It's expected the US Army soldier expends somewhere around 5400 cal in high intensity environments like Combat or even Ranger School. 6,000 calories a day or beyond for the same level of exertion in a cold weather, high stress, high intensity environment.

You can absolutely burn more calories that you take in, especially if stressed and not eating right. Or if simply... overtraining because of various psychological issues. Most people are not overtraining, nor are they in combat. Most.