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Nutrition and Metabolism


What is protein?

Protein is one of the macronutrients. It is rich in lean meats, cheese, egg white, beans, nuts and some plant seeds. In human body, old proteins are constantly broken down and new ones synthesized. Dietary proteins provide their building blocks amino acids to make new proteins in human body. Some of the amino acids from the digested proteins can also be used to provide energy.

What is the function of protein?

Proteins serve as structural and functional molecules in every part of human body. About 43% of the proteins in human body are skeletal muscles. Skin and blood each contain approximately 15% of the total proteins. The remaining proteins include enzymes, hormones, transporters, immunoglobulin etc. The building blocks of proteins, about 20 amino acids, also serve as precursors for neuron transmitters (i.e. serotonin), hormones (i.e. melatonin), nucleic acids (DNA and RNA), coenzymes etc. Amino acids can be broken down to provide nitrogen elements for making other nonprotein compounds such as glutathione, carnitine, creatin, carnosine, choline and nitric oxide. These compounds play important roles in regulating various metabolic pathways in human body. The carbon portion of amino acids can be converted to carbohydrates or fats when needed.

What are the essential amino acids?

There are 20 different amino acids that join together to make all types of protein. Human body cannot make all of them. The nine amino acids that we cannot make, thus have to get them from diets, are known as essential amino acids, some people also call them indispensable amino acids. In addition, six other amino acids are called conditionally indispensable as they are synthesized from other amino acids or their synthesis is limited under special conditions. Only five amino acids are truly dispensable (Table 1).

Table 1. Essential, conditionally dispensable and dispensable amino acids for human body.

Essential Conditionally Indispensible Dispensable
Histidine Arginine Alanine
Isoleucine Cysteine Aspartic acid
Leucine Glutamine Asparagine
Lysine Glycine Glutamic acid
Methionine Proline Serine
Phenylalanine Tyrosine  

What is a complete protein source?

In the dietary field, protein sources are classified according to how many of the essential amino acids they provide. If a food source provides all of the essential amino acids, it is considered as a complete protein source. In general, animal-based foods (meats, poultry, fish, milk, eggs, and cheese) are considered complete protein sources. In contrast, an incomplete protein source lacks one or more of the essential amino acids. For example, many grains are low in lysine and beans are low in methionine and cysteine.  Since beans are high in lysine while grains contain much of methionine and cysteine, by eating beans and grains together, one can make a complete protein source. Combining two or more complementary protein sources to create a complete one is important for vegans.

How much protein should I eat?

The Recommended Dietary Allowance (RDA) for both men and women is 0.80 g of complete protein per kg body weight per day. This translates into body weight dependent, thus age and a gender dependent, RDAs shown in Table 2.

Table 2. RDA for proteins (gram per day) by the Institute of Medicine of the National Academies.

Age RDA (g/day)
Male Female
1 – 3 years 13 13
4 – 8 years 19 19
9 – 13 years 34 34
14 – 18 years 52 46
19 years and older 56 46

Although there is no Upper Intake Level (UL) for total proteins, the Institute of Medicine of the National Academies recommends the amounts of proteins should contribute to 10–35% of your daily calories.  

Protein deficiency has adverse effects on all organs, growth, immune system, skin, hair, brain function, kidney function and death. Too much protein intakes have been implicated in chronic diseases such as osteoporosis, renal stones, cancer, coronary artery disease, and obesity. In the United States and much of the developed countries, protein deficiency is rare. Most people need to watch out to not to have too much proteins.

Should I take protein supplements?

You can easily get enough proteins in your diet. Protein supplements are for muscle builders and people who endure intensive physical exercises.  For average people, protein supplements may cause constipation, heart disease or liver and kidney problems. If you have certain medical conditions, you may want to limit or increase the intake of certain amino acids. This would require you understand the sources and compositions of the protein supplements well. You need to talk to your doctor before taking any protein supplements.

Do high-protein diets help you lose weight?

