Here’s a quick lesson in statistics: correlation is not causation but causation is correlation.
That means, something can be related even modestly – like grass grows better when it rains – but not have much effect on causation – like rain does not cause grass to grow.
Now you might be right to say, “Well, Dr Irv, when it rains in my neck of the woods, we get more grass.” And I’d say, “Surely if you live in the dessert you don’t get much grass growing every time it rains; or when it rains in December in Boston, you don’t get much grass growing.” And you would have to agree: If there’s not grass in the soil and if it’s not dormant at different times of year, you’re not going to see grass grow just because it rains. Which means, grass’s growth correlates with rain only when circumstances are properly aligned: seasonal, soil, etc.
Well, in science and medicine, even in exercise science and medicine, when two or more factors are correlated, they may or may not actually be causal, just contributory.
So, when a person exercises by running, typically they burn more calories than they would if they didn’t run – so you’d expect them to lose weight over time. But what if they don’t lose weight over time? In fact, what if their weight stays the same or goes up? Could you say exercise causes weight gain?
No. You’d take the data from many people who’ve been running, most of whom would lose weight, and argue that, since running helped most people lose weight, proving a solid correlation of running to weight loss, running clearly does not cause weight loss in everybody. If fact, the person who gained weight, you’d presume, must have eaten more calories or reduced activities elsewhere in their daily existence, so as to actually gain weight. Which, again, would prove that running does not cause weight loss.
You’d also demonstrate that eating more calories or burning off fewer during your normal daily activities, causes weight gain even if you’re running more – proving that eating more calories than you burn is not just correlated but even causes weight gain.
That said, when you read these two articles on the relationship of depression or taking anti-depressants and other diseases, please be careful: they may be correlated but not causal.
The first of these has to do with depression and heart disease. It points out that a cardio program consistent with US guidelines for minutes/wk of activity can reduce the effects of depression on heart health. Note that when you are depressed, you often lack the motivation to exercise, so your heart loses its strength and volume to beat hard enough to push out enough blood to make you fit enough to exert sufficiently hard enough to be called “fit”.
Depression correlates with poor heart health but in and of itself does not cause it.
So, when you start an exercise program, assuming something or someone or some program supervised by some organization motivates and supports you in does so even against your will, so to speak, you, too, will get the benefits anyone else without depression would be likely to get. In that respect, exercise – forced upon you somehow – confers the same benefits to hearts of both non-depressed and depressed people. That’s good news enough to maybe get some cooperation from someone who is depressed and knows they need to exercise for heart health. (Of course, this all assumes they exercise enough, with sufficient intensity and volume to get good results; and that their medicines do not have adverse affects on cardiovascular function.)
The same can be said for those who have been on anti-depressants and subsequently develop Alzheimer’s disease (AD). As this study shows, more people who took anti-depressants ended up with AD….but nothing says taking anti-depressants causes AD. Since we really do not know what causes AD, this merely speaks of a correlation.
However, we don’t know if this is a correlation of the disease, depression, or of the drugs that are used to treat the disease, most prominently SSRI’s, with the onset of AD.
Or does early onset AD cause some people to get depressed, requiring anti-depressants, and those people subsequently show several years later to have AD but that it was not diagnose-able when they first got depressed?
I write this not because exercise can stop or cure depression, let alone AD. There are many studies showing it helps treat depression and anecdotally there are many people who exercise a lot who never get depressed or manage it without drugs by doing so. Likewise for AD – many people get depressed but never get AD and many who get AD may have developed depression well before the diagnosis of AD.
And in the end, all we can know regarding exercise and depression, and AD, is that starting and continuing an exercise program – cardio, strength, or sports – may help manage the one and defer the other. After all, it’s one of the only medical interventions that has mostly positive effects if not overdone, and few serious negative effects if appropriately prescribed.