AT A DIFFERENT OCTANE
What happens, however, when we use a different type of fuel? Obviously, the efficiency levels would change, the fuel would burn in different ways, and you’d either increase or decrease your gas mileage. Add ethanol to a car, and your fuel economy decreases. Add other hydrocarbons, like anti-knocking agents, and it increases.
Try to transfer this logic to the human body, however, and most physique coaches’ eyes glaze over as though, in 2013, it’s still inconceivable that different ratios of macronutrients can cause diferent amounts of energy to be wasted in the human body. A calorie, after all, is still a calorie.
The body, however, disagrees, and it obeys the second law of thermodynamics. This means it varies in efficiency based on activity, hormonal status, and the type of fuel we provide. That’s why Wilbur Atwater, who introduced the 4-4-9 calorie values for carbs, protein, and fat, distinguished between physical fuel values and physiological fuel values.
The physical fuel value is the amount of energy you can get out of food by burning it with oxygen. You throw food in an oven, incinerate it, then record the total amount of heat released. The physiological fuel value is the amount of energy an organism can derive from the fuel, which can be higher or lower than the physical fuel value.
It has been found that the human body, in a growth stage, can get more than 11 calories per gram out of fat significantly more than the 9 grams listed on most nutritional labels. This is because different activities require diferent enzymes or other molecules. Different or accelerated avenues of metabolization can produce different amounts of energy, and a calorie of fat is clearly not a calorie of fat. The same concept holds true for protein, as well, and these values don’t match up there, either. About 2% of your ingested calories of fat, 7% of carbs, and 30% of protein is wasted as heat whenever you eat.