My own experience with this has been dramatic. I was pretty severely depressed for most of my life, so doing something about it was a huge deal to me. There were a couple of books I found particularly helpful:
Depression-Free For Life, by Gabriel Cousens
Potatoes Not Prozac, by Kathleen DesMaisons (also the creator of Radiant Recovery)
The second book in particular was the one that described how some
people respond differently to sugar, in a way that can cause
depression (I'll put up another rant soon about the problems of
oversimplification, like trying to say there's one optimal diet
everyone should follow).
I'm not going to go into too many details about how to tell if you have this problem, or how to alter your diet if you think you do, since you can (and should) read the website and book to find out about that. Instead, I'll give a brief account of my own experiences.
When I was at the University of Hawaii, before I was aware of this problem, I would stay off sugar on weeknights because I had a very heavy workload (taking 21 units and working 15 hours a week), and I didn't want the crash I had after eating sweets when I had so much to do. Didn't think much more of it at the time.
It took a good year after I read the book (probably mid-2003) to actually get my act together enough to try it. I was having a phase of realizing that I actually could do something to change all the things in my life that bothered me (which was wonderful), and I knew I should tackle this big, all-encompassing issue of debilitating depression. I gave up sugar for a while in October 2004, and wound up giving in and eating candy on Halloween, and gave up on it again for a while around that time. Starting in early November 2004, I gave it up basically for good, and I lost 10 pounds in a few weeks without even trying (and another 5 since), and all off my stomach (this is the same thing the South Beach diet talks about, that fat from refined carbohydrates goes to tummy fat). While it was nice to fit into my size 8 pants again, that was nothing compared to how much better I felt. I didn't get self-destructively miserable anymore. I was stable; I could handle small upsets in proportion, rather than taking everything like a disaster. Christmas break, I decided to let myself have a few pieces of Roger's Chocolates, and it made for a great controlled experiment, since I didn't have any other big things affecting my mood like I do during the semester. Of course, I got a big high from my chocolate, but the next day, I was just up and down all day. It was completely clear to me that the sweets had been the cause.
It's been hard to stay off it all this time, I've had a lot of cravings, and I've given in and had sweets several times since. On the most recent occasion, I had 4 cupcakes (one of my favorite childhood comfort foods), and I felt absolutely awful for about 4 days. I'm guessing my system has gotten more sensitive to sugar hits, after being off it for about 4 months at the time. After that incident, it's actually been easier to stay off sugar because of the strong, visceral negative reinforcement. I liken it to jumping off things: sure, it's fun to jump off high places, and if you have a parachute, it may not even be such a bad idea. But if you don't, you wouldn't even consider it, because you know what will happen when you reach bottom. You don't even consider it because you've got a really strong gut reaction of self-preservation to actively avoid falling from heights. Jumping off short things (over a handrail, for instance), isn't a problem. I'll eat the one cookie that comes with the Baggin's sandwiches, and I'm okay. But I'm not going to eat even one if there are a bunch around, because I won't be able to stop - much as you wouldn't jump off the roof of Gould-Simpson and plan to swing in a 9th-floor window.
The other big thing for me in getting rid of my depression was getting off hormonal birth-control pills. I experimented with going back on them for a while, and it was just as definite as the sugar. It took a few weeks to set in, and over a month to work its way out, so it's a longer timescale, and therefore harder to detect, especially if you're not paying attention to it.
This is one of the things I find most upsetting about all this: if you had asked me, when I first started taking the pill at age 18, if I had any side effect, I might have attributed a headache or two to it, but nothing else. Same for sugar: I'd have said I don't like the lack of energy after I crash, but nothing more than that. The average person is not aware enough of their own state to be able to make good judgements about how to treat themselves, and I'm sure this includes me too, even now, for everything I haven't had specific and direct reason to pay attention too. The pill is prescribed for all sorts of frivolous things: acne, cramps, even just making the cycle more regular (who cares??). We give sugar to children without even thinking about it. The average American teenager drinks over 600 sodas a year (about 2 a day). One soda alone is more sugar than even a healthy person should consume.
The other big thing I find upsetting is how people don't take any of this seriously. People still want to believe that depression is a weakness, and you should just ignore it or will it away, and certainly not talk about it. They certainly don't think sugar could be something so big or bad or important. Why is it so hard to believe that what you eat, the only things your body has to work with to make you and fix you other than air, can have a big impact on you? I don't know, maybe you could come up with some psychosomatic explanation, but why not accept that the brain has disorders too? If some one has a sprained ankle, you don't just tell them it's a weakness and tell them to keep running on it. You do something about it. You treat it. You go find the root that you tripped on cut it out. You don't keep doing exactly what you were doing before, ignore it, and hope it will go away on its own.
So, I have a screwed up body-chemistry, and I get mad about it. I'm jealous of people who can eat sweets and not feel like s**t afterwards. But at least I know and have found how to work with it. I feel really bad for all the millions of people out there, who have the same sensitivities I do, and are as unaware (and miserable) as I was a couple of years ago. Now there's something to take on for a life's mission....
Whatever the reason, I've been enjoying my newfound freedom to eat sweets and feel only a mild dip in energy, although, at this moment (January 2007), I'm actually off sugar again, for the pure vanity of wanting to lose weight. I do, however, have more energy, and feel sort of "clearer," so it does still have real physical benefits.