Whilst protein can be used for energy it is not a healthy method to use in the long term, it is more of an emergency source for the body. The reason being when protein is broken down for energy it releases an element called 'nitrogen', which needs to be removed from the body putting excessive strain on the liver and kidneys. It can also lead to a build up of ammonia in the blood stream, that is toxic to the brain. Fat on the other hand is made up of the same three elements as carbohydrates; carbon, hydrogen and oxygen, which are broken down by the body in large quantities without any side effects. Our body naturally stores excess energy as fat, so it makes a lot more sense to feed it it's natural energy source which is fat.
Another thing to consider is that if you are trying to lose fat, then you want your body to become effective at using fat for fuel, rather than glucose and protein is broken down to glucose to be used as energy.
If you are still in the mindset that 'eating fat makes you fat' then have a read of my article on the subject here to clear up any confusion.
So I hope that clears things up and if you are doing a low carb diet make sure it's high fat.
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