[sergiosgc]

Your body can get energy/calories from carbohydrates, fat or protein. Whatever is available. You absolutely need proteins. Between fat and carbohydrates, the body can compensate. There is a clear limit on the ability to process carbohydrates.

[dredmorbius]

Minor nit: essential fatty acids must come from diet as they cannot be synthesized directly. These are alpha-linolenic acid and linoleic acid, with docosahexaenoic acid and gamma-linolenic acid as conditionally essential under some conditions. Though required quantities are minute.