I ate vegan for more than a year and felt great and lost all the excess weight that accumulated over time. I always try to eat as healthily as possible -- strictly no junk, etc.. In the spirtit of tweaking and researching, I came across the concept of the hunter-gatherer diet which tries to mimick the way our ancient ancestors used to eat. It's basically fruit, veggies, meat & fish, nuts and eggs -- no dairy products, no grains, no starches. Low-carbs but in a healthy way.
For quite some time I suspected milk to be the main offender when it comes to energy drains and sickness and related issues. I finally started the way of eating like a caveman which is technically vegan plus organic meat, fish & eggs; sans grains and starchy vegs. I never felt better. The meat gave me an even more powerful energy boost than the initial vegan experience.
I wrote about this on my own site, including coverage of the spiritual implications of giving up eating vegan