I have had a few discussion recently on the relative energy value of various macronutrients and some of the complexity regarding counting calories.
As a brief background, the Atwater values are the energy values given to the various Macros from extensive research conducted in the late 1800's by Wilbur Olin Atwater. In an attempt to address the wide spread undernourishment occurring at the time, Wilbur Atwater wanted to understand the different energy values of each food so as to make informed decisions about what should be fed to those starving given limited resources.
To do that he measured the amount of thermal energy in food by burning it in a bomb calorimeter, then fed the same amount of food to his subjects, then had the shitty job of burning their feces, urine and expelled gases in an attempt to estimate how much energy was extracted from each individual food.
Given the year this experiment was conducted, this was cutting edge science. After exhaustive research Wilbur Atwater settled on the following values for each major macro-nutrient:
Protein = 4 kCals/ gram
Carbs = 4 kCals/ gram
Fat = 9 kCals/ gram
In the flow diagram below, these values represent the Metabolisable energy of food or ME. To be painfully clear, the Metabolisable energy is the estimated energy extracted from a food, once Fecal Energy, Combustable Gas, Urine and small amount of energy lost to the internal surface of our intestines (SE) has been subtracted.
Not all metabolizable energy is available for the production of ATP however. You see, the atwater values intrinsically consider the human digestive system a closed loop system, where by all energy removed from a food was was made available to run biological systems. As you will see in a moment, this is not an accurate assumption.
A portion of all energy extracted from a food is utilized during its digestion, absorption and fermentation (called intermediary metabolism) and can be measured as heat production. This is often called Dietary Induced Thermogenisis (DIT) in scientific literature or the Thermic Effect of Food (TEF) by everyone else.
Once these values have been subtracted we are left with the NET Metabolisable Energy or NME. The NME values for each macro nutrient are:
Protein = 3.2 kCals/ gram
Carbs = 3.8 kCals/ gram
Fat = 9 kCals/ gram
From these values further losses are incurred due to human factor elements not speicfically related to the food itself, such as your hormonal environment, the Non-exercise Adaptive thermogenisis (NEAT), thermogenics compounds such as caffiene or other fat burnery, various drugs which affect substrate metabolism, your current level ofinflammation, whether you are currently fighting of an infection and so on.
This means that often the energy values we THINK we are eating are not the energy values we ACTUALLY have available to produce ATP.
NOW, you may be thinking, thats great Ella, super fucking fascinating.... WHY SHOULD I CARE?
Because the next time some one claims that counting calories wasn't working and that calories don't count, you can remember that JUST BECAUSE YOU THINK you ate a deficit or surplus, doesn't mean you really did. Any number of the huge amount of variables between the energy you counted going in and what your body was able to extract means that you very likely weren't consuming what you think you were.
It's not that energy doesn't count, we're just pretty terrible at counting it. This is actually the major problem with the "If It Fits Your Macros" (IIFYM) movement, but thats an article for another day.