The core drivers of human energy production: the physiological functions of carbohydrates and proteins in life activities and intake strategies.

Dietary carbohydrates fall into two categories: those that can be digested and absorbed by the body, and fiber, which cannot be digested and absorbed but is beneficial to health. Carbohydrates are essential nutrients, and fiber is an essential dietary component; both are crucial for human health. Carbohydrates are one of the three major energy-producing nutrients; the brain, blood cells, skin, and testes all use glucose as an energy source. In the dietary structure of Chinese residents, carbohydrates provide approximately 60% of total energy. Carbohydrates also provide the conditions for the metabolism of fatty acids, glycerol, amino acids, and other organic substances, participating in the formation of essential life substances. It is best to consume polysaccharides in a daily diet, as this generally provides other nutrients and fiber. For example, rice and vegetables primarily provide polysaccharides, while vegetables primarily provide various nutrients. Excessive intake of monosaccharides and disaccharides, i.e., excessive consumption of sweets, can cause tooth decay and induce cardiovascular disease and diabetes. Chinese residents mainly rely on grains and tubers as sources of carbohydrates, with starch being the primary form of polysaccharide. Most nutritionists consider this a suitable approach. Diseases caused by genetic defects or deficiencies in digestive enzymes, such as lactase deficiency, can lead to bloating and diarrhea after consuming milk, a condition with a higher incidence in Asians. Other deficiencies include sucrase and maltase, preventing the intake of corresponding disaccharides. Cellulose refers to indigestible carbohydrates in food, primarily derived from plant sources, mostly as plant support structures and cell walls. Research over the past 20 years has found a link between low dietary fiber intake and high rates of colorectal cancer and irritable bowel syndrome among urban residents of industrialized countries, highlighting the importance of cellulose in human health. Cellulose stimulates the intestinal wall, promoting peristalsis and possessing strong water absorption, increasing stool volume and facilitating defecation, thus removing harmful substances from the intestines. Cellulose also adsorbs carcinogenic and tumor-promoting substances produced by bacteria acting on bile acids, lowers serum cholesterol, reduces postprandial blood sugar, prevents excessive energy intake, and adsorbs harmful chemicals such as additives, pesticides, and detergents in certain foods, thus benefiting health. Protein is the most abundant solid component in the human body and the material basis of life. Therefore, protein is closely related to human growth, development, and health, and is an important nutrient. Proteins are the main organic components of various tissues, and there are many different types. Each protein has important physiological functions that cannot be replaced by carbohydrates or lipids. Muscle proteins contract to perform work, apolipoproteins transport lipids, receptor proteins receive biological information, hemoglobin carries oxygen, histones and non-histone proteins, along with DNA, form chromatin and regulate gene switching, oncoproteins and various tumor suppressor proteins regulate normal cell growth, division, and apoptosis, plasma proteins regulate osmotic pressure, transport metal ions and bilirubin, and growth factors promote cell proliferation and maturation. In short, no physiological activity can be separated from protein. The various amino acids produced from the breakdown of proteins in the body are the raw materials for the synthesis of important nitrogenous substances such as pyrimidines, purines, creatine, choline, adrenaline, carnitine, and taurine. Protein metabolism in the body is highly active, with different proteins having varying renewal times. A 70kg adult renews approximately 400g of protein daily. Each gram of protein, when completely broken down, releases 4 kilocalories of energy, making it one of the three major energy-producing nutrients. The human body's need for protein essentially refers to its need for amino acids, especially essential amino acids. The body's requirements for both protein and essential amino acids decrease with age, with a more pronounced decrease in the requirement for essential amino acids. The proportion of essential amino acids in total protein requirements is 43% for infants, 36% for children, and 19%–20% for adults. Besides quantity, the proportions of various essential amino acids are also important. Dietary protein can only be fully utilized by the body when its amino acid profile closely resembles that of human protein. Therefore, consuming a variety of dietary proteins allows for the exchange and complementation of amino acids, particularly essential amino acids, promoting optimal absorption and utilization. Chinese dietary habits emphasize diverse combinations, which is highly beneficial for maximizing the complementary effects of proteins and enhancing their nutritional value. Animal-based foods generally contain more protein than plant-based foods. Most animal proteins contain all the essential amino acids, and their content and composition are similar to human proteins, making them easily absorbed and utilized by the body; these are nutritionally termed "high-quality protein." Soy protein, a type of plant protein, is also considered high-quality protein and is an important dietary protein source for Chinese residents. Pork contains 15%–20% protein, while beef and mutton contain less fat, thus their relative protein content is higher than pork. Chicken, duck, and fish all contain around 20% protein.