When people don't get enough oxygen, they often begin to gasp for air. They may also feel dizzy and light-headed. Besides these symptoms of altitude sickness, others such as headache and fatigue may also occur.
I bet you could imagine yourself polishing off 1,500 calories of pizza and ice cream pretty easily, but 1,500 calories of just lean beef will leave you sweating and struggling to swallow in between deep breaths.