glycogen and fat burn in low intensity aerobic exercise

Question

I've heard two seemingly inconsistent claims about low intensity aerobic exercise (walking / zone 2 running) -

1. During low intensity activities the primary energy source is glycogen in muscles and ATP in the blood
2. During low intensity activities the primary energy source comes from burning fat

I presume both systems work simultaneously, however would like to know what the science says.

In particular, if there is a ratio (power from glycogen / power from fat burn) - what is it, does is change with activity length (maybe yes, at given power, as glycogen depletes), and at which conditions the fat burn is maximal (after 30 min? from the start?)

Good answer should include a credible reference.

Answer 1

Glycogen is the storage form of carbohydrates/glucose and as such is a relatively short-lasting but quick release form of energy. It is converted into glucose by a hormone called glucagon and is used to raise the blood sugar when necessary and provide the initial burst of energy during aerobic activity. You are correct that both systems work simultaneously but the focus is on glycogen during the first 30 to 60 minutes and once that store is depleted, you will then be burning primarily fat. Of course glycogen stores then depend on the amount of calories(specifically carbohydrates) taken in, therefore the lower your calorie intake is, the smaller the glycogen store meaning you will quicker get to burning fat.