At the end of this post is a link to several articles that tie our current inflammatory diet and lifestyle to the increased incidence of depression. None of the articles claim it is the only cause, simply one more factor that influences the bodies reactions, including neurological reactions.
In a simple illustration, here is how the inflammation/allergy/energy reactions work. When we eat something our body doesn't recognize, or when we inhale or ingest an allergen, our internal systems immediately target that "foreign" substance and try to eliminate it. If we inhale it, we sneeze, our noses run, our eyes water. If we ingest something our body reacts to it can cause vomiting, diarrhea, skin rashes, bloating, gas, and other uncomfortable symptoms. Any foreign or unrecognizable substance will cause an inflammatory reaction of some sort, with varying severity. These are auto-immune responses. Dealing with this inflammation is a major diversion of physical energy. You feel listless, lethargic, you have no desire to do anything, because you don't feel good.
Inflammation is at the root of virtually every disease we experience. Our modern western diet simply adds to the complications. We consume many processed foods with excess sugars and unrecognizable ingredients. We also consume many Genetically Modified Organism foods that produce unrecognizable elements in the plants and in the digested components. Food crop staples such as corn, soy, and rice are almost all genetically modified.
In addition to the high percentage of processed foods (many of which are also high in sugar), we tend to self-medicate our energy gaps with readily available processed sugar for a quick boost. Unfortunately, the boost is always followed by a prolonged crash. When we go for another sugar boost to compensate for the crash, it simply makes the cycle worse. It takes very little time for the processed sugars to enter the blood stream, but it takes a much greater amount of time for the body to deal with it. Our bodies secrete large amounts of insulin to try and extract the excess sugars and store them for future use (which our bodies never need). At the end of the day, we are exhausted, our body is still inflamed, we are heavier, and we aren't getting any better.
As humans, we have many hormones that raise sugar levels, but we only have one hormone that reduces them, insulin. The natural balance of our anatomical structure indicates that we are not designed for a high-sugar diet, yet the average person in the US consumes 170 lbs of sugar each year! In 1820, the average sugar consumption was only 5-7 lbs per person. That is a big change, and it is increasing the inflammation in our bodies.
If we want to reduce the inflammatory pressures on our body, we need to eat a less inflammatory diet. Our diet should be lower in sugar and higher in unprocessed, unmodified whole foods. Whenever possible, you should also focus on nutrient dense foods, grown in well-managed, healthy soils. Doing so will decrease body inflammation, increase health, and according to the linked articles, even reduce and reverse depression!
The first link is the first depression article we saw. It is a summary of the Nova and Guardian articles. The Guardian article seems to be the root article in the tree, with the most information.