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I would like to ask if there are some proteins that the human body needs and can being endowed only from meat?

According this:

Other protein sources, such as fruits, vegetables, grains, nuts and seeds, lack one or more essential amino acids. Vegetarians need to be aware of this. People who don’t eat meat, fish, poultry, eggs, or dairy products need to eat a variety of protein-containing foods each day in order to get all the amino acids needed to make new protein.

How a vegan can complete these missing proteins?

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    $\begingroup$ this might be better for the cooking exchange.cooking.stackexchange.com Otherwise this borders on medical advice or at least nutritional advice. $\endgroup$ – John Jul 19 '17 at 23:21
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Proteins are made up of amino acids. It isn't particular proteins that are necessary in diet, but particular amino acids. For humans, these come primarily from breaking down the proteins in foods we eat.

Essential amino acids are the amino acids that humans cannot synthesize; other amino acids can be synthesized from these, but they do not need to be part of the diet. Not all sources of protein have sufficient quantities of all of the essential amino acids. When people refer to a protein source as "incomplete" they mean that particular source is low on one or more of the essential amino acids.

Meat products are typically "complete" because they contain all of the essential amino acids together. Therefore, if you subsisted on various foods but only got your protein from one animal source, you would be okay.

Some plant products do not have all the essential amino acids in large concentrations. However, if you combine protein sources from different "incomplete" plant sources that together cover these deficiencies, there is no disadvantage compared to a single "complete" source.

You also have a fairly substantial reserve of amino acids in the form of all the proteins in the body, which are constantly broken down and recycled. The daily protein intake is just a tiny fraction of the total amino acid content in the body.

Here is a paper that talks about some of these issues, and addresses some of the myths about balancing proteins. The quick summary is that problems develop if you get your sole protein from a source that is low in a particular essential amino acids over a long time period, but there is no need to ensure every meal contain all the amino acids.

It is possible for people on unusual protein diets, such as vegans, to develop a particular amino acid deficiency. There are many many guides on the internet for protein sources to mix to avoid this problem. A common combination is a legume (beans, peanuts) and a grain (wheat, rice). There are also some vegetable sources that provide complete protein nutrition - soy, for example, mostly works as long as it is not truly the only source.

References:

Young, V. R., & Pellett, P. L. (1994). Plant proteins in relation to human protein and amino acid nutrition. The American journal of clinical nutrition, 59(5), 1203S-1212S.

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  • $\begingroup$ Does the human body know to construct all the needed proteins from the 20 amino acids? That sounds impossible. Why not to use the consumed proteins? Or it's impossible for them to penetrate the plasma membrane? $\endgroup$ – 0x90 Jul 19 '17 at 22:52
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    $\begingroup$ That's exactly the information contained in DNA: information on which amino acids to assemble and which order, and both directly and indirectly, how much of each protein to make. That's actually much simpler than trying to detect and incorporate foreign proteins. Foreign proteins could also be harmful, and they vary far too much between different organisms. This is how biology works from bacteria to plants and animals. Think about how similar cars are, and yet, if you have an engine from a BMW and a transmission from Toyota, good luck putting them together in that Ford body. $\endgroup$ – Bryan Krause Jul 19 '17 at 23:00
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You probably meant "how can a vegan consume all the needed amino acids".

Proteins are made of amino acids chained together in a dehydration reaction. Digestion breaks the proteins, allowing the amino acids to be reused.

The most obvious and easy way to get all the amino acids: meat.

For vegans, rice and beans is a common way to get all the needed amino acids. Even today it remains a staple in many poor countries where meat isn't always readily available.

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    $\begingroup$ If you can add a source or two, and possibly an example, then you'll get my +1. Your answer is accurate and to the point, but IMO you should elaborate and support your statements a little more. $\endgroup$ – user22020 Aug 4 '17 at 16:12

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