and regards chips point about calories from fat and sugars etc. as i mentioned, your body uses a combination of sugar, protein and fats as energy, but not in equal measures. i cant remember the exact ammounts, but its something like 50% glucose/sugar (or carbs), 25% protein and 25% fat.
so if you burn 1000 calories down the gym, 500 will be from glucose, 250 from protein and 250 from fat. so it takes more to burn the same quantity of fat than sugar.
(this is all taken from my basic understanding of things and may not be 100% gospel!
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