Well first off, doctors do not know of a singular cause for depression because one does not exist. It seems that a hormonal imbalance is often present, but environmental factors, aging, genetics and circumstance can also cause depression. There are however, 'triggers' that can cause people to have depression like the loss of a loved one or drug use for example.
The discussion around HFCS is that consuming this and other so-called 'toxic' foods including trans and hydrogenated fats can interrupt normal cellular brain function and damage brain cells and tissues. There is continuing research on this and the other effects of these foods, but safe to assume that ingesting too much HFCS (which, by the way is in a LOT of foods-you have to read the labels to make sure you don't see HFCS listed as an ingredient...or better yet...avoid eating things that need labels...) is not good for you. Plus, having any sort of stimulant (which sugar is to some extent and HFCS mimics sugar) can heighten the ups-and-downs felt with depression, and no one wants that!
I personally have completely eradicated the substance from my diet and not only do I feel physically better, but I'm slimmer than years ago when I did consume this, and other, processed substances.
Yes if I eat any thing like two slices of bread a day for three days or two licorice root tablets a day for 4 days or even 6 small sushi rolls in one day I get full blown depression within a couple days of ingestion. I was yelling at my brother ever morning at work for no reason other than simple disagreements. The cloud above my head was so dark I nearly found a tall cliff to fly from. Then I did the food elimination process and found corn or corn meal products caused serious IBS and the corn extracted sweetners as in Corn syrup and fructose and glucose in food caused the depression. Now I avoid all foods with those materials but I make mistakes and will experience a mild form of anxiety and depression by eating them by accident.
I have completely eliminated HFCS from my diet, and my depression is gone.(plus I lost 80 pounds) If I eat anything with HFCS in it, I get a full blown depression for two days. I don't know why, but IMO this stuff is dangerous, and I avoid it like the plague. I have talked to others whi have experienced anxiety, depression, and insomnia from HFCS. They mask it on food labels by calling it "corn syrup", "corn sweetener" or "Corn sugar". It IMO is no coincidence that since they introduced this chemical to our foods in the Eighties, depression has increased, we now have syndromes for everything like ADHD, and ADD in children... Soemone should do reseach in whether these chemicals are the cause of it. All I know is it caused my depression, without a doubt!