When I drink caffeine on Atkins, I find that I get a much smaller "up" from it, and I crash badly. Caffeine, as I understand, works by causing a release of sugar into the blood, raising your energy level. If, as on Atkins, your sugar levels are altered, caffeine will have far different effects than it would normally. I also find that I crave sugar badly in the afternoons after a big cup of coffee in the morning.