some questions.
With the 40 minute to 1 hour workouts are you exercising close to exhaustion? If so it is likely that you are running close to glycogen depletion.
there is (I believe) some misconceptions about glucose/fat metabolism during exercise.
your muscles will predominantly burn fat whenever they can. purely aerobic exercise will primarily utilise fats for energy. As exercise intensity rises and you move into the lactate zone (typically above 65% MHR), your glucose utilisation increases. Lactate is the end product of the pyruvate pathway, and pyruvate comes from glucose. High intensity exercise uses fast twitch muscles fibres to provide power, and unlike slow twitch muscle fibres which rely on fat, these rely heavily on glucose for energy. Physiologically I believe the primary reason is that the strong contraction of muscle fibres impedes blood flow, and the body becomes unable to deliver enough oxygen to completely burn fat.
The body's total glycogen supply is around 45 minutes running at 85% MHR (or thereabouts) - after this the individual becomes exhausted and is no longer able to maintain the intensity.
However got to agree with Bluesky - you only actually need to take enough glucose to get through the 45 minute workout, and ensure that you replenish your muscle glycogen.
Post exercise snacks are thought to be beneficial in this regard.
The other question concerns regime - are you on insulin? If you are diet controlled only then low-carbing will have no other effect than cause you to become tired if you fail to replenish your body's glycogen. If you are on insulin failing to replenish your glycogen reserves can have fatal consequences.
I found this rather small study somewhat amusing...
[Swimming for 12 hours leads to no reduction of ad...[Schweiz Rundsch Med Prax. 2007] - PubMed Result
The athlete in question manged to lose 1.1 kg of muscle mass after swimming for 12 hours, yet his fat mass was unchanged - hmm I wonder which fuel his body was using?
