Vitamin C: helper of our immunity? Part 2: all about how to use it

Now we have a better idea of how vitamin C contributes to the maintenance of our immune system. The next step is: TAKE IT IN!

How much vitamin C should I consume each day?

An adequate intake (AI) of vitamin C is 90 mg/day for males and 75 mg/day for females in Canada [1]. Smokers should consume an additional 35 mg/day because smoking promotes the production of free radicals, which can be neutralized by vitamin C as an antioxidant.

After reviewing different types of human studies (metabolic, pharmacokinetic, observational, randomized controlled), scientists suggest that 200 mg/day of vitamin C is the optimum intake to get its maximum health benefits [2]. A one-time dose of 200 mg vitamin C was found 100% absorbed in humans, but higher doses were absorbed much slower or not completely [3].

High doses of vitamin C (e.g. 1,000 mg as commonly seen in supplements) is generally considered safe, because the extra that our body doesn’t use is not stored, but excreted in the urine [3]. Nonetheless, you may experience digestive discomforts (e.g. nausea and diarrhea) with too much vitamin C, hence a tolerable upper intake level (UL) is set at 2,000 mg/day [1].

Overall, orally consumed vitamin C is very easily absorbed (within a range!), but consuming too much only makes your pee expensive.

Getting enough vitamin C from foods? SO EASY!

Many vegetables and fruits are excellent dietary sources of vitamin C. It is not difficult at all to achieve the recommended 200 mg/day intake, even without supplements. The figure below shows how much vitamin C is provided by common vegetables/fruits.

(data collected from USDA FoodData Central:

No time to prepare fresh vegetables or fruits? Don’t worry (hopefully not too often though)! Some processed foods are enriched with vitamin C for various purposes, such as making up the degradation of natural vitamin C in food ingredients, preventing microbial growth and color change, and adding flavor. For example, extra ascorbic acid (the chemical name of vitamin C) is commonly added to fruit juices. Fun fact: the vitamin C indicated on the nutrition facts label of fruit juices only reflects the added ascorbic acid, without considering the natural vitamin C in fruits. Also, cereals are often enriched with both vitamin C and iron, because vitamin C promotes the absorption of plant-based iron [4].

Nonetheless, don’t simply rely on processed foods for vitamin C intake – you may miss all the other good stuff in fresh produces, e.g. other vitamins and fibres. Oh, and don’t forget flavonoids. Flavonoids are polyphenolic compounds found in plants that have synergistic effects with vitamin C on reducing cellular oxidative damage [5]. Interestingly, flavonoids improved vitamin C absorption in cell culture [6],[7], although a review of seven human studies only showed comparable vitamin C uptake from plain vitamin C and flavonoid-rich fruits/veggies [8].

What about supplements? Which one is better for absorption?

Supplements are designed to provide the nutrients in a concentrated form without food matrices, and there are continuous efforts making them more bioavailable (i.e. can be used by our body!). Nevertheless, for a nutrient so accessible in foods like vitamin C, think about your diet first before taking supplements to avoid unnecessary or overconsumption.

Supplement forms

Vitamin supplements are available in different forms, such as powder, tablets/caplets, capsules, and gummies. Wonder which one is best at delivering vitamin C to our body? They aren’t much different. For example, a recent clinical trial in healthy adults showed 1000 mg vitamin C in gummies were absorbed as efficiently as those in caplets [9]. It is more important to choose the form that you like enough to maintain regular consumption.

Slow-release formulations vs. plain vitamin C

There are also slow (or “timed”) release formulations on the market, which claims to provide better absorption than plain vitamin C. However, stronger evidence is required. Although one study in 1969 showed better incorporation of vitamin C in tissues after the consumption of slow-release formula versus regular capsules [10], two more recent studies didn’t find any significant difference in absorption efficiency [11],[12].

Vitamin C formats: ascorbic acid vs. mineral ascorbate

Vitamin C can also be supplemented as ascorbic acid (natural or synthetic – no significant difference in their physiological impacts [8]), or mineral ascorbate, which are salts of ascorbic acid. Sodium and calcium ascorbates are the most common mineral ascorbates. They are less acidic than the plain ascorbic acid, hence gentler on the gastrointestinal tract. Nevertheless, minerals are well-absorbed in this form, so keep an eye out for mineral total intake to avoid overconsumption.

Is intravenous injection of vitamin C (IVC) a magical cure to COVID-19?

Pumping vitamin C into your vein?! Sounds aggressive, but you read it right. It is especially a hot topic these days as a potential treatment for critically ill patients affected by COVID-19.

Why intravenous? The intestinal uptake of vitamin C has a limit, which will occur with oral consumption – that’s why too much vitamin C ends up in the urine. In contrast, intravenous injection allows vitamin C to be delivered directly into the blood (i.e. completely bioavailable).

