I suspect the numbers posted stating Fat was 97% Efficient are based on storage and a High-Carb Diet. I've seen it said quite often that Fat is the most efficiently stored fuel followed by Carbs and Protein [which is why they claim a diet should be Low in Fat and high in Carbs]...But, that's based on a typical High Carb Diet, and only comes into play when you eat too much. What it means is that if you eat a typical HC American Diet, that only 3% of your excess Fat Calories will be lost storing it as Body Fat versus 10% or more for Carbs and Protein.

But, that's only how efficiently its stored. What we're really concerned with is how efficiently its used as fuel and more importantly how the Macronutrient composition (LC vs. HC) effects that efficiency. Ketosis/Lipolysis is believed by many to be highly ineffcient in that fuel is outright wasted through sweat/urine/etc...A person on a typical HC Diet is not going to be in Ketosis/Lipolysis, but instead will be burning Carbs and storing extra Fat.

A person on a HC Diet will be using Carbs for energy and storing Fat for later use. Carbs are an efficient source of energy, and Fat is an efficient means of Storage. So, this is the optimal state to be in if you want to eat less, and gain weight. OTOH, when someone eats a LC Diet, they go into Ketosis/Lipolysis. In this state, they use Fat for fuel and generally store little or no new Fat. Fat is very inefficient when its actually used as fuel. Alot of it gets wasted.

Protein is not generally going to be used to an extensive degree for fuel. Some is needed to be converted to Glucose for those cells that can't use Ketones. But, most gets used to build new tissue, and for other needs that the body might have for it...and what is left over is not very easily converted and stored as fat, so Ketosis/Lipolysis is a very inefficient state, making it ideal for someone who wants to lose Weight/Fat.