Sadly, depression seems to be an ever-more common occurrence in the population. It can be attributed to many external forces such as stress, physical, mental or emotional trauma, and even child birth. More recently however science has confirmed a connection between diet and depression. The following link to the Harvard School of Public Health points that out quite clearly.
In particular foods that are known to trigger inflammation, including sweetened and diet sodas, red and processed meat, hydrogenated oils and margarine and refined grains are now all associated with depression. The risk of depression is further increased if anti-inflammatory foods and food nutrients , such as omega-3 rich fish, red wine, coffee, olive oil and carotenoid rich green leafy and yellow vegetables are uncommon or nonexistent in the diet.
I found this article interesting and further testament to the importance of good nutrition to health; physical, mental and emotional. The old adage that "you are what you eat" now has a deeper meaning, don't you think?