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All about proteins - Plant vs Animal sources of proteins. Complete and Incomplete proteins.g

Updated: Jan 26

Proteins and its importance:

Protein is an indispensable nutrient for good health. No life exists without proteins. Its name was aptly derived from a Greek word Proteos, "meaning primary" or "taking first place". All the basic functions of life depends on proteins. In fact there is no physiological function in which they do not participate.

Building blocks of our body: Each and every cell of our body is made of proteins. From muscles, bone, teeth, skin, tendons, cartilage, blood vessels to hair, and nails everything is made of amino acids.

Catalysts: Thousands of biochemical reactions that take place in our body needs enzymes and enzymes are proteins. Even to digest the food we eat we need digestive enzymes like lactase, sucrase, amylase etc...

Messengers: Hormones and neurotransmitters are also proteins. They act as chemical messengers in our body. Hence proteins are not only essential for brain functions but also for the functions of other organs in our body.

Buffers: Proteins, because of their constituent amino acids, can serve as buffers in the body and thus help to regulate acid-base balance. The pH of the blood and other body tissues must be maintained within an appropriate range. Blood pH ranges from about 7.35 to 7.45, whereas cellular pH levels are often more acidic. The protein hemoglobin, for example, functions as a buffer in red blood cells.

Fluid balancers: Proteins influence fluid balance through their presence in the blood and in cells. More specifically, proteins help attract and keep water inside a particular area and contribute to osmotic pressure. When protein concentrations in the blood are less dense than normal, fluid “leaks” out of the blood and into interstitial spaces causing swelling.

Immunoprotectors: Protein is essential for even immune functions. Immune cells that protect us from diseases such as cytokines, macrophages, neutrophils, Immunoglobulins (IgG, IgM etc) all are made of proteins.

Transporters: Proteins also acts as means of transport that caries substances such as vitamins, minerals, and other nutrients, within the blood, into cells, out of cells, or within cells. For example, the protein hemoglobin, found in red blood cells, transports oxygen and carbon dioxide.

Energy providers: Protein also provides energy to our body. 1 gram of protein gives 4 calories. But our body uses proteins for energy only in situations like long fasting, strenuous exercises and low calorie intake.

Essential amino acids and non-essential amino acids

We all know that proteins are made up of amino acids. There are 20 amino acids in human body. Though amino acids are classified based on different attributes, the most commonly spoken classification is based on body's ability to synthesize them. Based on this amino acids are classified into essential amino acids (EAAs) and non-essential amino acids (NEAAs).

Essential amino acids

Non - Essential amino acids

Conditionally essential amino acids






















Aspartic acid



Glutamic acid


Non Essential amino acids: Out of 20 amino acids 11 amino acids are non-essential amino acids. Non- essential amino acids are those which are produced inside our body and hence its not necessary to include them in our diet. But under certain conditions like stress, illness and injuries, the body fails to produce few of them in adequate amounts. These non essential amino acids becomes conditionally essential in such circumstances. There are 6 conditionally essential amino acids and these are to be supplemented through diet under such circumstances. Please refer the table above for the names of amino acids.

Essential amino acids:

Essential amino acid is an amino acid that cannot be synthesized by our body and hence need to come from diet. There are 9 essential amino acids. Refer the table above for the names of these amino acids.

Protein quality- Complete and incomplete protein:

Based on the availability of all essential amino acid in a protein source, it is classified as complete or incomplete protein. When all the 9 essential amino acids are available in a protein source then its considered as a complete protein (high quality proteins). If one or more amino acids are lacking or are present in too little quantity then the source is said to be incomplete protein (low quality proteins).

Plant vs animal proteins:

Usually animal sources of protein such as milk, yogurt, cheese, eggs, meat, fish, and poultry are complete proteins as they provide all the 9 essential amino acids in adequate quantity. Where as plant sources of protein like cereals, legumes and grains usually lacks 1 or more essential amino acids and hence are incomplete proteins. The word limiting amino acid is used to describe the essential amino acid that is too less in a protein source.

