the metabolism of proteins

What is protein metabolism?

What does ‘protein metabolism’ mean?

Protein is the primary metabolic actor in the body. They do a lot of things including make your muscles grow.

Proteins are a large molecule composed of one or more basic units called amino acids.

There are a total of 20 amino acids in the body. Amino acids are the basic building block of protein.

The term ‘building block’ is a very good way to describe amino acids because they are used by your body for building biological substances such as specialized proteins.

If these amino acids are placed in a different order then you get different proteins. There are proteins that are specific to every organ in your body.

What is the protein metabolism process?

How can you use protein to power up your metabolism?

The body’s protein mass not only provides architectural support for cells but also serves vital roles in maintaining their function and survival.

The process of protein metabolism consists of breaking down proteins you ingest by enzymes into amino acids and then putting them back together in a different order to create a new specialized protein, which will become different things such as enzymes, antibodies or hormones, they might also become structural proteins like muscle protein or collagen found in connective tissue.

What are catabolic and anabolic proteins?

The breakdown of protein is called proteolysis or catabolic process, and protein synthesis is an anabolic process. The process of skeletal muscle metabolism is moderated by two hormones, insulin, and glucagon.

Skeletal muscle protein synthesis and repair occur locally at the muscle and that’s how it’s going to work in all the other organs pretty much. In order to prevent, or at least minimize the breakdown, you have to increase your protein intake to mitigate more damage to skeletal muscle.

That would provide your body excess amino acids to rebuild the muscle that you may have worked out, it also provides excess amino acids that can be turned into glucose and to help prevent your body from targeting your skeletal muscle.

That’s why most diets include a lot of protein because the excess protein will minimize the effects of protein breakdown while you were dieting.

What is the protein metabolism cycle?

Protein digestion begins in the stomach with the action of an enzyme called pepsin. Pepsin is the active protein-digesting enzyme of the stomach.

When pepsin acts on the protein molecule, it breaks the bonds that hold the protein molecule together called peptide bonds. When these bonds are broken you get chains of amino acids linked together called polypeptides.

These polypeptides then move into your small intestine where digestion will be completed. In the small intestine, pancreatic enzymes called trypsin, chymotrypsin and carboxypeptidase will break down the polypeptides. These enzymes will enter the duodenum via the pancreatic duct.

What are peptides and polypeptides?

The peptide bonds holding the polypeptides together continue to be broken down and result in smaller units called peptides. Peptides are just two or more amino acids linked together.

Enzymes continue to break down polypeptides into amino acids because amino acids are very small they are able to be absorbed through the small intestine lining and into your bloodstream.

What is the connection between protein metabolism and the liver?

Digested nutrients that leave the digestive tract to take a detour to the liver before entering the bloodstream. This detour allows your liver to have the first claim at nutrients coming from the digestive tract.

Capillaries in the wall of the digestive tract pick up the amino acids. These amino acids along with other digested nutrients then move to the liver through a unique system of veins called the hepatic portal system. This is the system of veins that are responsible for directing blood from the digestive tract to the liver.

The hepatic portal system can be considered as the pathway to the liver. After the liver takes its share of the nutrients, the blood enters the general circulation through the hepatic veins, which drain the liver.

The remaining amino acids can now circulate to your body cells for them to use them to build the specialized proteins.

amino acids pool layout

Is protein metabolism beneficial for weight loss?

How can you use protein to power up your metabolism?

Well, see, not all calories are created equally, especially when it comes to carbs and protein.

A study found that eating protein for breakfast improves all the hormones that control your weight, they reduced a hunger hormone-like ghrelin, and it slows the emptying of your stomach.

The exact same calories from carbohydrates actually increase the hunger hormones and make your stomach empty faster.

It’s not only the calories you eat, but it’s also the information you eat what’s so important

Now remember protein can be nuts and seeds, beans, and whole grains, it can be Omega 3 eggs and wild fish, lean poultry, and other meats but it is by no means as steak and cheese.

A diet protein it seems is also more thermogenic, which means protein burns hotter than other food sources so at the end of the day you burned off more calories when you eat protein rather than storing them.

In one study the participants burn 71 more calories a day than those who eat low protein diets, like 36% vs. 15% protein. Now it may not seem like much but over a year it’s equivalent to an extra seven point four pounds of weight loss.

