Diet

That's misleading. An insulin spike does no such thing - but it does drive nutrients into cells that need it, which is why most recovery drinks should be 3:1 or 4:1 carb:protein ratio. Put simply, its the carbs that create an insulin spike to drive the protein into the cells that need it, whilst also replacing used glycogen.

If however, and this may be what you meant, your stores are fully loaded, you will indeed then begin to store excess carbohydrate as fat....

I meant both. Insulin drives nutrients into your tissues, the process of which will make you feel tired. I wasnt suggesting that taking carbs while doing vigorous exercise will make you fat/should be avoided. Just that to beware of large amounts/high GI stuf. And as you know, excess stuff will be turned into fat - but that wasnt my point. Hormones are serious business, if you get to the point where large quantities of carbs/simple sugars no longer cause insulin spikes or where that insulin is no longer having much of an effect, you are basically on your way to diabetes - if im not mistaken.

Most athletes use a mix of HIGH GI (simple sugars) and Protein post-training, intentionally spiking insulin as it drives those nutrients hard and fast to where they need to be, in particular to muscle tissue. But they wouldnt do this before or during a sports event or training. Some people (namely bodybuilders) actually inject insulin to exploit its powerful anabolic effects, but hae to be very careful in their diets not to get fat.