Yes for some people. Human body need to spend more time and energy to digest protein than to digest carbohydrates containing the same amount of calories.  Therefore, high-protein foods make you feel full longer than high-carb foods. However, the long-term effects of a high-protein, low-carb diet are not well understood yet. People with certain genetic background or certain medical conditions may not benefit from a high-protein diet. See later section on genetic makeup and dietary protein response.

What genetic makeup influence dietary responses to protein?

Different genetic backgrounds cause people respond to dietary protein differently.  For example, The TCF7L2 gene encodes a transcription factor involved in the development of type 2 diabetes. When it comes to weight control, studies suggest that carriers the HapA variant of the TCF7L2 gene, distributed in about 53% of the general population, benefit more from high-protein diets while the non-carriers are better suited with low-protein diets.  HapA carriers gain more weight when protein intake is less than 17% of the total energy but are protected from weight gain when protein intake exceeds 17% of total energy. Another gene, the FTO gene, is related to satiety. Variations of the FTO gene have been associated with increased fat mass and obesity. Studies on weight loss in obese population show a particular variation of the FTO gene (SNP rs1558902) make its carriers lose more weight when taking high-protein diets.  On the contrary, non-carriers of this variation lose more weight when taking low-protein diets. This variation, more frequent in Caucasians and rarely seen in Asian and Africans, distributed in 24% of the general Caucasian population but showed a much higher frequency in obese Caucasians (46%).

What foods are rich in proteins?

Proteins are rich in meats, poultry, fish, bean, tofu, and dairy products. The following table listed the most common foods rich in proteins.

Table 3. Common protein-rich foods

Beans, Soybeans
Per 100 g (g) 40
Per Serving (g) 68
Serving Size 1 cup (172 g)
Seeds, Pumpkin and squash seed kernels
Per 100 g (g) 30
Per Serving (g) 8
Serving Size 1 oz (28 g)
Peanuts, roasted
Per 100 g (g) 24
Per Serving (g) 7
Serving Size 1 oz (28 g)
Cheese, parmesan
Per 100 g (g) 42
Per Serving (g) 12
Serving Size 1 oz (28 g)
Cheese, mozzarella
Per 100 g (g) 28
Per Serving (g) 8
Serving Size 1 oz (28 g)
Cheese, Cheddar
Per 100 g (g) 25
Per Serving (g) 7
Serving Size 1 oz (28 g)
Per 100 g (g) 31
Per Serving (g) 26
Serving Size 3 oz (85 g)
Beef, lean
Per 100 g (g) 26
Per Serving (g) 22
Serving Size 3 oz (85 g)
Fish, tuna
Per 100 g (g) 23
Per Serving (g) 20
Serving Size 3 oz (85 g)
Per 100 g (g) 21
Per Serving (g) 18
Serving Size 3 oz (85 g)
Sea food, Lobster
Per 100 g (g) 21
Per Serving (g) 18
Serving Size 3 oz (85 g)
Fish, Anchovies
Per 100 g (g) 20
Per Serving (g) 17
Serving Size 3 oz (85 g)
Fish, salmon
Per 100 g (g) 20
Per Serving (g) 17
Serving Size 3 oz (85 g)
Chicken, breast
Per 100 g (g) 18
Per Serving (g) 15
Serving Size 3 oz (85 g)
Sea food, shrimp
Per 100 g (g) 14
Per Serving (g) 12
Serving Size 3 oz (85 g)

How to improve your dietary proteins?

First you are recommended to use the GB Food Calorie and Nutrition Calculator to get the most accurate estimate of your current protein intake. Depending on your genetic background and your health status, you may want to maintain, increase, or decrease your protein intake.

To increase your protein intake, you may want to choose from the common protein-rich foods shown in Table 3. Animal based protein-rich foods are complete protein sources. But you have a risk of getting extra saturated fat or cholesterol along with them. Plant based protein-rich foods are free of cholesterol.  However, you need to combine complementary sources to make the protein complete.

Exercise caution when you are attempted by protein supplements. An average healthy American get all the protein he or she need from diets. If you do want to get protein supplements, ask your health providers first.


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