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Vitamin C: Oral vs. Intravenous, Immune Effects, Cancer, Exercise Adaptation & More | Found My Fitness with Dr. Rhonda Patrick

Key Takeaways

  • Vitamin C is a potent antioxidant and supports many physiological processes including immune support, wound function, fatty acid metabolism, neurotransmitter production, blood vessel formation
  • Oral vitamin C and Intravenous (IV) vitamin C differ significantly in biological activity – oral vitamin C is good for immune function while IV vitamin C is clinically therapeutic
  • Higher doses of vitamin C are associated with reduced risks of developing numerous acute and chronic diseases
  • Low vitamin C intake and deficiencies impair biological processes and pose an increased risk to health, such as decreased fat utilization and increased neurological disorders (e.g., Alzheimer’s)
  • Variations in administration, dose, duration, additional supplementation, and study design cause inconsistencies in studies on effect of Vitamin C and health


Dr. Rhonda Patrick (@foundmyfitness) provides a comprehensive discussion about vitamin C

Link to Rhonda’s 28-page paper on the topic with 190 references, Vitamin C

Vitamin C‘s Importance

  • Vitamin C serves as an antioxidant and has benefits beyond its ability to support immune function and prevent or shorten the common cold
  • It reduces the risk of a wide array of viral infections, ranging in severity from chicken pox to HIV
  • Intravenous (IV) vitamin C has been used in therapeutically in the treatment of sepsis and a supplemental treatment for cancer patients alongside chemotherapy   
  • Vitamin C influences fatty acid oxidation (AKA fat burning) the method which uses fat as an energy source, suggesting that higher vitamin C status might be necessary for fat utilization and weight management
  • Vitamin C increases fertility in men due to its antioxidant activity

How to Obtain Vitamin C Through Diet?

  • Most animals can synthesize their own vitamin C but humans have to get it from our diet or supplements
  • Highest amounts in guava, kiwi, bell peppers
  • Cooking and exposure to oxygen can ruin vitamin C potency in foods so best to eat vitamin C rich foods raw and immediately after cutting
  • Meat is a poor source of vitamin C and a carnivore diet could feasibly lead to vitamin C deficiency

Oral Vitamin C – Recommended Daily Amount (RDA)

  • Vitamin C dose depends on age, sex, and life stage
    • Generally, needs are high in infancy, decrease in childhood, increase in again in adulthood
  • General guideline for oral vitamin C: 90mg/day for men; 75mg for women
  • Tolerable upper intake limit when consuming orally is 2000mg/day
    • Some GI issues have been noted above that range but for most people, it’s well-tolerated and safe even above RDA

Special Considerations for Dosing Vitamin C

  • Pregnancy and breastfeeding increase need  
  • Smoking warrants higher needs because it increases oxidative stress, increasing antioxidant needs
  • Alcohol consumption increases urinary vitamin C losses by almost 50% so higher intake is needed to prevent deficiency
  • Medical conditions also increase a person’s needs, especially kidney failure and inflammatory bowel diseases like Crohn’s or ulcerative colitis
    • Single dialysis sessions reduce vitamin C plasma levels by 50% as compared to baseline (right before dialysis)

Bioavailability of Vitamin C: Oral vs IV

  • When taken orally, the extent to which vitamin C enters the body’s circulation is frequency and dose-dependent since it is absorbed in the small intestine
  • Excess vitamin C gets excreted in the urine, but plasma levels still increase a modest amount
  • Maximum plasma concentration of vitamin C achieved orally (220 micromoles/L blood)
    • 3g oral vitamin C taken 6x/day can sustain maximum plasma levels whereas a single 3g dose cannot
  • Concentration in different tissues varies with the lowest amount in muscles, heart, kidneys, and the highest amount in the brain and adrenal glands  
  • IV vitamin C bypasses intestinal absorption and other transport mechanisms which have lower saturation levels

