Losing 9lbs per week is ridiculous, no wonder you felt weak.

If you go onto a low-carb diet you are quite likely to find yourself unable to sustain intensive efforts of any great duration. But that isn't necessarily a problem, if you have the time for long rides. In my experience, riding for longer at lower intensity has the advantage of burning the same amounts of calories as a shorter, harder, effort without producing quite as great an urge to overeat afterwards.

Personally, though, I wouldn't go in for any fancy diets, or exclude any food groups - not even carbs. I'd accept the pain of having to count calories, work out my base metabolic rate (there are calculators on-line) and from there I'd be able to work out the balance between consumption, and exercise, that should enable me to lose a couple of pounds per week. If a couple of pounds per week doesn't sound like much, trust me - what you are trying to do is make changes that you might be able to sustain, and in any event, how good would you feel about yourself if in six months time you'd lost 50lbs?

Forget about sports nutrition. Eat a balanced diet. You aren't entering the Tour de France.