Cars need to burn gasoline as energy to start, airplanes need to burn diesel as energy to fly, and people need to use food as energy to survive. From the perspective of physics, the human body is no different than a machine that consumes energy. It consumes energy (eating) and consumes energy (activity and survival) every day. Until the end of life, it no longer eats (intake energy) nor activities and Survival (consumption of energy).

　　As we all know, although low-quality gasoline (energy) can also drive car engines, it will damage the performance of the car and shorten the service life of the car. Why is it not so? Ingesting poor energy can make people survive, but it will reduce human vitality and damage its lifespan. On the contrary, ingesting high-quality energy will promote health and maintain longevity at or near the maximum design value in the process of keeping the human body running (currently, the maximum design value of human life is genetically determined). So, what is the high-quality energy for the human body?

　　From the perspective of energy metabolism, sugar (also known as carbohydrates) is undoubtedly the most suitable energy source for this machine. In the course of evolution, people have developed an efficient sugar metabolism system, and this set of sugar metabolism systems can be said to be the basis or hub for the metabolism of other substances. In human cells, sugar metabolism is rapid, direct, fast, and very clean. The final product is only water and carbon dioxide, and does not produce any substances that are harmful to health. If you want to talk about the shortcomings, the amount of sugar stored in the body (blood sugar or glycogen) is very small, so it must be supplied regularly or in time. Because of the excellent performance of sugar, it is generally recommended that more than half of the energy required by the body (55% to 65%) be provided by sugar. Taking 2,000 kilocalories of energy throughout the day as an example, 275 to 325 grams of sugar should be ingested.

　　 carbohydrates are mainly found in plants, from photosynthesis of plant leaves. Cereals, potatoes, fruits and vegetables are natural sources of sugar, and refined sugars (such as white sugar, syrup, etc.) are widely found in various cereal processed foods (such as biscuits, bread, etc.) and beverages. Because it is difficult for animals to store more sugar, animal foods such as meat, eggs, fish and seafood are extremely low in sugar. But milk is an exception. Milk contains lactose, a sugar specially prepared for breast-feeding young children. It does not come from food, but from the secretion of the mother''s breast.

　　 sugars include many types, such as starch, sucrose, glucose, fructose, maltose, lactose, etc., their chemical structures are different, but they are the same as the basic process of energy metabolism in the body. To maintain basic life, all kinds of sugars can play roughly the same role. However, this does not mean that they have the same health effects. On the contrary, different foods have different effects on health. Coarse grains (such as whole wheat, brown rice, millet, corn, buckwheat, sorghum rice, barley, etc.), carbohydrates (such as red beans, mung beans, kidney beans, etc.), sugars in potatoes, fruits and vegetables, digest and absorb faster Slow, less stimulating effect on insulin secretion from the pancreas, it is a high-quality sugar energy source. And sugar in fine grains (such as white buns, white rice, white porridge, white bread, white noodles, etc.), sweets (such as snacks, cakes, snacks, ice cream, etc.) and beverages (such as cola, juice drinks, milk drinks, etc.) , The rate of digestion and absorption is faster, and the stimulation of insulin secretion by the pancreas is strong, which can easily cause insulin resistance. Insulin resistance is the common pathological basis of chronic diseases such as atherosclerosis, type 2 diabetes, dyslipidemia, hypertension, fatty liver, and obesity. Therefore, they are low-quality sugar energy sources. I am afraid that long-term intake will damage the performance and life span of this energy machine.

　　Fat is another energy source that can be used by the human body and is widespread in food. It is characterized by high caloric value, and the energy produced by the metabolism of 1 gram of fat in the body is more than twice that of carbohydrates. Another distinguishing feature of fat is that it can be stored in the body in large quantities. The fat stored in the body comes from excessive intake of fat, sugar and so on. The process of metabolizing fat in the human body is much more complicated and “difficult” than that of carbohydrates. Moreover, the metabolic process of fat cannot be separated from the support and help of carbohydrates. Without the help of sugar (such as in a hungry state), fat cannot be smoothly metabolized into carbon dioxide and water, and "ketone bodies" will be produced. The ketone body is acidic, and a small amount of ketone body is not a problem, but a large number of ketone bodies will cause serious consequences-ketoacidosis (this is one of the main reasons for people starving to death). In general, fat is not a high-quality energy source for the human body. It is generally claimed that the energy provided by dietary fat accounts for 20% to 30% of the energy required throughout the day. Taking 2000 kilocalories of energy as an example throughout the day, 45 to 67 grams of fat should be consumed.

