From what I understand the vast majority of people have very low metabolic health and their ability to use fat as energy during exercise is not well developed. Meaning they are almost exclusively using glucose and glycogen.
From personal experience, when i was untrained, i could feel if i was fasted or not. Additionally if i did not eat some form of carbs 40 minutes into my workouts i would crash and burn but if i had a small gel pck i could go for another hour. I dont know the mechanism for how to glucose would even be absorbed that quickly but i did feel a noticeable boost in energy as soon as i consumed the gel.
I think there is some connection between the digestive system and the brain that signals that you can use more glucose immediately after consuming food.
This can be replicated without consuming any glucose, by simply washing something carbohydrate around the mouth. It's called the "Carbohydrate Mouth Rinse"[1].
If I can summarise, from memory, it tricks the brain into thinking more fuel is being added so it's okay to release more of the stored fuel it was holding back.
> Routinely rinsing a carbohydrate-containing solution around the oral cavity for between 5-10 s has been shown to improve high intensity endurance performance lasting 30-70 min.
> The magnitude of the performance benefit is similar to that of carbohydrate ingestion, although athletes who are prone to gastro-intestinal (GI) distress during high intensity exercise may want to consider rinsing because of the reduced risk of GI discomfort.
> These effects appear to be specific to carbohydrate and independent of taste or sweetness.
From personal experience, when i was untrained, i could feel if i was fasted or not. Additionally if i did not eat some form of carbs 40 minutes into my workouts i would crash and burn but if i had a small gel pck i could go for another hour. I dont know the mechanism for how to glucose would even be absorbed that quickly but i did feel a noticeable boost in energy as soon as i consumed the gel.
I think there is some connection between the digestive system and the brain that signals that you can use more glucose immediately after consuming food.