But the experimental design was simply this: Trained runners were switched to hi-carb or hi-fat diets for 4 weeks, then VO2 max, time trials, and other running related measurements were taken. Also, small pieces of muscle fiber were taken out before and after. Result: the change in diet produced (a) no statistically significant alteration in athletic performance, and (b) the hi-fat group, over 4 weeks, significantly increased the number of fat burning enzymes in their muscles.
Obviously such a study can't be done "blind", as the athletes know what they're eating. Nevertheless, the results are interesting. If anything, a placebo effect would have favored the carbs, since conventional wisdom at the time was, "runners should carbo-load".
Prior, similar studies had claimed to show that going high-fat hampered running performance, but, those studies were of much shorter duration. The authors concluded that it took several weeks to fully switch over the enzymatic pathways. But the pathways are there, and can be turned on at will.
Turning "fat into muscle", on the other hand, is pure marketing B.S.