Because you are making a fundamental shift in energy substrate there is metabolic «re-tooling» going on so trying to train through is a recipe for failure. (vespapower.com)
Your body is constantly using a mix of energy substrates (fats, carbs and sometimes amino acids) for fuel. (onnit.com)
Just understand the order in which energy substrates are used by the body: ATP and creatine phosphate during short, intense bursts of activity (5 - 15 seconds of actual activity), glucose for medium duration activity (20 seconds to a minute or two), and fat stores once effort is extended out beyond that. (completehumanperformance.com)