Ruth's personal trainer tells her not to walk so fast on the treadmill that walking becomes uncomfortable. Discomfort is her body's signal that she is not getting enough oxygen to burn fat. Instead, it begins to burn the simple sugars that are immediately available in her blood, called

Question

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The simple sugars immediately available in the blood for energy are known as glucose. This is the primary source of energy that the body uses before it starts to burn fat, especially during high-intensity exercises where oxygen is scarce.

Ruth's Personal Trainer Tells Her Not to Walk So Fast