If you do IF you may be a little worried about training on days you haven't eaten. I've done it and haven't found it a problem even after a 24 hour fast. There is all sorts of science about training in a fasted state (or a carbohydrate restricted state which is similar.......)
In any case this study might be of interest - short-term training elicits similar adaptations in peak O2 whether carried out in the fasted or carbohydrate-fed state. Obviously normal caveats - e.g. the abstract doesn't explain what "fasted" means.
Matt had some posts a while ago reflecting on the fact that for a hunter exercise would generally be in the fasted state - i.e., when you ran out of food and got hungry you had to exercise (hunt) to get some food. Hence exercise would often be in the fasted state - so it is something we should be adapted to.
Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake
Skeletal muscle gene response to exercise depends on nutritional status during and after exercise, but it is unknown whether muscle adaptations to endurance training are affected by nutritional status during training sessions.
Therefore, this study investigated the effect of an endurance training program (6 wk, 3 day/wk, 1–2 h, 75% of peak O2) in moderately active males. They trained in the fasted (F; n = 10) or carbohydrate-fed state (CHO; n = 10) while receiving a standardized diet [65 percent of total energy intake (En) from carbohydrates, 20%En fat, 15%En protein].
Before and after the training period, substrate use during a 2-h exercise bout was determined. During these experimental sessions, all subjects were in a fed condition and received extra carbohydrates (1 g·kg body wt–1 ·h–1). Peak O2 (+7%), succinate dehydrogenase activity, GLUT4, and hexokinase II content were similarly increased between F and CHO. Fatty acid binding protein (FABPm) content increased significantly in F (P = 0.007). Intramyocellular triglyceride content (IMCL) remained unchanged in both groups.
After training, pre-exercise glycogen content was higher in CHO (545 ± 19 mmol/kg dry wt; P = 0.02), but not in F (434 ± 32 mmol/kg dry wt; P = 0.23). For a given initial glycogen content, F blunted exercise-induced glycogen breakdown when compared with CHO (P = 0.04). Neither IMCL breakdown (P = 0.23) nor fat oxidation rates during exercise were altered by training.
Thus short-term training elicits similar adaptations in peak O2 whether carried out in the fasted or carbohydrate-fed state. Although there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training, fat oxidation during exercise with carbohydrate intake was not changed.