There has been a huge body of research in the past decade as to the relationship between the circulatory system and depression. Here's what we know:
There is a strong relationship between heart disease and depression.
Inflammation is strongly related to depression.
Many supplements that are good for circulation/inflammation are also helpful with depression, especially fish oil.
There are two reasons that come to mind for these relationships:
A poor circulatory system causes poor blood flow to the brain and more "silent infarcts". It is actually considered normal now to have many small areas of dead brain tissue on MRI! I have had several patients bring in brain MRI's showing these multiple infarcts with the explanation "my doctor said it is normal with aging." The only reason its considered normal is that it happens to most Americans. This line of reasoning could leave one to believe that being fat, having heart disease, and being depressed is "normal". When your brain is less active, it becomes less effective at inhibiting, or "turning down" your sympathetic, or "fight or flight" system. This can lead to chronic stress and depression.
A poorly functioning circulatory system and inflammation go hand in hand. For regular readers, you know that the root of heart disease is inflammation, not cholesterol, and that fish oil (a potent anti-inflammatory) is helpful in the treatment of virtually all degenerative disease (read my article on omega-3's ). Inflammatory chemicals have receptors in your brain that promote "sickness behavior"--this means lying in bed and being generally cranky. This occurs so that if you have an injury that is undergoing an inflammatory process you'll engage in behaviors that help you to heal. With chronic inflammation, however, the end-result is depression.
So here's what you can do with this information:
Learn to avoid inflammatory foods and use a high dose fish oil like the one in my store (read more on how and why here ).
Exercise--putting demand on your system is necessary to improve your circulatory system--read more on my exercise page.