Your Metabolic Engine
There's no such thing as a 100% efficient mechanism, and the human engine is no exception.
When it comes to metabolic efficiency, the most efficient metabolisms extract calories and store them more easily, losing less of that energy as heat. A less efficient metabolism doesn't extract calories as well and loses more of them as heat. If it's fat loss you desire, then a LESS efficient metabolic engine is what you want.
Much of this metabolic efficiency comes down to genetics and metabolic hormones. For example, those with normal thyroid function produce more metabolic heat and are less efficient. Those with lower thyroid function produce less heat and are more efficient. This is one of the reasons those with low thyroid function respond more slowly to diet.
Certain parts of the body are more productive at storing fat and less efficient at losing it. These include the stubborn body fat areas like the hips, butt, and thighs of women, and the love handles of men.
These areas of fat are more insulin sensitive (more likely to store and less likely to release fat) and have more alpha than beta receptors. Betas are like fat burning garage doors. Alphas are like tiny kitchen windows; fat can barely squeeze through.
The Law of Metabolic Efficiency overlaps with the Law of Metabolic Compensation. Dieting makes the metabolism more efficient. That is part of what adaptive thermogenesis is doing. Annoying, I know. There are also things we've learned about macronutrients, toxins, and gut microbes that impact efficiency.
When it comes to macronutrients, not all calories are created equal:
Protein is the most satiating and most thermogenic of the macronutrients. That's science speak for "it's a less efficient fuel." In other words, substitute protein calorie-for-calorie in place of carbs and/or fat and the metabolism will burn off more energy. Protein is the hardest macronutrient to store as fat.
Carbs are the next most satiating and thermogenic, and they're highly variable. Carbs with lots of fiber are very inefficient. More refined carbs with less fiber are more efficient. Glycemic index and insulin kinetics related to carbs can be viewed as an efficiency measure.
Also, starches can have varying degrees of resistant starch. A cold potato eaten whole with the skin on is more inefficient compared to a hot mashed potato without the skin. One interesting study showed the useable calories in rice can be reduced by 50% when cooked with coconut oil, cooled, and reheated again. This is an example of making food more inefficient in the cooking process.
Despite what the popular pseudoscience and biased blogosphere says, fat is the least thermogenic and least satiating macronutrient. In other words, calorie for calorie, it's the most efficient fuel you can eat and store. However, when combined with protein, its satiating potential is more pronounced and this combination flips the switch to metabolic inefficiency.
Two more interesting and emerging pieces of info related to metabolic efficiency have to do with toxins and bacteria living in our colon (what is euphemistically referred to as "bugs").
I hate the word toxins because it means nothing without clarification. In the context of metabolic efficiency, I'm talking about POPs or persistent organic pollutants.
These compounds accumulate in the environment (pesticide residues, plastic leaching, industrial pollutants, etc) and concentrate in the fat tissue of animals. This is known as bioaccumulation, where animals eat the plants that harbor the compounds and end up with the highest concentrations. This is the same reason the large predatory fish of the ocean have the highest mercury levels.
So these POPs are mainly in the fatty meat you eat. Yes, even the organic, grass fed, Shangri-la steak the primal peeps swear by. Of course that's better, but lower fat options might be even better if you're dealing with this issue. Also coffee, the most heavily sprayed crop on the planet, and butter. If you're dealing with these concerns, you may want to look into the POPs issues here as well.
Bacterial populations in your digestive tract are impacting your metabolic efficiency too. These "bugs" act like that annoying friend that keeps snatching french fries off your plate.
The amount and types of these bugs you have can determine much about your metabolic function. Not only do they use some of your calories, they send constant signals into your body and adjust your metabolic thermostat as a result. There is no more exciting area of research right now in medicine than this area of inquiry.