Vitamin C is mostly known for its role in boosting immunity and preventing scurvy. It works as an antioxidant in the body AND plays an important role in the production of collagen, fibrous protein that hold together your connective tissue present in muscles, ligaments, tendons, bones, blood vessels, skin and eyes. These two properties are responsible for the majority of vitamin C health benefits.
This is how vitamin C is believed to support your heart health, maintain the integrity of your bones and keep your skin, gums and eyes healthy. Recently, intravenous administration of high-dose vitamin C is also garnering a lot of attention as a promising cancer therapy. In this context, a recent study found that Vitamin C is up to ten times more effective at stopping the growth of cancer cells than pharmaceutical cancer drugs. 
This study provides the first of its kind evidence that vitamin C can be effectively used to target and destroy cancer stem cells (CSCs) – specific cells known to drive cancer formation in the body.
What are cancer stem cells (CSCs)?
Research shows that all cancer cells are not same. Some behave as stem cells and have the ability to self-renew and thrive. This is the reason why certain cancer cells develop resistance to chemotherapy and radiotherapy, eventually leading to treatment failure in patients with advanced cancer. Drug resistance in cancer cells is one of the major hinderances in chemotherapy. In addition, CSCs are also implicated in tumor recurrence and cancer regrowth down the road.
In the study, researchers analysed compounds like vitamin C, milk thistle, caffeic acid phenyl ester (CAPE, a honey-bee derivative) and three experimental pharmaceuticals (actinonin, FK866 and 2-DG).
The team investigated how these natural and pharmaceutical compounds affected the natural ability of cancer stem cells to survive and flourish. The idea was to single out mechanisms that allow cancer stem cells to sustain themselves, with a purpose to disrupt their energy metabolism and starve them.
The team proposed that cancer stem cells are able to increase the number and mass of their mitochondria to generate more energy. In cancer cells, the energy (ATP) production process switches from oxidative phosphorylation (a process through which healthy cells make their energy in the mitochondria) to glycolysis. This metabolic flexibility allows these cells to meet their increased energy requirements and continue to thrive.
This makes glycolysis an attractive target for cancer treatment. The study found that vitamin C behaves as an inhibitor of glycolysis, thus halting energy creation in cancer cells and starving them.
Other compounds such as 2-DG, silibinin (milk thistle) and stiripentol also showed similar anti-glycolysis effects. But what stood out was that vitamin C is ten times more effective than the experimental cancer drug 2-DG. Though stiripentol was 50-100 times more effective than 2-DG, the use of stiripentol is largely limited to the treatment of epileptic seizures in children and not for cancer. These results predict a promising role for natural agents like vitamin C in the war against cancer.
In another study, the same team also found that vitamin C, when combined with antibiotic doxycycline, becomes even more effective in eradicating cancer stem cells. 
Doxycycline can kill CSCs in multiple cancer cell lines, but these cells can eventually become resistant to the antibiotic drug. Professor Michael Lisanti, who designed the study, explained: "We now know that a proportion of cancer cells escape chemotherapy and develop drug resistance; we established this new strategy to find out how they do it. We suspected the answer lay in the fact that certain cancer cells -- which we call metabolically flexible -- are able to switch their fuel source. Thus, when the drug treatment reduces the availability of a particular nutrient, the flexible cancer cells can feed themselves with an alternative energy source." 
The study found that the combination of vitamin C and doxycycline prevented the cancer cells from switching to an alternative source of energy, making then metabolically inflexible. With their relatively non-toxic profile, this combination therapy could significantly cut down the dangerous side effects associated with conventional cancer treatments.
Vitamin C and cancer
A 2017 study published in Frontiers in Oncology found that a diet high in vitamin C and selected carotenoids (for example, β-carotene, α-carotene, β-cryptoxanthin, and lycopene) may protect against lung cancer, even for individuals who smoke heavily.  Another recent study from Japan showed that that vitamin C can cut down mortality risk in women with breast cancer by 25 percent.
Vitamin C is a powerful anti-oxidant and is the key reason why it plays such an important role in preventing cancer. Free radicals cause oxidative damage to cellular structures such as lipids, DNA, proteins and enzymes – resulting in altered chemistry and function of these critical biological molecules. This eventually leads to chronic inflammation and several health conditions including cancer, cardiovascular disease, cataracts, diabetes etc.
