Oh also - yes, it's possible to have less energy. It actually makes a lot of sense. Say you eat a cup of table sugar, for example (doubt you do - but to make a point). Your blood sugar will spike very high very quickly. Consequently, your pancreas will release a LOT of insulin into the bloodstream. This signals adipose tissue (i.e. fat cells) to take up sugar from the bloodstream and use it to make fat, which is stored.
The problem is that the body is evolved to expect sugar intake to be slower and steadier over time. If you eat low glycemic index whole grains, for example, it takes a while to get at the sugar, so only a bit gets into the bloodstream initially, and then it continues to trickle in with time - sort of like a time release medication.
In the first case, with the cup of sugar, even after all that sugar enters the bloodstream, and insulin spikes to remove it, the insulin remains in the bloodstream. This drops blood sugar low - which gives you less energy. The body expected that the sugar that was coming in would continue at that rate for a time. The pancreas can rapidly upregulate insulin release, but once it's there, it takes some time for it to go away.
With a "time release" carb like a whole grain, insulin only goes up a bit The sugars continue to trickle in over time, so there is no precipitous drop in blood sugar - they remain roughly in balance.
This is why you should strive for low glycemic index carbohydrates. You want to avoid the sudden spike followed by the prolonged crash which increases weight gain and reduces energy.
