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What is nutrition? What nutrients does the human body need?
Nutrients refer to the nutrients contained in food. There are many kinds of nutrients in food, about 50 of which are needed by human body. According to their chemical properties or physiological functions, they can be divided into six categories, namely protein, lipids, carbohydrates, minerals, vitamins and water.

According to the human body's demand for nutrients and their contents in the body, it can be divided into macro nutrients and micro nutrients. Macronutrients refer to protein, lipids and carbohydrates (sugars), while micronutrients refer to minerals (inorganic salts) and vitamins. In addition, protein, lipids and carbohydrates can release energy by oxidation in the body, and these three nutrients are also called productivity (heat) nutrients.

Protein is the foundation of all life, accounting for16% ~19% of human body weight. Every day, about 3% of human protein is updated, among which the intestine and bone marrow are the fastest. Protein's main function in the body is to form and repair human tissues; Physiologically active substances such as enzymes, hormones and antibodies participate in regulating physiological functions; Providing energy, each gram of protein is oxidized in the body to generate 16.74 kilojoule energy. Its main food sources are meat, fish, eggs, milk and soybeans.

Lipids include neutral fats and lipids. Neutral fat is commonly called oil, including animal oil and vegetable oil. Its main functions are to store and supply energy, form biofilm, increase satiety and delicious food, supply essential fatty acids, and promote the absorption of fat-soluble vitamins. There are two kinds of fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are often added to health food because they can promote the development of brain and retina and reduce blood fat.

Carbohydrate, also known as sugar, is the main component of rice and wheat. Its main functions are to provide human energy, help the liver detoxify, save protein and promote ketone oxidation. Dietary fiber is a kind of carbohydrate that can't be digested and absorbed by human body, which mainly comes from vegetables and fruits. Its physiological functions are as follows: ① enhancing gastrointestinal peristalsis and facilitating fecal excretion. Middle-aged and elderly people should increase their intake of dietary fiber. ② Control weight and lose weight. Dietary fiber can't be digested and absorbed, but it can produce satiety, reduce energy intake, and achieve the effects of weight control and weight loss. ③ Lowering blood sugar and blood cholesterol, dietary fiber reduces the absorption of sugar by small intestine, and absorbs bile acid and fat, so as to reduce its absorption and achieve the effect of lowering blood fat.

Minerals, also known as inorganic salts, are the general names of other chemical elements except carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) in human body. According to the content in the body, it is divided into macro elements and trace elements. The weight of macroelements in the body is greater than 0.0 1% of the body weight, including seven elements: potassium, sodium, calcium, magnesium, phosphorus, sulfur and chlorine. Others such as zinc, iodine and iron are all trace elements. The minerals most closely related to the growth and development of children and adolescents are mainly calcium, iron, iodine and zinc.

Calcium is the most abundant mineral in human body. Calcium deficiency can cause rickets, osteoporosis, osteomalacia and muscle spasm. The China Nutrition Society recommends that the daily intake of calcium for adults is 800 mg. The best food sources of calcium are milk and dairy products. In addition, beans, dried shrimps, kelp, oil seeds and vegetables are also rich in calcium.

Iron is the most abundant trace element in human body, which is mainly involved in red blood cell composition and oxygen utilization. Iron deficiency leads to iron deficiency anemia, mainly manifested as loss of appetite, irritability and fatigue, pallor, palpitation and shortness of breath, dizziness and decreased immunity. Iron deficiency affects children's intellectual development and behavior. The human body only absorbs bivalent iron, but does not absorb trivalent iron. Some plant foods contain high iron, but the absorption rate is too low to achieve the effect of iron supplementation. Good foods for iron supplementation are animal liver, whole blood, lean meat and fish. It should be pointed out that the content of iron in dairy food is low, so it cannot be supplemented with iron.

The content of zinc in human body is 2 ~ 3 grams, which is one of the important trace elements in human body and closely related to the growth and development of children and adolescents. Zinc deficiency is characterized by slow growth, even stagnation, hypoplasia of sexual organs, inhibition of sexual maturity, hypoplasia of secondary sexual characteristics and sexual infantilism; Loss of taste and smell, loss of appetite and even pica; Low resistance, easy infection and slow wound healing; It can also be manifested as dry and rough skin, facial acne and recurrent oral ulcers. Animal food is a good source of zinc, especially seafood, red meat and animal liver. Plant food contains less zinc and its absorption rate is low.

Vitamins are a kind of low-molecular-weight organic compounds necessary to maintain normal metabolism and physiological functions of the body, which can be divided into fat-soluble vitamins and water-soluble vitamins. Fat-soluble vitamins include vitamin A, vitamin D, vitamin E and vitamin K. Water-soluble vitamins include vitamin B 1, vitamin B2, vitamin B6, vitamin B 12, vitamin C, nicotinic acid, pantothenic acid, folic acid and biotin.

Vitamin A deficiency can lead to decreased adaptability, and in severe cases, it can cause night blindness. Dry keratosis of conjunctiva can form dry eye, which can seriously cause blindness; Dry skin; Children are slow in growth and development, susceptible to infection, impaired in hemoglobin synthesis and metabolism, and have low immunity. High-dose intake of vitamin A can cause acute, chronic poisoning and teratogenic toxicity. Foods rich in vitamin A include animal liver, cod liver oil, fish eggs, whole milk and poultry eggs. Dark vegetables are rich in β -carotene and can also provide a certain amount of vitamin A.

The main function of vitamin D is to regulate the metabolism of calcium and phosphorus in the body, which is beneficial to the normal growth and development of bones and teeth. Foods rich in vitamin D include cod liver oil, cream, chicken liver, eggs, etc., and vitamin D can also be obtained by sunbathing. High-dose intake of vitamin D can cause poisoning.

Vitamin C deficiency can lead to scurvy, which is characterized by gingival swelling and bleeding, conjunctival bleeding, hair follicle keratinization, subcutaneous ecchymosis, purpura and joint pain. Vitamin C mainly comes from vegetables and fruits, and there are many kinds of hot (sweet) peppers, tomatoes, cauliflower and all kinds of dark leafy vegetables, and there are many kinds of fruits such as citrus, lemon, jujube, hawthorn, kiwi fruit and Rosa roxburghii.

The deficiency of vitamin B 1 causes beriberi. Foods rich in vitamin B 1 include cereals, beans, dried yeast, nuts, animal offal, eggs and lean pork.

Vitamin B2 is one of the nutrients that are easy to be deficient in our traditional diet. Deficiency of vitamin B2 can cause angular stomatitis, blepharitis, scrotum (labia) dermatitis, seborrheic dermatitis on both sides of the nose and so on. Because vitamin B2 deficiency can cause inflammation of oral cavity and scrotum at the same time, it is called "oral-reproductive syndrome". Foods rich in vitamin B2 include animal liver, kidney, heart, egg yolk and milk. The content of green vegetables and beans in plant food is high, but there are few grains.

Folic acid deficiency can cause megaloblastic anemia, glossitis and gastrointestinal disorder. In recent years, folic acid deficiency is related to neural tube defects (including anencephaly and spina bifida) in newborns. Foods rich in folic acid include liver, kidney, eggs, fish, green leafy vegetables, nuts and soybeans.

Water is the most basic substance to maintain life activities. The water content in adults accounts for 50% ~ 60% of body weight, about 60% for men and 50% for women. Water has the functions of regulating body temperature, lubricating joints, intestines and respiratory tract, promoting substance metabolism and maintaining tissue morphology. The human body needs about 2500 ml of water every day, which can be obtained from food (about 1000 ml of water), drinking water or beverage (about 1200 ml) and metabolic water (about 300 ml).