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Because I have always had some scientific knowledge of health preservation, I knew that vitamin C can help fight colds. This happened again, so I checked the relevant information. The following is a useful part of what I extracted:
High-dose intake (thousands of milligrams) may cause diarrhea, but as long as the intake is reduced immediately, it will not be harmful to the human body.
Those who approve of a drug alternative (taking vitamin C without taking drugs) declare that the lowest dose sufficient for diarrhea is the true vitamin C requirement.
Cathcart and Cameron demonstrated that patients with terminal cancer or severe influenza who consume up to 200 grams of ascorbic acid will not have any diarrhea. As the symptoms worsen, the human body will increase the intake of vitamin C to fight the virus and other highly stressful crises. After recovery, the saturation of vitamin C is reduced to 4-15 grams per day.
Vitamin C is an essential ingredient in the process of producing antibodies (IgG) and complement. Therefore, vitamin C can help eliminate viruses, but it is also an essential ingredient for the human body to supplement nutrition, neutralize toxins produced by viruses and repair physique damaged by viruses. , When vitamin C in the body is depleted, the condition will deteriorate rapidly.
However, when fighting against bacterial infections that multiply rapidly (such as pneumococcus or mycoplasma), high amounts of vitamin C can only fight (prevent the disease from getting worse) but cannot be cured. It can only be combined with the use of drugs (such as antibiotics). To achieve the maximum effect.
The United States officially defines the Tolerable Upper Intake Level (Tolerable Upper Intake Level) for a 25-year-old man as 2,000 mg per day. (2g)
The recommended intake is for general healthy individuals. When the body produces a large amount of antibodies and complement, the demand for vitamin C, which is one of the necessary reactants, is bound to increase. (Please refer to the reaction process of antibody and complement production in immunology and biochemistry)
If you want to take a lot of vitamin C to treat diseases, calcium ascorbate should not be used, because excessive calcium will consume vitamin C to be excreted from the body. If you take a lot of vitamin C orally, you still need to use pure vitamin C (ascorbic acid).
Vitamin C tablets are very common in the market. In the early years, they were extracted from natural fruits, but now they are completely synthesized from glucose using chemical and fermentation methods. Synthetic vitamin C is exactly the same as natural vitamin C due to bacterial fermentation, and has the same right-handed optical rotation.
The main work of the immune system is done by white blood cells and lymphocytes. The content of vitamin C in the two is 30 times the content of vitamin C in the blood. White blood cells and lymphocytes must have enough vitamin C to swallow filtering viruses and bacteria, so the body’s immunity is closely related to the stock of vitamin C. Vitamin C has a strong reducing ability. Many biochemical reactions in the body need the help of vitamin C to complete. Vitamin C can prevent white blood cells from being damaged by auto-oxidation, so it can strengthen the immune system.
Taking a lot of vitamin C will increase the concentration of antibodies such as IgA, IgG and IgM in the blood. These antibodies attach to foreign viruses and bacteria, directing white blood cells and lymphocytes to destroy them.
Vitamin C is an extremely strong anti-oxidant substance, which can be effective against free radical substances produced by the human body in long-term exposure to bad environments (lipid peroxide, smoking, drinking, insect bites and many chemical toxins) The removal. The medical profession recognizes that free radicals are related to the occurrence of cancer or aging. Nitrite is a carcinogen. If there is enough vitamin C in the body, it can prevent the conversion of nitrite used for curing meat to produce nitrite. 
Vitamin C participates in many biochemical reactions in the human body. When vitamin C is lacking, these reactions cannot proceed smoothly, causing lesions in many related organs. Most animals can produce vitamin C on their own in the liver, so they rarely catch common colds, coronary artery blockages, and cancers that are unique to humans.
Hemilä H, Louhiala P. Vitamin C for preventing and treating pneumonia. Cochrane Database Syst Rev. 2007, (1): CD005532. PMID 17253561. doi:10.1002/14651858.CD005532.pub2.
Cathcart, Robert. Vitamin C, Titrating To Bowel Tolerance, Anascorbemia, and Acute Induced Scurvy. Orthomed. 1994[2007-02-22]. (Archived from the original content on 2013-04-28).
Then say what the two scientific documents I read say:
The first article pointed out in the conclusion that the effectiveness of vitamin C in the fight against pneumonia still needs further proof. but! In their experiments, high-dose vitamin C is effective in treating severe pneumonia. (In other words, although they saw the effect, they felt that their samples were too few)
The second article says that when vitamin C is used for treatment, the highest dose that can be taken can be simply determined as “the dose that just doesn’t have diarrhea.”