"Dietary Reference Intakes for Chinese Residents"
The Dietary Reference Intake is a set of reference values for the average daily intake of dietary nutrients. It includes Estimated Average Requirement (EAR), Recommended Nutrient Intake (RNI), Adequate Intake (AI), and Tolerable Upper Intake Level (UL).
The EAR is the average requirement for individuals in a population, calculated from individual requirement research data; it can meet the needs of 50% of individuals in a specific gender, age, and physiological status group based on certain indicators; this intake level cannot meet the needs of another 50% of individuals for the nutrient; it serves as the basis for establishing the RNI.
The RNI is equivalent to the traditional RDAs and can meet the needs of the vast majority (97%-98%) of individuals in a specific gender, age, and physiological status group. Long-term intake at the RNI level can satisfy the body's need for the nutrient, maintain health, and keep an appropriate reserve in tissues. The RNI is established based on the EAR and is mainly used as the target value for individual daily intake of the nutrient.
When research data on the individual requirements of a certain nutrient are insufficient to calculate the EAR and thus cannot estimate the RNI, the AI can be set to replace the RNI. The AI is the observed or experimentally obtained nutrient intake level in healthy populations. It can also be used as a target for individual intake and can meet the needs of almost all individuals in the target population.
The UL is the highest average daily intake level of the nutrient that can be tolerated. "Tolerable" means that this intake level can be endured without harming health for almost all individuals in the general population. When intake exceeds the UL and continues to increase, the risk of harm to health also increases.
**Chapter One...**
**Composition of the Human Body and Digestion and Absorption of Food**
**Section One: Composition of the Human Body**
The human body is an organism based on matter. According to the extent of people's understanding of the body, the human body can be understood from five levels: atomic level, molecular level, cellular level, tissue level, and finally the overall level.
**One, Atomic Level**
At the atomic level, there are more than 130 known elements, among which the human body contains more than 60 elements, mainly oxygen, hydrogen, carbon, nitrogen, calcium, and phosphorus, etc. Among them, the content of oxygen is about 65%, carbon is about 18%, hydrogen is about 10%, nitrogen is 3.0%, calcium is 2.0%, phosphorus is 1.0%. Oxygen, carbon, hydrogen, and nitrogen account for 96% of the total weight of the human body. Although other elements account for a small proportion in the human body, it does not mean that they are unimportant, such as hemoglobin, which is the carrier of oxygen in the body, and iron is an important component of hemoglobin.
**Two, Molecular Level**
At the molecular level, the human body is composed of proteins, lipids, carbohydrates, water, and minerals. Taking a 65kg male as an example, the water content in his body is about 40kg, accounting for more than 60% of body weight; lipids are about 9kg, accounting for 14% of body weight, of which about 1kg is essential for life activities, and the rest is energy storage, which can change according to the activity status of the body; proteins are about 11kg, accounting for 17% of body weight, most proteins exist in the body as basic components, and a loss exceeding 2kg will lead to serious physiological dysfunction. Carbohydrates in the body mainly exist in the form of glycogen and can be consumed in reserves not exceeding 200g.
**Three, Cellular Level**
At the cellular level, the human body is composed of cells, extracellular fluid, and extracellular solids. Cells are the main components of the body's functions. Cells are usually divided into muscle cells, fat cells, epithelial cells, nerve cells, etc., according to the tissues in which they exist.
**Four, Tissue Level**
At the tissue level, the human body is composed of tissues, organs, and systems, so body weight equals the sum of fat tissue, skeletal muscles, bones, blood, and other visceral organs. Fat tissue includes fat cells, blood vessels, and some supportive structural components, which are the main places for storing fat. There are more than 400 skeletal muscles, and the proportion of body weight varies depending on gender and age. Adult males account for about 40%, and adult females account for about 35%. Limb muscles account for about 80% of the total muscle weight, with lower limb muscles accounting for about 50% and upper limb muscles accounting for about 30%. The total blood volume of a normal person accounts for about 8% of body weight. A 50kg person has approximately 4000ml of blood, and only 70%-80% of the total blood participates in circulation. The rest is stored in the liver, spleen, and other human blood banks. When the body experiences slight blood loss, the blood stored in the human blood bank is immediately released to replenish it. The skeleton is the body's support system, consisting of 206 bones, and the weight of an adult skeleton is approximately 9kg.
**Five, Overall Level**
It should be noted that the composition of the human body at various levels is a dynamic process. For an individual, the body composition will show certain changes during different periods such as fetus, infant, toddler, adolescence, adulthood, and old age, and it will also undergo certain changes under conditions such as disease and stress. However, under normal circumstances, within a specific period of time, such as measured in months or years, the composition of the human body at various levels is relatively stable, meaning that there is a stable quantitative relationship between each part. Therefore, the composition of the body at various levels can be determined by human measurements at the overall level. This is also the reason why anthropometric indicators such as height, weight, skinfold thickness, and Body Mass Index (BMI) are widely used in evaluating the nutritional status of the human body.