must be clear that vegetables contain a certain amount of protein. In fact, according to the current general knowledge of biochemistry, any living individual, whether animals, plants (such as vegetables), or bacteria, viruses, must have protein in the body.

　　 Although the percentage of protein content in vegetables is not high, such as 2.6g protein per 100g spinach and 2g protein per 100g potato, but because the consumption of vegetables is relatively large (one kilogram per day, that is about 500g), A pound of spinach contains 13g of protein, this amount is relatively large, roughly equivalent to the protein contained in 2 eggs. In this way, vegetables seem to be an important source of protein, but this is not the case.

　　 First, let’s analyze the protein content of vegetables (this article is represented by spinach, and other vegetables are similar). The current general method for detecting protein in food is the so-called "Kjeldahl method", which first determines the content of "nitrogen" in food, and then converts it into protein according to a certain coefficient. This method has two premises: first, it is determined that the "nitrogen" element in food only exists in protein, and other ingredients do not contain "nitrogen"; second, the "nitrogen" element in protein (in addition to nitrogen, protein molecules also have Contains carbon, hydrogen, oxygen and other elements) accounted for 16%. It now appears that both of these premises have significant loopholes. Nitrogen is not only found in proteins, but also in small amounts of other nitrogen-containing compounds in natural foods (not to mention the artificially added nitrogen-containing compounds in food processing, such as melamine). In addition, the proportion of "nitrogen" elements in some food proteins is not 16%, for example, soy protein contains 17.5% nitrogen elements.

　　In short, the data of food protein content determined by the Kjeldahl method commonly used at present is not very accurate. For example, 100g of spinach does not really contain 2.6g of protein. As we all know, protein molecules are made up of many amino acids, and the components actually digested, absorbed and actually used by the human body after ingesting proteins are also amino acids. Therefore, detecting the amount of amino acids contained in food ("amino acid analysis method") can more accurately reflect the nutritional value of food, such as 100g spinach contains 2.1g amino acids (a total of 18 kinds of amino acids). In other words, 100g of spinach with 2.6g of protein is actually overestimated by 24%. Of course, the error caused by the detection method will also appear in meat or egg foods. But the degree of error is obviously less than vegetables. Egg protein content was 12.8g and 11.7g with Kjeldahl nitrogen determination and amino acid analysis, respectively, which was only overestimated by 9%; beef was 19.9g and 19.3g, which was only overestimated by 3%.

Secondly, considering the 10.5g of various amino acids contained in 500g of spinach, although the amount seems to be close to the various amino acids (11.7g) contained in 100g of eggs, its nutritional value is far less than that of eggs. Why? Compared with eggs, the ratio of 8 essential amino acids in spinach (protein) is not suitable for human needs. Based on the amino acid score (the index for evaluating whether essential amino acids are suitable for human needs, the smaller the value, the lower the nutritional value). Only 0.40, while the egg is 1.08. When measured by indicators such as biological value and efficacy ratio, the same conclusion will be reached: the nutritional value of protein in spinach is far less than that of eggs.

　　Finally, the cell structure of plants is different from that of animals. The cell membrane is surrounded by a relatively dense cell wall mainly made of cellulose “woven”. Cellulose is a large molecular substance that cannot be digested and absorbed, and will affect the digestion and absorption of other nutrients. Therefore, the protein digestibility of plant foods such as spinach is generally lower than animal foods such as fish, egg and milk. The digestibility of protein in kidney beans is only 78% (spinach was not found at the moment, temporarily use kidney beans to explain), and the protein digestibility of eggs is as high as 97%.

In summary, although spinach and other vegetables (the same is true of fruits) contain some proteins, because of the large detection error (higher), the ratio of essential amino acids is not suitable for human needs, low digestibility and other practical factors, Therefore, it can not be used as a good source of protein.