been on a low carb diet (KETO) since January and am in the best shape I have been in for years.
The first week or so sucked--I don't typically eat a lot of donuts, but I was freaking dreaming about them. I finally figured out that I needed more natural fats from avocado, butter, etc. and when I get a good balance of those, I feel great.
The cool thing about Ketosis is that you can track your progress with urine testing strips which can reveal if your body is burning its own fat or carbs:
http://en.wikipedia.org/wiki/Ketogenic_diet