Razor said:
But doesnt it give you as sustained level of energy unlike sugar which spikes and dies.
No, that's the point I was pointing: baked potato gives you even
more of a sugar spike than sugar does. If you want something that
doesn't give you a short-lived blood-glucose rush followed by a blood-glucose crash, you need something that actually
does have a low glycaemic index. People wrongly assume that a "complex" carbohydrate must also have a low GI, but it simply isn't true.
Incidentally, if you're planning on doing a lot of aerobic training, you should eat fat as well. Muscle comes in two sorts: fast muscle, and endurance (or "slow") muscle. Fast muscle is good for one-off explosive force, while endurance muscle is good for repeated actions. So a sprinter, for example, or a long-jumper, or weight-lifter, would focus on building fast muscle (lots of resistance training - weights, etc.) while a marathon runner would focus on optimising endurance muscle (whole-body aerobic exercise).
Fast muscle burns glucose, so sprinters and bodybuilders need lots of carbohydate for energy. But endurance muscle actually burns the products of fat digestion (fatty acids and glycerol) rather than burning glucose - so good aerobic performance comes from having fat in the diet as well.
One of the things that happens when a person gets fitter is that the ratio of fast to endurance muscle in the body shifts (annoyingly slowly) towards a larger amount of endurance muscle. This is one reason why you need to take it easy if you're not very fit and just starting exercise: most of an unfit person's muscle is fast muscle, so exercise causes an almost immediate blood-glucose crash, and he feels dreadful. As he gets fitter, more of his muscle is running on fatty acids and glycerol, so exercise no longer causes low blood sugar.
Obviously most good training regimes will include both resistance and aerobic training - even long-distance runners need a sprint finish - but don't neglect the fat. (Aside from anything else, including fat in the meal helps to lower the effective glycaemic index of everything else you eat at the same time - you also can't absorb a number of vitamins without it).