Your body relies on all kinds of antioxidant systems; both endogenous (that are produced within) and exogenous (supplied from outside) to prevent as well as control this oxidative damage. Vitamin C, owing to its amazing capacity to neutralize free radicals, is an important player in this fight against cellular oxidation.
A 2011 study found that women who used vitamin C shortly after they were diagnosed with breast cancer had reduced mortality and recurrence risk.  A 2014 meta-analysis also suggested that “post-diagnosis vitamin C supplement use may be associated with a reduced risk of mortality” among women with breast cancer. 
In fact, vitamin C is also known for its ability to destroy free radicals produced by exposure to ionizing radiation, thus preventing cellular and DNA damage in exposed individuals. A study found that vitamin C, along with other antioxidants, lowered DNA damage and reduced overall risk of developing cancer in workers exposed to ionizing radiations at the Japan’s Fukushima nuclear power plant. 
However, recent studies demonstrate that it might not be just the vitamin’s antioxidant properties that are responsible in its anti-cancer benefits. There may be other mechanisms involved. For example, the scientists from the University of IOWA showed that high levels of vitamin C selectively destroy cancer cells, leaving healthy cells unharmed at the same time. 
In this case, the researchers explained that vitamin C works as a pro-oxidant and shows selective toxicity towards cancer cells. What happens is that vitamin C tends to quickly disintegrate into hydrogen peroxide (H2O2), which is a free radical. We already know how increased levels of reactive oxygen species (ROS) including H2O2 are believd to play an important role in cancer growth and development.
Normal, healthy cells have various devices at their disposal to quickly and efficiently eliminate H2O2, which helps control excessive accumulation of free radicals. The enzyme catalase, which is found in high amounts in healthy cells, is one such important mechanism through which normal cells can get rid of H2O2.
Cancer cells don’t make enough catalase, which limits their ability to remove high amounts of hydrogen peroxide. When cancer cells are swamped with high doses of vitamin C, it leads to overwhelming production and build-up of hydrogen peroxide free radical in cancer cells, which turn fatal.
The results of a 2018 study also suggested a potential role of vitamin C in breast cancer prevention and treatment, thorough mechanisms that are distinct from vitamin C’s antioxidant abilities. 
Intravenous administration helps the body to achieve higher amounts of circulating vitamin C in comparison to oral consumption. And while you need small doses of vitamin C to prevent scurvy, you certainly require high levels of vitamin C to kill cancer cells. Taking high quality liposomal vitamin C is another effective way to raise your levels of vitamin C in the blood.
- Bonuccelli et al. NADH autofluorescence, a new metabolic biomarker for cancer stem cells: Identification of Vitamin C and CAPE as natural products targeting “stemness”. Oncotarget, 2017
- Francesco et al. Vitamin C and Doxycycline: A synthetic lethal combination therapy targeting metabolic flexibility in cancer stem cells (CSCs). Oncotarget. 2017
- University of Salford. Vitamin C and antibiotics: A new one-two 'punch' for knocking-out cancer stem cells. ScienceDaily. 2017.
- Shareck, et al. Inverse Association between Dietary Intake of Selected Carotenoids and Vitamin C and Risk of Lung Cancer. Frontiers in Oncology. 2017.
- Nechuta et al. Vitamin supplement use during breast cancer treatment and survival: a prospective cohort study. Cancer Epidemiol Biomarkers Prev. 2011
- Harris et al. Vitamin C and survival among women with breast cancer: a meta-analysis. Eur J Cancer. 2014
- Yanagisawa A. Orthomolecular approaches against radiation exposure. Presentation Orthomolecular Medicine Today Conference. Toronto 2011 http://www.doctoryourself.com/Radiation_VitC.pptx.pdf
- Garry R. Buettner et al. Tumor cells have decreased ability to metabolize H2O2: Implications for pharmacological ascorbate in cancer therapy. Redox Biol. 2016.
- Sant et al. Vitamin C promotes apoptosis in breast cancer cells by increasing TRAIL expression. Sci Rep. 2018