When we are digesting food containing protein, carbohydrates, and fats, we’re doing it for the purpose of creating energy in our cells. Protein, which contains amino acids, is broken down by enzymes in our mouth, stomach and small intestines.
But what happens to the amino acids after digestion and how do we metabolize them into energy?
Proteins are broken down into amino acids in the small intestine and absorbed via the bloodstream to the liver. In the liver, they are a part of a process, called the citric acid cycle, which is responsible for metabolizing protein, carbohydrates, and fat and turning it into energy in our cells. Amino acids are only responsible for 10-15% of our energy production, but they also depend on a fasted versus non-fasted state. Most energy is made from glucose and fatty acids and since amino acids support glucose synthesis, ingesting amino acids for overall energy production is important.Â
What Is The Purpose Of Amino Acids?
Amino acids are chains of molecules found in protein sources. When eating protein, our enzymes in the mouth, stomach, and small intestine, break down the amino acids, so we are able to absorb them to use for energy.
Even though there are more than hundreds of amino acids found in nature, we only need around 20 non-essential and essential amino acids. The non-essential amino acids are synthesized in our bodies. The essential amino acids we’re not able to create ourselves, why we need them from an outside source, such as protein.
The non-essential amino acids
- Alanine
- Arginine
- Aspargine
- Aspartic Acid
- Cysteine
- Cystin
- Glutamic Acid
- Glutamine
- Serine
- Taurine
- Glycine
- Proline
The essential amino acids
- Histidine
- Branched Chained Amino Acids (Isoleucine, Leucine, Valine) also called BCAA’s
- Methionine
- Lysine
- Phenylalanine
- Threonine
- Tryptophan
Amino acids are responsible for many processes in our body. This includes the synthesis of important molecules such as nucleotide bases that are used to form DNA molecules, RNA molecules and energy production (ATP) in our cells.
Other than that, amino acids function as a part of the:
- Protein synthesis
- Building muscle
- Repairing tissue
- Immune system support
- Signaling between cells (hormones and neurotransmitters)
- Function as antioxidants
- Sustain healthy digestion
The purpose of amino acids is to support our body in different processes and are a part of our metabolism – also the metabolism of carbohydrates and fatty acids.
What happens after the digestion of amino acids and how our metabolism is structured to create energy, we’ll dive into down below.
Amino Acid Metabolism
Amino acid metabolism creates energy in our cells through protein synthesis via the citric acid cycle. The citric acid cycle is the cellular respiration in the cells, in which we metabolize the energy consumed from carbohydrates, fats, and protein. This means the substances we consume must be absorbed in our intestines, travel through the bloodstream to the liver, and be converted to energy in our cells.
But how much of our energy production is amino acids responsible for?
Amino acid catabolism, how we break down amino acids, only accounts for 10-15% of total energy production (ATP) in our mitochondria. The mitochondria are part of our cells, where ATP is created via the citric acid cycle.
The rest of the energy production is through carbohydrate catabolism, which is how we break down glucose, also called glycolysis. Fatty acid catabolism, which is how we break down fatty acids, are called fatty acid oxidation.
This means that amino acids count for 10-15% of energy production whereas the rest stems from carbohydrates and fat.
But, how is that connected, you might think?
What Happens When We’re Eating Protein?
When we ingest food, the enzymes in our small intestine will break the protein down into amino acids. When broken down, the amino acids travel to our liver through our bloodstream.
Insulin and glucagon are two of the primary hormones in the metabolism of food. When we are eating a meal, our hormone insulin is released in response to higher blood glucose which elevates glucagon in the liver. In a fasted state, our insulin levels are decreased, which will result in lower blood glucose. This will increase the hormone glucagon to release energy.
In the liver, the amino acids can be used directly in protein synthesis or the liver can use the excess amino acids and convert these into glucose and fatty acids. Glucose is stored in the liver as glycogen. Glycogen is used when we haven’t been eating for a while and we need energy. Fatty acids are stored in the connective tissue as triacylglycerols. The same thing is relevant here – when we have been fasting, we’ll release the triacylglycerols to the liver, where they can be used for energy production in our cells.
Conversion Of Excess Amino Acids To Glucose And Fatty Acids
Glucose is metabolized through glycolysis in the citric acid cycle. The precursors in the citric acid cycle are called pyruvate and oxaloacetate. Pyruvate and oxaloacetate help assist the citric acid cycle and the intermediates process in metabolizing glucose.
The intermediates are known as:
- Citrate
- Isocitrate
- α-ketoglutarate
- Succinate
- Fumarate
- Malate
All of these intermediates, which are a part of the citric acid cycle, help glucose metabolism in combination with essential and non-essential amino acids.
The precursor for fatty acids are called Acetyl-CoA. Acetyl-CoA is in balance with another molecule called acetoacetyl-CoA. Acetyl-CoA is made in the mitochondria by metabolizing fatty acids and the oxidation of pyruvate or Acetyl-CoA. When our body has excess ATP, the energy in Acetyl-CoA can be stored as fatty acids.
The amino acid molecules can be interconverted (change of state from one phase to another) and metabolized directly into the molecules in the precursor molecules and directly converted into pyruvate, oxaloacetate, Acetyl-CoA and acetoacetyl-CoA. These molecules are a part of the citric acid cycle that is responsible for energy production.
Ketogenic And Glucogenic Amino Acids
Another classification of the amino acids is whether they are ketogenic (fatty acid metabolism) or glucogenic (glucose metabolism) amino acids.
What it means is that the ketogenic amino acids feed into the precursor molecules such as Acetyl-CoA and acetoacetyl-CoA that are then converted to fatty acids. The glucogenic amino acids feed into the precursor molecules such as pyruvate, oxaloacetate or intermediates of the citric acid cycle.
Two amino acids are classified exclusively ketogenic which are lysine and leucine – which means when you ingest too much of these amino acids you´ll make fatty acids.
When amino acids enter the liver they can be used for protein synthesis or be converted into other storage forms like glucose and fatty acids.
What Happens When We Are Fasting?
As mentioned earlier, after the enzymes break down what we ingest in the small intestines, the liver is responsible for the metabolism of amino acids, carbohydrates, and fatty acids. In the fasted state the fatty acids are released from connective fat tissue and being sent to the liver. Here it’ll be oxidized and will fuel the synthesis of glucose for energy production. When we have been fasting for, let’s say 1-2 days, we’ll start producing ketones. Ketones are a breakdown of fatty acids.
Amino acids are a part of the process as well. They are being released from our muscles via the bloodstream to the liver. Amino acids, which are used for energy production in times of fasting, can enter a diverse array of metabolic pathways.
This means, when fasting, the glucogenic amino acids can contribute to its precursors (pyruvate and oxaloacetate) of gluconeogenesis and help support the production of glucose when fasting.
Final Thoughts
When we break it down, literally, it means that proteins are broken down by enzymes in our mouth, stomach and small intestine. Here, the amino acids will cross the barrier and travel via the bloodstream to the liver. Once absorbed in the liver, the amino acids will either be used as a part of protein synthesis or be stored as excess energy.
Amino acids are only responsible for 10-15% of our energy production in our cells, but the process depends on a fasted versus non-fasted state. Most energy is made from glucose and fatty acids, but since amino acids support glucose synthesis, ingesting amino acids for overall energy production is important.
