Watching the best athletes in the world in the Olympics, with their fit and trim physiques, has me thinking about exercise. We all know it is great for cardiovascular health, but what about for mental health? Does swimming five miles a day put Michael Phelps in a good mood?
An article in this month's "Archives of General Psychiatry" suggests that, as we might expect, regular exercise is associated with reduced symptoms of depression and anxiety.
But there is a bit of a surprise: The researchers also found that exercise did not appear to cause decreased depression and anxiety. Instead, common genetic factors seemed to underlie both a predisposition to exercise more and a predisposition to experience less depression and anxiety.
The study took advantage of a Dutch sample of almost 6,000 twins. For any given individual, exercising more when measured at a given time point correlated with fewer mood symptoms at the same time point.
But for identical twins, who are genetically the same, more exercise in one twin predicted fewer mood symptoms in the other. This did not hold for fraternal twins, who are only halfway similar genetically.
It was also noted that for any individual, more exercise at one time point did not predict fewer mood symptoms at a later time point.
The message: happy people are exercise-friendly people.
But what about the "runner's high"? Many people have heard about how exercise increases beta-endorphin, a natural opiate in the brain that is associated with feeling good. A study published earlier this year that scanned the brains of runners did show increased release of this natural buzz chemical and a correlation of its release with perceived euphoria.
There is also evidence that exercise can increase levels of the brain chemicals serotonin and norepinephrine, neurotransmitters which have long been known to play a role in mood regulation. The antidepressants all boost levels of these same chemicals.
A group at Yale University led by Ronald Duman examined the "runner's high" in running mice. In a paper published last year in Nature Medicine, they observed that a week of exercise in mice led to changes in the expression of genes in their brain, including an increase in one called VGF, which promotes growth of brain cells.
They then went on to show that injecting the VGF gene product into mouse brain produces an antidepressant response in the animals.
So doesn't this suggest that exercise should help people's moods? There have been many studies that have experimentally tested the hypothesis. They have compared groups of depressed patients assigned to an exercise regimen to others getting treatment as usual or placebo.
One review found that eight out of 18 of these studies showed a 50 percent reduction in depressive symptoms in those who exercised. Another review of 14 studies found that exercise did appear to reduce depressive symptoms, but the quality of the studies was not sufficient to be confident in the conclusions drawn from the data.
The trouble with prescribing exercise to depressed patients is that they are often too depressed to want to exercise. It can be like prescribing reading exercises to strengthen the vision of a blind person. With no energy or motivation, a severely depressed person may have no chance of being able to follow through.