Typically, they say that you need to train for some time until your body can start actively burning fat.
But then, to my understanding, while doing ketogenic diet, you can "reprogram" your body to consume fat in larger portions as energy source.
What does that mean for fat burning during training, does it start immediately on the rate as desired to achieve for the "fat burning training"?
The substrate of energy that your body utilizes during exercise essentially has zero impact on your body’s overall fat stores. This is because any fat that is utilized for the purpose of providing energy will later be replenished the next time you eat - unless you are in a calorie deficit.
The reason that your body uses different energy sources in the first place is because there are advantages for using more of one than another. At rest, your body is basically using fat exclusively as it’s fuel source. This is because the breaking down of fat is a (relatively) longer process that requires a good amount of oxygen to be present. As exercise intensity increases, the body uses more and more carbs for fuel. The breaking down of carbs produces energy much faster and may occur in an oxygen rich or oxygen poor environment. The problem with an oxygen poor environment (your body when you are short on breath) is that it makes energy production more difficult fat utilization will be minimized and using carbs in an oxygen poor environment produces a lot of lactic acid and other metabolic byproducts that make your muscles feel sluggish.
As a long term training adaptation that is only noticeable after many training training sessions, the body will be able to continue using more fat for fuel as exercise intensity rises. This is useful because fat doesn’t produce the byproducts that slow us down like carbs without oxygen can. Essentially it allows a person to keep going without getting tired as much. I very much doubt that such a training adaptation happens automatically while in ketosis. And while ketosis does seem be useful in some endurance sports, it is harmful to performance in basically everything else presumably because the body is lacking it’s faster energy system.