Why is IVC suggested for COVID-19? The immune system of critically ill COVID-19 patients is undergoing a battle so tough that its defensive responses also damage the body. Similar reactions occur in critically ill cancer patients as well. Human studies have applied high doses of IVC ranging from 0.5 to 200 g/day (= 500 to 200,000 mg per day, the latter is 1,000 times of the recommended optimal intake!), and some of them demonstrated beneficial effects on cancer patients’ quality of life, symptom control, and/or survival time [13],[14]. These benefits are postulated to link with vitamin C’s immune-supporting properties (please tell me you do remember how vitamin C fights against infections in the last post ☝☝☝) [13],[15]. Also, the high plasma vitamin C concentration after intravenous injection was similar to the concentration toxic to cancer cells in culture [16].

However, the following points should be noted:

  1. IVC has been tested mostly as a supportive therapy. When used alone, it was only tested in terminal/incurable cancer patients. There is NO evidence that intravenous vitamin C, alone, can cure cancer.
  2. When used in conjunction with chemotherapy, IVC was found to reduce toxic side effects of chemotherapy and improved patient quality of life. In contrast, limited data is available yet to evaluate the use of IVC with radiotherapy.
  3. Many early studies showing the benefits of IVC are not “controlled”, meaning that there were no control subjects under similar conditions that didn’t receive IVC – how can we tell if the improvement is from the treatment, or being in the trial, or other random factors?
  4. Not all the studies reached show promise of IVC. More studies are needed to figure out how to best use IVC, such as the timing and dosing of IVC administration.

SOOOOOO…what about IVC as a treatment for COVID-19?! Given the short history of COVID-19, it is expected that scientists are still working on it. According to (as of today), there are currently 3 registered clinical trials (2 in US and 1 in Canada) investigating the effect of high-dose IVC on the symptoms of COVID-19, but none of them are completed yet [17]. We are excited to see how it goes and certainly hoping for a positive outcome; of course, while keeping you updated!


Bottom line: vitamin C can help us be our best selves to embrace life, be it sunshine and rainbows or the occasional sticks and stones.

But to expect vitamin C to be the magic pill that will make you perform beyond your bodily limits is unrealistic. Rather, it may be a better approach for us to appreciate the immune system that was built-in with our Homo sapiens bodies from the moment when we were born: powerful, intricate, our always on-call personal responders. Perhaps it is a chance that we look within and examine our everyday life: are we treating ourselves with the love and care that our body and soul deserve? Are we bringing the best out of ourselves? Is the fact that, vitamin C can indeed make us feel better, a sign that maybe… just maybe, the inherent wellbeing in us is much greater than what we’re currently experiencing?

If you’re interested in including more whole-food vitamin C rich recipes in your daily routine PLUS a comprehensive list of vitamin C rich food (what we have here is a sneak peek 😉), be sure to check out our following posts!




[2] Frei, B. et al. Authors’ perspective: What is the optimum intake of vitamin C in humans? Crit Rev Food Sci Nutr. 2012;52(9):815-29.

[3] Levine, M. et al. Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance. Proc Natl Acad Sci U S A. 1996;93(8): 3704–3709.

[4] Tuecher, B. et al. Enhancers of iron absorption: Ascorbic acid and other organic acids. Int J Vitam Nutr Res. 2004;74(6):403-19.

[5] Noroozi, M. et al. Effects of flavonoids and vitamin C on oxidative DNA damage to human lymphocytes. Am J Clin Nutr. 1998;67(6):1210-8.

[6] Sorata, Y. et al. Cooperation of quercetin with ascorbate in the protection of photosensitized lysis of human erythrocytes in the presence of hematoporphyrin. Photochem Photobiol. 1988;48(2):195-9.

[7] Skaper S. et al. Quercetin protects cutaneous tissue-associated cell types including sensory neurons from oxidative stress induced by glutathione depletion: cooperative effects of ascorbic acid. Free Radic Biol Med. 1997;22(4):669-78.

[8] Carr, A. & Vissers, M. Synthetic or food-derived vitamin C—Are they equally bioavailable? Nutrients. 2013; 5(11): 4284–4304.

[9] Evans, M. et al. Vitamin C bioequivalence from gummy and caplet sources in healthy adults: A randomized-controlled trial. J Am Coll Nutr. 2019;2019 Nov 20:1-10.

[10] Richards, T. et al. Effect of sustained release versus regular multivitamin supplement upon vitamin C state. Int Z Vitaminforsch. 1969;39(4): 407-15.

[11] Viscovich, M. et al. Vitamin C pharmacokinetics of plain and slow release formulations in smokers. Clin Nutr. 2004;23(5):1043-50.

[12] Nyyssonen, K. et al. Effect of supplementation of smoking men with plain or slow release ascorbic acid on lipoprotein oxidation. Eur J Clin Nutr. 1997;51: 154-163.

[13] Carr, A. & Cook, J. Intravenous vitamin C for cancer therapy – Identifying the current gaps in our knowledge. Front Physiol. 2018;9: 1182.

[14] Klimant, E. et al. Intravenous vitamin C in the supportive care of cancer patients: a review and rational approach. Curr Oncol. 2018 Apr; 25(2): 139–148.

[15] Magri, A. et al. High-dose vitamin C enhances cancer immunotherapy. Sci Transl Med. 2020;12(532).

[16] Park, S. The effects of high concentrations of vitamin C on cancer cells. Nutrients. 2013;5(9): 3496–3505.