Why are we so sure that vitamin C is good for us?
And not just good, but extremely health-promoting. Is that really the case?
The answer is, there is a good reason why lemons are so strongly associated with vitamin content and health: their high vitamin C content of 53 mg in 100 g (66% of the daily requirement).
Most people take vitamin C in winter for cold prevention, either in the form of tablets or lemon juice.
But: Vitamin C has more than one effect on health
First and foremost, vitamin C is a powerful antioxidant. In all cells and tissues of our body it acts as a balancing helper against free oxygen radicals.
Its ability to transfer electrons to other molecules makes it a central switch of oxidation balance.
Through these and other important tasks within the body and since it cannot be produced by humans, we call it vitamin. In chemical terminology, it is called ascorbic acid.
Sailors in the past often suffered from scurvy, a massive weakening and damage to the connective tissue, which was later identified as vitamin C deficiency. On long cruises, fresh fruits were missing in the diet.
It was discovered later on that vitamin C was also needed for the synthesis of collagen.
Ascorbic acid is therefore essential for a healthy body. But:
Does the vitamin have a preventive or even healing effect on diseases?
Vitamin C regulates a healthy immune function
When the influence of vitamin C on the immune system was investigated for the first time, it was only established that vitamin C was present in a great concentration in most white blood cells important for the immune system.
It turned out that vitamin C acts as a cofactor of important enzymes in white blood cells (immune cells). It is therefore essential for their metabolism and function in the fight against pathogens1.
In vitro analyses confirmed the antimicrobial properties of the molecule. In addition, vitamin C favoured immune cell division. During an infection, ascorbic acid is consumed to a high degree, i.e. it is used for a specific purpose in supporting the immune defence2.
The European Food Safety Authority (EFSA) confirms an effect between vitamin C intake and optimal immune function in adults and children under the age of three3.
Presumption of a much stronger effect
For some time now, there has been an unconfirmed belief that vitamin C can be used against cancer. Particularly in the USA, highly concentrated intravenous vitamin C has already been advertised by various suppliers.
The only approved treatment methods for tumor diseases are operation, chemotherapy and radiation therapy. Suppliers of the vitamin C "remedy" put patients at risk by promising what they may not be able to provide. There is no clinical data to confirm effective treatment with high levels of the vitamin.
However, it is interesting to assess the thoughts behind this claim systematically.
Because although it should not be used for treatment, vitamin C still has proven beneficial for our health.
So where does one draw the line and what has actually been studied?
Background of the treatment with vitamin C
The preventive effect of vitamin C on chronic diseases can be attributed to its antioxidant properties and the support of a healthy immune system.
It is also suspected that vitamin C may have an antiproliferative effect on cancer cells in very high concentrations4. This means that vitamin C influences the ability of tumor cells to multiply.
It has also been observed that very high doses of vitamin C can make cancer cells more sensitive to certain cytostatic agents5. Both of these points affect the cell division of cancer cells and might slow down growth. Which hasn't been proven in humans.
Chemotherapies have massive side effects that weaken the patient because they are designed to combat fast-dividing cells, which include cancer cells as well as cells of the immune system that are also destroyed by the therapy.
In order to curb the side effects of chemotherapy, vitamin C was tested as a complementary therapeutic and was able to reduce the toxicity of some chemotherapeutics in animal experiments, but above all to support the immune system6,7.
Chemotherapy and vitamin C deficiency
In general, tumor patients easily get a vitamin C deficiency due to their altered metabolism8. The low serum concentration of the vitamin in cancer patients, despite adherence to the recommended daily dose, could be caused primarily by the increased need.
During cancer treatment, there is a massively increased consumption of the antioxidant for the detoxification of reactive oxygen radicals, which are formed during surgery, chemo, and radiation.
Oxidative stress and wound healing thus consume a great deal of vitamin C to keep the body in balance despite the high stress9,10,11.
Why the FDA banned intravenous vitamin C in America
The FDA is the US Food and Drug Administration under its Department of Health. At the end of 2010, this agency sent a reminder to the Californian company McGuff, which at that time manufactured intravenous vitamin C preparations, demanding that production be discontinued. At first, there were massive outcries.
So what happened?
The FDA is investigating whether drugs are marketed without their prior approval. To be approved there needs to be an examination of safety and efficacy.
In contrast to conventional vitamin C preparations, which are bought and used to compensate for deficiencies or to support health in stressful situations, the intravenous variant is a drug. It's marketed as remedy. But this use is not confirmed.
The verdict: the preparations, which are highly concentrated, are not designed for diffuse health care, but "were meant to treat a disease which should neither be self-diagnosed nor treated without a therapist "12.
