Rog, I'm not suggesting that I'm right or wrong, just that perhaps you need to know how you react to
different ingredients before you can "blame" one over another for a spike. I'd have assumed the combination of carbs in that
whole slice were the cause rather than the icing on its own, unless I knew whether I reacted more to a particular ingredient. The cake itself will have included both starchy and sugary carbs, (flour, sugar
and the jam filling) and it would also depend on whether it was a fatless sponge or included fat which would cover the effect of the carbs a little. It's worth knowing how your body reacts to the different types of carbs - you then have the option of being selective over your choice of higher carb "treats".
Nothing's ever simple....
Robbity