No matter what you eat a meal your body is going to store some as fat to use it up later.
Just because some gets stored as fat after a meal doesn't mean it can't be broken down for energy later. In fact storing energy as fat then breaking it down isn't 100% efficient and results in some energy loss.
Insulin spikes. Unless you are do diabetic you won't have a problem with that. Your body regulates blood sugar in a fairly narrow range. If you are getting huge swings in blood sugar causing highs and crashes go see a doctor.
I agree that the human body is incredibly efficient in storing fat but it does matter what you eat and insulin just corresponds to that as it responds in a bigger way to processed calories or those high in sugar than it does to the same calories from whole foods which are more resistant to digestion than processed foods .
Insulin spikes are very important to factor into what type of calories are eaten as there are many many skinny people who are pre diabetic or diabetic and have obesity type illnesses . Which means that calories are not equal .
Personally Im not having spikes or crashes but have seen good results from cleaning up my diet without having to change too much
