Some problems with the digestive system make it very difficult for the body to digest enough food. Why would it be harder to exercise when you have a digestion problem like this? Your cells would not work as well because they would not have energy to break down oxygen from the air.
Your cells would not work as well because they would not have energy to break down oxygen from the air.

You would not get any of the molecules your cells need to exercise.
You would not get any of the molecules your cells need to exercise.

You would need to breathe in a lot more air because your body would have fewer molecules from food.
You would need to breathe in a lot more air because your body would have fewer molecules from food.

Your cells would not work as well because even though they would have enough oxygen molecules from the air, they would not have enough glucose molecules from food.