Vitamin C and Immunity

  • Vitamin C is highly concentrated in immune cells which actively participate in eliminating bacteria and viruses from the body
  • Vitamin C regulates cytokine levels which are produced in response to inflammation and infection
  •  “Vitamin C’s immune-boosting and antioxidant properties mediate the body’s inflammatory response, reducing the symptoms or risk of various diseases” -Rhonda  
  • Vitamin C interferes with the replication of viral particles
  • Studies show that vitamin C might be protective against respiratory diseases and protect the lungs by boosting immune function

Reducing Incidence of the Common cold

  • Administration of vitamin C prophylactically instead of therapeutically is more effective at slightly reducing the duration of the common cold
  • Study results have been mixed, largely due to inconsistencies in study design, age, dose, frequency, and supplementation

The Complex Relationship Between Vitamin C and Exercise

Evidence of vitamin C as performance enhancer

  • Supplemental vitamin C can mitigate some of the effects of exercise-induced bronchial-constriction (narrowing of airways in response to rigorous exercise)
  • Vitamin C might enhance exercise performance by reducing negative consequences of excess reactive oxygen species
  • Some studies show that supplemental doses of vitamin C decreased markers of muscle damage and reduced soreness

Can Vitamin C Blunt Effects of Exercise?

  • Reactive oxygen species (the thing vitamin C removes) appear to provide beneficial training adaptations such as enhanced immune function, glucose sensitivity and utilization, increased mitochondrial number and function
    • This means vitamin C might blunt beneficial training adaptations that reactive oxygen species induce!
  • Repeated studies with lower doses suggest vitamin C has no effect on physical adaptation (i.e., muscle soreness, fatigue, damage) to exercise

Vitamin C and the Brain

  • Vitamin C is found in high concentrations in the brain, especially in the hippocampus and frontal cortex which are important for memory consolidation, learning, and aspects of executive function
  • The brain retains vitamin C during times of deficiency at the expense of other tissues
  • Vitamin C antioxidant capacity might be crucial to decrease risk of neurodegenerative diseases like Alzheimer’s, Parkinson’s, Huntington’s
  • Vitamin C is important in the regulation of neurotransmitters and regulation of neurocircuits

Intravenous (IV) Vitamin C and its Therapeutic Uses

  • “Oral and IV vitamin C are worlds apart in biological activity because of sheer concentrated of increase in plasma levels when you take something intravenously” -Rhonda
  • Historically, IV vitamin C has been used as a treatment for symptoms of shingles, cold sores, chickenpox, mononucleosis, among others
  • Recent research suggests that IV vitamin C can reduce blood pressure in patients diagnosed with pre-hypertension
  • Evidence suggests IV vitamin C might be an effective treatment for sepsis, a life-threatening condition that can arise when the body is responding to bacterial infection
  • In clinical studies, IV vitamin C alone and in combination with hydrocortisone and thiamine – reduced number of hospital deaths related to sepsis and risk of organ failure
  • IV vitamin C has been studied as a supportive treatment of cancer and as treatment of side effects of chemo
    • Positive trend observed in overall survival rate in patients administered IV vitamin C in conjunction with standard therapy
  • IV vitamin C seems to improve cancer patients’ quality of life
    • Patients reported improvements in physical, emotional, and mental status, as well as reduction in fatigue, nausea, vomiting, pain, appetite loss

Safety When Supplementing with Vitamin C

  • Not a good idea in medical conditions related to having high levels of iron
  • IV vitamin C is well tolerated and has low toxicity – most common side effect is mild to moderate nausea, headache, dry mouth
    • Less common side effects are fatigue, hypertension, loss of appetite, hyperglycemia
    • More serious side effects have been reported in patients with cancer and certain predispositions to hemolysis

Misleading Information on Kidney Stone Risk with High Oral Dose

  • Some studies have suggested that vitamin C is a risk factor for kidney stones due to the formation of oxalate which is an end product of vitamin C metabolism
  • Findings are misleading because the risk was only found after statistical data manipulation
  • The more likely answer is that people with kidney impairment are at higher risk, but vitamin C poses relatively low risk in most healthy people
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