Food sources of incomplete protein

Possible limiting amino acids

Wheat, rice, corn and other grains

Lysine, Threonine (sometimes), tryptophan (sometimes)



Legumes, peas, lentils

Methionine (sulphur containing amino acids)

Nuts and seeds

Lysine and Threonine

Few nearly complete plant proteins:

  • Quinoa

  • Soy products like Tofu, tempeh, edamame

  • Buckwheat

  • Hemp

  • Chia seed

  • Spirulina

  • Tempeh

  • Amaranth

Recommended protein and different amino acids intake per day:

For a healthy adult 0.83 grams per kg of body weight is the recommended daily intake. But the need of protein varies with age, physical activity and other pathological conditions. Older adults above 60 years of age need1 - 1.5 grams of protein per kg body weight to preserve muscles mass.


Protein recommendation in g/kg/d

Adult men


Adult women


Pregnant Women Second trimesters


Third trimester

(body weight x 0.83) + 9.5


(body weight x 0.83) + 22

Lactating mothers (0 - 6 months)


(6 - 12 months)

(body weight x 0.83) + 16.9


(body weight x 0.83) + 13.2

Recommended daily intake of essential amino acids in mg/kg body weight

Amino acids

RDA in mg/kg body weight (WHO)









Methionine + Cysteine

10.4 + 4.1



Phenylalanine + Tyrosine

25 (total)





How to get all essential amino acid from food we eat:

People who include foods of animal origin such as milk, yogurt, cheese, eggs, meat, fish, and poultry in their diet usually get all the essential amino acid in adequate amount if they manage to get daily recommended intake of protein. For example an adult who weighs 60 Kg needs 49.8 grams (60 x 0.83) of protein a day and if this 49.8 grams of protein mostly comes from animal source then all the essential amino acids are usually met easily. Its recommended that one should divide their total protein requirement per day by number of meals and try to include the value of protein derived in each meal. For example if the total requirement of protein per day is 49.8 grams and a person takes 3 meals a day then he should aim to take 16.6 (49.8/3) grams of protein in each meal.

Lacto-ovo vegetarians ( people who eat egg, milk products and follow vegetarian diet otherwise) also manage to take adequate essential amino acids if they meat daily recommended intake of protein. Lacto vegetarians (people who include only milk products) in their diet can also manage to get all essential amino acids if they pair legumes, grains and one milk product in their meal. But pairing only legumes and milk can be less in sulphur containing amino acids (Methionine and cystine). So its important to pair milk products with grains and legumes or pulses. Including soy protein in diet is also an excellent way to get all essential amino acids for this group.

Pure vegetarians who eat only plant based foods should be cautious and plan their meal to get adequate essential amino acids as plant based proteins are not only incomplete proteins but their digestive scores for proteins are lesser than animal proteins. But this doesn't mean that they should start eating animal based foods. Pairing different kinds of plant based foods can easily give adequate amount of essential amino acids. This practise is known as mutual supplementation.

Pairing cereals with legumes and pulses:

Legumes and pulses are high in lysine but low in methionine and cystine while cereals are high in methionine and cystine. Hence pairing grains with legumes and pulses is an excellent way to manage essential amino acid requirements.

Pairing soy products with sesame and other seeds and nuts:

Soy products are good sources lysine but they are deficient in methionine and cystine while seeds and nuts are good sources of methionine and cystine. Hence adding seeds and nuts to soy products will supplement each other.

Take away:

Protein is crucial for good health. Recommended protein intake for an adult is 0.83 grams per kg body weight. But the protein requirements changes with age and other physiological and pathological state. Older people usually need 1 to 1.5 grams of proteins per kg body weight. There are 9 essential amino acids in our body and these are not produced by our body and hence should come from diet. For healthy living not only the recommended total protein intake but also consuming adequate individual essential amino acids is crucial. While its easy for people who consume animal based foods to meet their recommended amino acids requirements, people who consume only plant based foods should include variety of foods in their diet to meet essential amino acids requirements. Pairing grains with legumes or pulses and pairing soy products with nuts and seeds are some of the examples of good pairing.

- Sangeetha Duraiswamy

B.P.T., C.S.P.T., M.I.A.P., M.B.A. (HRM),

M.Sc. (Diet and food service management)

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