It also seems that amino acids, those building blocks that form protein, send messages to areas in your brain to signal you that you’re full.

How important is exercise and protein metabolism for muscle growth?

Generally, protein synthesis is suppressed during exercise and protein breakdown is increased to support glucose synthesis.

When you exercise your muscles are damaged with micro-tears. In order to rebuild them and to get bigger and stronger to hypertrophy, your muscles need protein.

What is hypertrophy?

Hypertrophy is an increase in the volume of organs or cells due to the enlargement of cellular components. Atrophy is actually the opposite when the skeletal muscle fibers actually shrink.

After exercising the catabolic activity slows down since the glucose demands decrease, and the net protein breakdown slows, so the protein synthesis increases. This is when hypertrophy occurs.

Eating your body weight in grams of protein per day should provide sufficient protein for muscle growth. Proteins can contribute approximately to 5-15% of the fuel required during exercise.

A research from the Journal of Applied Physiology show that essential amino acids and an acute bout of resistance exercise independently stimulate human skeletal muscle protein synthesis and it also appears that ingestion of essential amino acids following resistance exercise leads to an even larger increase in the rate of muscle protein synthesis compared with the independent effects of nutrients or muscle contraction.

How to improve your protein metabolism?

Your metabolic state is dictated and partially determined by many things like your dietary composition (net calories, fat, protein, carbs), the ratio of circulating insulin and glucagon, other hormones (epinephrine, cortisol, etc), recent exercise (resistance, endurance, etc), body composition (lean muscle, body fat), your basal metabolic rate and many other.

What are the different body types?

Metabolism is different for everyone so you should be eating according to your type.

There are three metabolic types, the endomorph or protein type, the ectomorph or carbohydrate type, and the mesomorph or mixed type.

Metabolic body types


The endomorph body type is solid and generally soft and gains fat very easily.

Their muscles are strong, especially the upper legs. They also tend to be insulin dominant, leading to a greater propensity of energy storage.

Endomorphs have a lower tolerance for carbohydrates.

The protein types’ ideal intake will be around 25% of carbs, 35-40% protein and the rest of fats.

The person with endomorphic traits will tend to function best if they remain lean for their frame, probably maintaining between 15-20% body fat for men and 20-25% body fat for women.

Endomorphs are about strength and size

They tend to excel at endeavors involving strength and size, impact sports, or in unusual activities such as cold-water swimming.

They often have difficulty in sports that require a certain amount of leanness and aesthetics such as bodybuilding, or in those activities that require weight-bearing endurance such as in running.

One of the common problems that occur with those people with endomorphic tendencies is that they often do not participate in physical conditioning activities and as a result, decondition.

Stay active

This lack of activity tends to cause them to put on significant amounts of body fat while losing muscle mass.

These people more frequently end up suffering from obesity and obesity-related problems such as diabetes and arthritis than their ectomorphic and mesomorphic counterparts.


Ectomorphs are skinny with a small frame, low body fat, lightly build and lean muscle.

They have a fast metabolism making this body type the most resistant to weight gain.

The carb type ideal diet would be about 2/3 carbs in his/her meal, such as grains, fruits, and vegetables.

The last third would be a protein and healthy fat source, so that could be meat or fish.

A person with ectomorphic traits will tend to function best if they remain lean, probably less than 8-10% body fat for men, and 12-15% body fat for women.

Ectomorphs are about endurance

They will do well to train and compete in endurance events like triathlons (running, cycling, swimming) and utilize mild strength activities as this goes with their natural constitution.

Ectomorphic trait people often have difficulty when they want to be a competitive powerlifter, bodybuilder, or compete in impact sports with larger individuals.

Of course, there are always notable exceptions, but these pursuits are often met with disappointment because they are not best suited to this body type.


Mesomorphs have a medium-sized bone structure, athletic body and usually a considerable amount of lean mass.

They tend to be testosterone and growth hormone dominant, which leads to a predisposition for muscle gain and the maintenance of lower body fat.

The mixed type would follow a more balanced diet with about 40% of carbs, 30% of proteins and 30% of fat.