The FDA does not rule out efficacy forever. It merely protects patients from an unconfirmed treatment method that may be ineffective or even pose additional risks.
Vitamin C as a high-dose supplement
In fact, even in higher dosages, the vitamin is safe for most adults!
Various studies13 on vitamin C as a dietary supplement showed no significant side effects14 even at higher doses between 500 and 2000 milligrams per day. The NIH (USA) defined a Safe Maximum Intake per day of 2000 mg for adults.
Side effects that occurred occasionally included: gastrointestinal problems, mild diarrhea or nausea caused by the osmotic effects of unabsorbed ascorbic acid in the digestive tract. Vitamin C cannot be completely absorbed in this high concentration and attracts water in the intestine.
The feared negative effects on iron uptake and metabolism could not be confirmed.
Orally administered vitamin C in conventional form, however, is difficult to get into the bloodstream in large concentrations, which is why IV administration probably occurred in the first place.
Less invasive dietary supplements include liposomal ascorbic acid or combinations with bio-enhancers. Like that, enough of the substance reaches its site of action, making it very suitable for short cures.
 Ang, Abel et al. “Vitamin C and immune cell function in inflammation and cancer” Biochemical Society transactions vol. 46,5 (2018): 1147-1159.
 Wintergerst ES, Maggini S, Hornig DH (2006). "Immune-enhancing role of vitamin C and zinc and effect on clinical conditions". Annals of Nutrition & Metabolism. 50 (2): 85–94. doi:10.1159/000090495.
 EFSA Panel on Dietetic Products, Nutrition and Allergies (2015). "Vitamin C and contribution to the normal function of the immune system: evaluation of a health claim pursuant to Article 14 of Regulation (EC) No 1924/2006". EFSA Journal. 13 (11): 4298. doi:10.2903/j.efsa.2015.4298.
 Fromberg A, Gutsch D, Schulze D, Vollbracht C, Weiss G, Czubayko F, Aigner A, Ascorbate exerts anti-proliferative effects through cell cycle inhibition and sensitizes tumor cells towards cytostatic drugs. Cancer Chemother Pharmacol 67(5): 1157-1166, 2010.
 Kurbacher CM, Wagner U, Kolster B, Andreotti PE, Krebs D, Bruckner HW, Ascorbic acid (vitamin C) improves the antineoplastic activity of doxorubicin, cisplatin, and paclitaxel in human breast carcinoma cells in vitro. Cancer Lett 103(2):183-189, 1996.
 El-Merzabani MM, El-Aaser AA, Osman AM, Ismael N, Abu el-Ela F, Potentiation of therapeutic effect of methanesulphonate and protection against its organ cytotoxicity by vitamin C in Ehrlich ascites carcinoma bearing mice. J Pharm Belg 44(2): 109-116, 1989.
 Prasad SB, Giri A, Arjun J, Use of subtherapeutical dose of cisplatin and vitamin C against murine Dalton's lymphoma. Pol J Pharmacol Pharm 44(4):383-391, 1992.
 Vollbracht, C., Schneider, B., Leendert, V., Weiss, G., Uerbach, L., Beuth, J., Beuth, J., Intravenous Vitamin C Administration Improves Quality of Life in Breast Cancer Patients during Chemo-/Radiotherapy and Aftercare: Results of a Retrospective, Multicentre, Epidemiological Cohort Study in Germany, In Vivo, 11-12/2011, vol. 25, no. 6, 983-990.
 Beuth JH N, van Leendert R, Basten R, Noehle M, Schneider B, Safety and efficacy of local administration of contractubex to hypertrophic scars in comparison to corticoid treatment. Results of a multicenter, comparative epidemiological cohort study in Germany. In Vivo 20: 277-284, 2006.
 Akinloye O, Adaramoye O, Kareem O, Changes in antioxidant status and lipid peroxidation in Nigerian patients with prostate carcinoma. Pol Arch Med Wewn 119(9): 526-532, 2009.
 Esme H, Cemek M, Sezer M, Saglam H, Demir A, Melek H, Unlu M, High levels of oxidative stress in patients with advanced lung cancer. Respirology 13(1):112-116, 2008.
 Warning Letter an McGuff Compounding Pharmacy Services Inc.: https://www.fda.gov/ICECI/EnforcementActions/WarningLetters/2015/ucm473759.htm.
 Hathcock JN, Azzi A, Blumberg J, Bray T, Dickinson A, Frei B, Jialal I, Johnston CS, Kelly FJ, Kraemer K, Packer L, Parthasarathy S, Sies H, Traber MG. Vitamins E and C are safe across a broad range of intakes. Am J Clin Nutr. 2005 Apr;81(4):736-45.