If fat is stores of energy, then why aren’t fat people more energetic than skinny people?
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In a starvation situation, people with more fat reserves will be more energetic as their reserves will last longer than someone with less body fat.
Your question presents a false premise, that body fat makes you energetic. It’s stored energy, not magic energy sauce.
Fat has more than twice the number of calories per gram as carbohydrate or protein does which makes it very energy dense, however it’s hard to burn. Think of it like a big log of wood. you can’t just hold a match to a log and light it on fire. It takes time to either chop it down, or build up kindling around it to get a fire going. Fat is the same way, it is great for long term energy storage, but it takes time for the body to make use of that energy, so you can think of it as ‘slow but steady’ where as carbohydrates are more ‘fast energy that lasts half as long’.
When you fast for long periods, the body starts out with a normal fat storing/carb burning metabolism but as the liver and muscles are depleted of glycogen and fast energy, and no new carbohydrates are digested to replace them, eventually your energy levels sag and you feel very lethargic starting about day 2. During this time your body starts to break down the fat into ketone bodies, which your body and brain are able to use as energy. Just like splitting a log into pieces to fit in your fireplace.
The longer the body is in fat burning mode, the better it gets at it and eventually your energy levels return to something approaching normal only the body is getting energy from burning fat instead of carbohydrates.
Ironically once in fat burning mode, if you eat carbs, the body tries to go back to carb burning, and it can make you feel MORE tired and temporarily reduce your energy level until fat burning ramps up again.
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