The person with mesomorphic traits will tend to function best if they remain lean, probably maintaining between 10-15% body fat for men, and 15-20% body for women.

Mesomorphs are natural athletes

They tend to do particularly well and excel in sports involving sprints up to middle distances (running, cycling, rowing, swimming), bodybuilding, and impact sports.

People with mesomorphic traits tend to do well in most of the common sports that the majority of people participate in and are often considered by those observing as being “natural athletes”.

Mesomorphic trait people often have difficulty if they force themselves to become excessively lean or if they allow themselves to gain too much body fat.

According to the American Nutrition Association, the metabolic typing diet starts from the premise that we are each biologically and metabolically unique, and therefore a one-size-fits-all diet will never work for all people.

amino acids metabolism layout

Protein metabolism and its effects on blood glucose levels

When you start dieting, either decreasing calories, carbs or both what happens is that your blood sugar will go down as a consequence.

Your body will try to raise your blood sugar by breaking down muscle proteins back into amino acids. These amino acids can then be converted into sugar.

Your body specifically targets muscle protein when you are fasting or dieting because compared to most other organs it is expendable.

Better your skeletal muscle than your brain, for instance.

After depleting your glycogen stores and have no oral intake for 12 hours or so, your body shifts from what we call fasting to more of a starvation state, and that’s where you begin to mobilize more fat, this is what’s called ketogenesis.

What is the hormonal regulation of protein metabolism?

While all the hormones have regulatory effects on the rates of protein synthesis and breakdown there is a complex interaction between them in this control process.

In humans, insulin appears to act primarily to inhibit proteolysis.

Growth hormone and testosterone have an important role during growth but continue to be required to maintain body protein during adulthood.

What are the thyroid hormones?

Thyroid hormones are also required for normal growth and development.

The hormones glucagon, glucocorticoids and adrenaline are all increased in catabolic states and may work in concert to increase protein breakdown in muscle tissue and to increase amino acid uptake in the liver for gluconeogenesis.

While increased glucocorticoids result in reduced muscle mass the effects of glucagon may be predominantly in the liver resulting in increased uptake of amino acids.

A study held by the Department of Medicine of United Medical School at St Thomas’ Hospital, London and published in PubMed, shows the hormonal control of protein metabolism.

Another study held by the Department of Internal Medicine, Endocrine and Metabolic Sciences at the University of Perugia, Italy and published in the European Journal of Endocrinology, allows the rates of the whole-body protein synthesis and breakdown to be estimated together with amino acid oxidation and the fractional synthetic rates of mixed muscle proteins or of single plasma proteins.

What are some protein metabolism disorders?

About one out of 1.250 newborns are born with a metabolic disorder, also called ‘inborn error of metabolism’. It is inherited and passed down unknowingly by the parents who have not been screened before and may carry the defective gene copy.

In the case of an inherited metabolic disorder, the person lacks a particular enzyme to perform normal metabolic functions.

The affected baby becomes sick with various symptoms, such as poor feeding, vomiting, seizures, weak muscle tone, development delays or lethargy.

Some serious developmental issues and lifelong complications such as mental retardation, physical disability or in some instance death may occur if the affected baby is left untreated.

What are some metabolism diseases?

1. Maple Syrup Urine Disease

Maple syrup urine disease (MSUD), also called branched-chain Ketoaciduria. It is caused by a deficiency of the branched-chain a-keto acid dehydrogenase complex (BCKDC).

This means that the body has a problem breaking down protein which needs to grow and repair itself.

2. Glutaric Aciduria Type 1

Glutaric aciduria type 1 (GA-1), is an inherited neurometabolic disorder that results from gene mutation of the Glutaryl-CoA dehydrogenase enzyme (GCDH). GA-1 causes GCDH deficiency, which may affect neuronal energy production, it causes carnitine depletion, which is important for intramitochondrial regulations.

GA-1 can be defined as two clinical entities; Before, macrocephaly, and after, motion difficulty, encephalopathic crises.

3. Hartnup Disorder

Hartnup disorder affects the neutral amino acid transporter in kidneys and intestine. It affects the absorption of non-polar amino acids such as tryptophan which is excreted in the urine.

Symptoms are mostly caused by a deficiency of serotonin and niacinamide.

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