apparent even from childhood when you have the full form; but there are phenotypic differences in people who only have one of the mutations, such as being resistant to some parasites which are blocked by mucosal thickening(!). The implications of the single-mutation variant on health, though, are AFAIK completely unexplored in medical literature.)

No, my heart rate goes up (like everyone else's) but doesn't come back down for a while (10 minutes?). I have no endurance and resting between workouts doesn't recharge me.

Random reply from someone on the internet: Do no take this as medical advice :-)

Obviously, Your two sentences is probably not enough for me to help you, but I'll give it a shot anyway. Having a high heart rate that you can sustain is, in my experience, actually good for endurance sports. In fact, my father pointed out the other day that he just can't get his heart rate up any more (he's 73 and still rides his bike over mountains). This means he can't get enough oxygen into his muscles to keep a high pace. As I've grown older, I've also noticed this ceiling that is limited by how high I can sustain my heart rate and for how long.

So, rather than assume that your observation is the problem, I think perhaps you might have something you can work with, whatever your underlying problem actually is. I've done sports my whole life (running, a small bit of weight lifting, karate, and now cycling). Especially now that I'm getting older, I need to concentrate on "base miles" on my bike. This means going out at a very slow pace (annoyingly so, for me) and riding for very long time. Cycling is good for this as it is relatively easy to dial the pace to whatever you need -- just make sure to have a lot of low gears if you tend to have problems.

I suspect that if you ride a bike at a very slow pace for an hour a day for about 6 months that you will find a really marked difference in your ability to exercise. Keep your pace so that you could have a conversation easily and that you aren't out of breath. If you run into difficult situations (hills, wind, whatever), just go into a lower gear. You may need to go to a bike shop to get especially low gears, but it won't be outrageously expensive.

Hope that helps.

Completely anecdotal but low blood sugar does that to me.

Having a good sleep and some sugar before improves the condition.

Hypertension, perhaps? Or some sort of adrenergic overproduction that could be treated with beta blockers.

3% of white people in the U.S. are affected to varying degrees by a genetic disorder called a Alpha 1 Antitrypsin Deficiency. I found out I had the moderate form(SZ) from 23andme.com. I've never been able to exercise as well as anyone else.

It is a genetic liver disease that causes harm to your lungs.

Anecdotally, yes, I think you may be on to something.

For example, I suffer from persistent but relatively mild skin problems which make getting the right amount of exercise difficult without causing more pain.

I know I walk less than I should do, but I can't do more without causing more problems for myself.

Causality often gets confused particularly by the time the results make it to the popular press. Studies of alcohol intake against mortality often show the effect. Scientists find drinkers drop dead slower than non drinkers, the the newspaper reports it as a glass of wine a day protects your health. The reality is people like drinking but you've got to be reasonably fit to take the damage it causes.

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