It can be said that the human body needs nutrition all the time from the embryonic period to the end of life, and human growth and development, health level, working ability and life expectancy are closely related to nutrition.

Nutrition is related to brain function, malnutrition will have a disastrous effect on the development of the brain of infants and children, resulting in poor intellectual and physical development, even in adulthood can not compensate, but also to pass on the defects to the next generation. Adults, such as malnutrition, poor stamina, fatigue, attention and memory loss, thus affecting the efficiency of physical and mental work.

Nutrition has a direct or indirect relationship to the occurrence and development of many diseases, such as iodine deficiency can suffer from goiter, iron deficiency can suffer from anemia, lack of vitamin D and calcium can suffer from rickets and so on. Malnutrition, the body's immunity is reduced, will increase the prevalence of infectious diseases, prolong the course of the disease, affecting recovery. Nutritional imbalance often becomes obesity, cardiovascular disease and certain tumor causes, seriously affect human health, and even threaten life. Malnutrition also affects endocrine function, including gonadal function.

Women will experience amenorrhea and men will have hypogonadism, thus affecting fertility. Malnutrition in pregnant women can easily cause premature delivery, congenital nutritional deficiency, congenital malformation or stillbirth in the fetus. Overall, the combined impact of nutrition on health is still most profoundly reflected in mortality and average life expectancy.

Proper nutrition is a basic and important part of health care and disease prevention. It can not only promote growth and development, enhance physical fitness, improve intelligence, improve efficiency and improve the level of competition; can also promote tissue renewal, enhance the body's resistance to disease, reduce the rate of disease and reduce mortality. Even if the unfortunate disease, during the illness, through dietary allocation, can also shorten the course of the disease, accelerate recovery.

For each individual, nutrition is related to functional status, intelligence, work capacity, immune function, eugenics and life expectancy. For a nation, nutrition is related to the overall quality of the nation, to the health level of a generation or even generations, and to the future of the country and the nation. All of this is a warning to people at all times and everywhere: for health, we must pay attention to nutrition.

Specifically, a balanced diet refers to the establishment of a balanced relationship between dietary nutrition supply and the physiological needs of the body in four aspects at the same time, namely: amino acid balance, caloric nutrient composition balance, acid-base balance and the balance between the intake of various nutrients, so that there is no shortage, no bias, no excess and no disorder, only in this way is conducive to the digestion, absorption and utilization of nutrients. If the dietary relationship is out of balance, that is, the diet is not adapted to the physiological needs of the human body, it will have adverse effects on human health, and even lead to certain nutritional diseases or chronic diseases.

Amino acid balance, the nutritional value of food protein depends largely on the amount and proportion of the eight essential amino acids contained in food. The ratio of the eight amino acids provided in food is close to the ratio required by the body to effectively synthesize human tissue proteins, and vice versa, it will affect the utilization of protein in food. The World Health Organization has proposed a ratio of 8 essential amino acids required by the human body, and the closer the ratio is to it, the higher its physiological value is. When the physiological value is close to 100, i.e., 100% is absorbed, it is called the total amino acid balance. A protein that can achieve the full amino acid balance is called a complete protein. We can use this criterion to score the amino acids of various food proteins.

Eggs and human milk have amino acid ratios that are extremely close to those of the human body, and therefore can be called amino acid balanced foods. Most foods, on the other hand, have an unbalanced amino acid composition, so the nutritional value of the protein is compromised. For example, the high leucine in corn affects the utilization of isoleucine; the high arginine in millet affects the utilization of lysine. Therefore, the diet based on plant foods, should pay attention to the reasonable combination of food to correct the imbalance of amino acid composition ratio. For example, mixing cereals with beans, making soybean corn flour, soybean millet flour, etc., can improve the utilization rate and nutritional value of protein.

Calorie nutrient composition balance, carbohydrate, fat and protein can provide heat for the body, they are called calorie nutrients. When the total heat provided by caloric nutrients is balanced with the energy consumed by the body, when the ratio of the intake of three caloric nutrients is 6.5:1:0.7, and the heat provided to the body is about 60% to 70% for carbohydrates, 20% to 25% for fats and 10% to 15% for proteins respectively, the special roles of each can play and promote and protect each other, and this total Calorie balance, the ratio of calories (or the ratio of calorie nutrient intake) is also balanced, which is called calorie nutrient composition balance.

Too much calorie nutrient supply will cause obesity, hyperlipidemia and heart disease; too little will cause malnutrition and also induce many diseases, such as anemia, tuberculosis and cancer. The three caloric nutrients affect each other, and when the total caloric balance is unbalanced, the ratio will also affect health. When carbohydrate intake is too much, it increases the burden on the digestive system and kidneys and reduces the opportunity to consume other nutrients. When too many protein calories are provided, it affects the normal function of protein, causing protein depletion and affecting the ammonia balance in the body.

When carbohydrate and fat calories are not supplied sufficiently, the protective effect on protein is weakened. To achieve the normal caloric demand for life and work, usually, the caloric distribution of three meals a day should be 30% for breakfast, 40% for lunch and 30% for dinner to ensure the heat balance of the day.

The balance of nutrient intake is difficult to grasp because of the intricate relationship between various nutrients and the different physiological states and activities, and the different nutrient requirements. Therefore, the Chinese Nutrition Society has formulated the daily supply of various nutrients. Accordingly, the intake of various nutrients in our diet should be kept within a range of not more than 10% error above and below the standard supply within a certain period. This ratio between each other can be called the basic balance between nutrients. It is recommended in the Chinese balanced diet pagoda that each person should eat 300-500 grams of cereals, 400-500 grams of vegetables and 100-200 grams of fruits, 125 grams of animal foods such as fish, poultry, meat and eggs (50 grams of fish and shrimp, 50-100 grams of poultry meat, 25-50 grams of eggs), 100 grams of dairy foods, 50 grams of beans and soy products, and no more than 25 grams of oils and fats. Not more than 25 grams of oils and fats.

Acid-alkaline balance, under normal circumstances, human blood due to its own buffering effect, the PH value is maintained between 7.3 to 7.4. People consume the right amount of acidic and alkaline foods will maintain the acid-base balance of body fluids, but if the food is not properly matched, it will cause a physiological acid-base imbalance. When the food is not properly matched, acidic food in the diet exceeds the required amount, it will lead to acidic blood, blood color deepening, viscosity increase, and in serious cases, it will also cause acidosis, and also increase the consumption of calcium, magnesium, potassium plasma in the body and cause calcium deficiency. This phenomenon is called acidic body, will affect the health of the body. Common acidic foods include: egg yolk, rice, flour, chicken, eel, carp, pork, beef, dried squid, beer, peanuts, etc.