Final answer:
During physical exercise, glycogen is the nutrient that gets depleted the most rapidly, supplying energy for up to 90 seconds of high-intensity activity before the body starts using other sources like fatty acids and proteins.
Step-by-step explanation:
The nutrient that becomes depleted most rapidly during physical exercise is glycogen. Glycogen is a form of carbohydrate stored in our muscles and liver and is utilized to make adenosine triphosphate (ATP) through glycolysis. Though creatine phosphate can supply energy at a high rate, it is only for a brief period, about 8-10 seconds. After this, glycogen becomes the primary source of energy, producing ATP for up to 90 seconds of high-intensity activity without the need for oxygen, leading to the production of lactic acid as a by-product. During prolonged physical activity, the body transitions to fatty acids and proteins as additional energy sources once glycogen stores are depleted.