　　 Unlike carbohydrates, which mainly come from plant foods, fats mainly come from animal foods. This is because the animal body can synthesize (using sugars, etc. from plants as raw materials) and store large amounts of fat (if it has enough food). Foods such as meat, eggs, milk, fish and seafood all contain varying amounts of fat. Plant foods (such as those mentioned above) usually contain very little fat. But the seeds of many plants contain a lot of fats, such as soybeans, certain nuts (peanuts, sunflower seeds, cashew nuts, almonds, walnuts), rapeseed, cottonseed, pumpkin seeds, etc. These plant seeds or fruits (such as palms, olives, etc.) are also used as oil to extract refined edible oils. With the development of technology and economy, the consumption of edible oil has been increasing. Not only is it directly used for cooking, but it is also added in a large amount to various processed foods, such as biscuits, instant noodles, snacks, bread, snacks, fillings, etc. . In China, edible oil has surpassed animal foods, becoming the largest source of fat, and is the main cause of excess fat (more than 30% of the reasonable upper limit).

　　Protein is also an energy that the human body can metabolize. Protein is the most important nutrient required by the human body. It is not only the key component of our body (viscera, bones, skin, muscles, blood vessels, nerves, etc.), but also the main component of physiologically active substances (such as hormones, enzymes, antibodies, etc.) ingredient. So the recipes for humans (all animals are the same) must contain a certain amount of protein. Generally, it is appropriate for adults to consume 0.8-1 g/kg body weight of protein per day. Take a person with a weight of 70 kg (fat and thin) as an example, 56-70 grams of protein should be ingested every day.

　　However, when used as an energy source (more protein intake, or starvation), protein is a poor energy source. This is because protein molecules contain unique "nitrogen" (N) elements, which in the body are metabolized in addition to carbon dioxide and water, but also produce nitrogen-containing wastes such as urea, creatinine, and ammonia. A small amount of metabolic waste can be detoxified in the liver and excreted through the kidneys (urine), which is harmless. However, if a large amount of nitrogen-containing metabolic waste is generated, it will undoubtedly increase the workload of the liver and kidney. Therefore, ingesting a large amount of protein for energy use is not worth the gains and has always been opposed by mainstream nutrition. It is generally believed that the energy provided by protein should account for 10% to 15% of the energy required throughout the day. This proportion is significantly lower than sugar (55%~65%) and fat (20%~30%).

　　 The main sources of protein are animal foods such as fish, meat, eggs, milk and soy products. Cereals, nuts, etc. can also provide some protein. Or we can put it another way: In addition to vegetables, fruits, and potatoes (these foods have a high water content, which is the main reason for the low protein content), other natural foods can provide more protein. Protein is the foundation of life, and it is widely found in all animals and plants. From this, it is not difficult to understand that when eating natural foods and sufficient food resources, protein is not easy to lack. This may also be one of the necessary conditions for the animal world to evolve smoothly. Of course, only mankind is eligible to "enjoy" a large amount of artificial junk food, which is another matter.

　　Alcohol (ethanol) is also an energy source that the human body can use. 1 gram of alcohol in the body will be metabolized into 7 kilocalories of energy, more than sugar and protein (both 4 kilocalories). But alcohol has no nutritional value, so nutrition is called "empty heat", that is, empty energy with no physiological value. Intermediate products such as acetaldehyde produced during the metabolism of alcohol can damage the liver, kidneys, and brain. The degree of damage is related to alcohol intake and the individual’s ability to metabolize alcohol, ranging from almost no damage to fatal. The greater the alcohol intake, the worse the liver''s ability to metabolize alcohol (closely related to genetics), the more severe the damage.

　　 It is precisely because alcohol has almost no positive effect, so it is impossible to talk about the recommended intake. On the contrary, there are many authoritative recommendations on the alcohol limit, generally the maximum is not more than 20-25 grams of alcohol per day.