Gluconeogenesis: How Protein Is Converted To Energy (Calories)
When your body has excess sugar (carbohydrates) that it doesn’t need, it has a way to save it for later.
It connects each individual sugar molecule together into chains. These chains of sugar are called glycogen, and they’re stored in your muscles and liver.
When your cells need more energy, and there is no more sugar in your bloodstream, your body breaks these glycogen chains back down into individual sugars and releases them into the bloodstream for your cells to use.
And if your body has so much excess sugar that it has filled all of the body’s glycogen stores and still has more, then it connects these sugars to fatty acids, forming triglycerides, and stores them in our fat cells as body fat.
This is also how the extra fat we consume is stored. If the body has more fatty acids than it can use for energy or cellular structure, then it connects the fatty acids up to sugars and stores this as body fat in our fat cells.
Your body has no such storage ability for amino acids. They are synthesized into new proteins or cannot be used.
When we consume proteins, and they’re fully broken down into amino acids, these amino acids are released into our bloodstream for the cells to use.
But these amino acids only stay in our bloodstream for a few hours on average. If they’re not used in that time, because they’re in excess of what is needed, your body has to do something with them.
Amino acids are molecules that can themselves be broken down. And when they can’t be used to synthesize new proteins, that’s what happens.
“If amino acids exist in excess, the body has no capacity or mechanism for their storage; thus, they are converted into glucose or ketones, or they are decomposed.”
An amino acid molecule contains an amine group, a carboxylic acid group, and a side chain that is specific to each amino acid.
“Most of the carbons from amino acid degradation are converted to pyruvate, intermediates of the TCA cycle or acetyl CoA. During fasting, these carbons are converted to glucose in the liver and kidney, or to ketone bodies in the liver. In the well-fed state, they may be used for lipogenesis [the metabolic formation of fat].”
This is where “protein has calories” comes in. Calories measure how much energy could potentially be produced by the breakdown of the amino acids in a protein, or the breakdown of a carbohydrate or fat.
But that’s potential.
Calories are the amount of energy released when your body breaks down (digests and absorbs) food.
If the amino acids were used to build new proteins, then they were not broken down and no energy was released.
It’s only the excess amino acids, the ones that could not be used to build new proteins, that then go through gluconeogenesis and are broken down, releasing glucose (sugar) and ketones, or which are directly oxidized as fuel — the energy measured in terms of calories.
But if you’re eating protein, and building muscle, then obviously at least some of the amino acids in the protein you’re consuming are being converted to new protein instead of being used as energy.