I have done an isolation system because doing that is a part of the
directions for the Atkins process and I decided to follow the
principles and do more than is strictly in the instructions.  As such,
among the foods allowed for Induction of Atkins at some point I
tried a week without them and then reintroduced them.

Since when I started Atkins aspartame was not yet banned for
those first two weeks, I used aspartame during Induction.  At the
time I don't recall if splenda had come out yet.  It was several
months down the line that I learned that aspartame was a problem
for many and went without for a week.  The result was a big
difference for my wife (weight started dropping, fierce headaches
started) and zero difference for me.  Sure enough, everyone has
their own reactions.

So when splenda came on the market or became noticed by
me I tried it and checked whether it had any effect.  In my case
it didn't.

So splenda *or* aspartame.  One suspect at a time in my opinion.
It's how systematic isolation systems figure out problem foods.