Dear Andy, 

fat oxidation reglation is definitely sensitive to CHO ingestion. However, the question becomes much more complicated if You want to know exactly how strong is the fat metabolism inhibiton and at what level it occurs. One thing is that the kind of exercise you are talking about is very important, the insulin concentration in the blood follows somewhat of a U curve with high concentrations at both very low and very high intensity. Insulin inhibitory effect on adipose tissue triacylglycerol lipase and hormone sensitive lipase seems to be very important and potentially the rate limiting step in fat metabolism at mentioned exercise intensities. CHO ingestion and rise in circulating insulin can also cause downregulation of fat transport across muscle membrane and limit the fat availability and oxidation in this way too. As I said earlier ,the precise answer is very complicated, because of the interaction of various other endocrine signals and intracellular regulatory mechanisms, but I believe that, the insulin increase would very much point at the possibility of fat oxidation decrement during CHO ingestion. 

I hope this articles will be useful:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4008806/

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3253005/

Best regards,

Jan

Hi Andy, Just seen your question - a good starting point is to look at the paper by Horowitz et al 1997. They had participants ingest 0.8g/kg carbohydrate as glucose or fructose and observed blunting of lipolysis and subsequently lower fat oxidation. However, the picture is not so clear cut as some studies do not show such marked changes in fat oxidation with carbohydrate feeding which likely relfects that insulin response or potentially differences in use of IMTGs. Cheers, Stuart.