B25024 | Written By Ryan Gobble, PharmD Candidate, 2026

Reviewed By M. Faisal Hossain, BPharm, MPharm, PhD

Vitamin C, also known as ascorbic acid, is one of the most widely consumed vitamins in the world. It is promoted for its antioxidant effects and ability to fight infections. From oranges, strawberries, and cherries to high-dose effervescent tablets, it’s often one of the first things people seek for natural help at the first sign of a cold or flu.

But how effective is it? Can we meet our needs through diet alone? And what does modern research say about its role in immunity and viral infections?

Let’s take a closer look at the science, the myths, and the evidence-based facts.

Natural Sources of Vitamin C: Are Oranges the best source of Vitamin C?

Vitamin C is found in a wide variety of fruits and vegetables, many of which far surpass oranges in concentration. It’s also sensitive to heat and water, meaning cooking methods can significantly affect content.

Food Source	Vitamin C (mg/100g)	Notes
Amla	250–450 mg	Extremely sour, widely used in Ayurveda
Acerola Cherry	~1,650 mg	Popular in supplements
Guava	125-250 mg	High content, common in South Asia
Kiwi	51 mg	Excellent for immune health
Bell Peppers (raw)	80–130 mg	Among the top vegetable sources
Strawberries	59 mg	Rich in antioxidants
Orange	40-55 mg	Well-known, but not the highest
Lemon	40–50 mg	Used in drinks and culinary applications
Pineapple	~48 mg	Refreshing, mild source
Watermelon	~8–10 mg	Hydrating but low in vitamin C
Papaya	~60 mg	Digestive and immune support
Brussels Sprouts	~85 mg	Better when lightly steamed
Broccoli (cooked)	~56 mg	Retains vitamin C when steamed

Tip: Steaming vegetables preserves more vitamin C than boiling. From 10% to 50% of vitamin C can be lost during prolonged cooking, depending on the product [8].

Supplemental Forms of Vitamin C: Which One Works Best?

Vitamin C is available in various supplemental forms. While all are effective at moderate doses, differences emerge at higher levels or in specific populations.

• Ascorbic acid: This is the standard and most commercially available form.
• Liposomal vitamin C: Encapsulated in lipids, this form is marketed for better absorption at high doses, but often comes at a much higher cost.

Bioavailability 

According to Carr & Vissers (2013), absorption of vitamin C is similar for all oral forms (ascorbic acid, sodium ascorbate, calcium ascorbate, and with bioflavonoids) at doses of $\le$200 mg/day, with absorption efficiency typically 70–90% [1].

However, as the oral dose increases, gastrointestinal absorption becomes saturable due to active transport limitations:

• ~70–90% absorption at intakes of 30–180 mg/day
• Drops to ~50% at 1 g/day

The main mechanism for absorbing the reduced form (ascorbic acid) is via sodium-dependent vitamin C transporters (SVCTs), particularly SVCT1, which is a high-capacity, saturable transporter. The oxidized form, dehydroascorbic acid (DHA), is absorbed via glucose transporters (GLUTs) and then rapidly converted back to ascorbic acid inside the cells. This complex system ensures tight control over plasma and tissue concentrations.

Liposomal and esterified forms may offer improved plasma levels at higher doses (>500 mg) or in individuals with poor gut absorption (e.g., chronic illness, malabsorption syndromes), but data is still emerging and not yet conclusive [1].

Extended-Release Vitamin C Tablets and Absorption

Extended-release (ER) vitamin C tablets are formulated to release ascorbic acid gradually over several hours, aiming to:

• Reduce peak concentrations that may cause gastrointestinal upset (e.g., diarrhea) at high doses.
• Maintain more stable blood levels compared to immediate-release tablets.

However:

• Studies (Carr & Vissers, 2013; Levine et al., 1996) show that overall absorption (AUC) is similar between ER and standard tablets [1, 3].
• The body still uses saturable transport in the small intestine, so bioavailability does not exceed the physiological limit (~200–400 mg plasma saturation).
• ER tablets may benefit people sensitive to high single doses (less stomach upset), but they do not significantly increase total vitamin C uptake compared to divided doses of regular tablets.

How Much Do You Need?

NIH Recommended Daily Allowance (RDA):

Group	RDA
Adult Men (18+)	90 mg/day
Adult Women (18+)	75 mg/day
Tolerable Upper Limit (UL)	2,000 mg/day

The normal diet provides enough to cover the RDA of Vitamin C. However, in instances of illness, high stress, or poor diet, supplementation may help.

Clinical Evidence: What the Studies Say

Cold Prevention and Duration

A Cochrane Review of 29 trials involving over 11,000 participants concluded that:

• Routine supplementation ($\ge$200 mg/day) does not reduce cold incidence in the general population.
• However, it does reduce duration by 8% in adults and 14% in children.
• High doses (1–2 g/day) taken at the onset of symptoms may slightly reduce severity [3].

Can You Get Enough from Food Alone?

Yes. A simple day’s diet can easily exceed the RDA:

In addition to vitamin C, whole foods offer phytonutrients and fiber that supplements do not.

Do You Need a Supplement?

You may consider supplementing if you:

• Smoke or are exposed to pollutants
• Are you recovering from wounds
• Have digestive or absorption issues (e.g., Crohn’s)
• Eat very few fruits and vegetables

Otherwise, most healthy individuals do not require supplements if eating a balanced diet [5].

High Doses: Are They Safe?

Long-Term Safe Range: Up to 2000 mg/day is safe for most people. Exceeding 2,000 mg/day may lead to:

• Diarrhea
• Kidney stones 
• Increased iron absorption 

Always consult a healthcare provider before high-dose use, especially in children, pregnant women, or people with chronic illness.

Vitamin C + Zinc: Better Together?

 Synergistic Effects:

• Vitamin C enhances white blood cell function and tissue repair [2].
• Zinc is crucial for T-cell function, antiviral defense, and wound healing [6].

 Clinical Insight:

• Zinc lozenges may reduce cold duration by 33% if taken within 24 hours of symptoms [7].
• Combo use may help prevent colds in people under physical or emotional stress [3].

Final Takeaway

• Vitamin C is essential for immune defense, antioxidant function, and healing.
• While it won’t prevent colds, it can reduce their duration and severity.
• For most people, a diet rich in fruits and vegetables is enough.
• Supplements may help in times of stress, illness, or low dietary intake.
• Stay under 2,000 mg/day unless supervised by a healthcare provider.

 References

1. Carr AC, Vissers MCM. “Synthetic or food-derived vitamin C—are they equally bioavailable?” Nutrients. 2013;5(11):4284–4304. doi:10.3390/nu5114284
2. Carr AC, Maggini S. “Vitamin C and Immune Function.” Nutrients. 2017;9(11):1211. https://doi.org/10.3390/nu9111211
3.  Hemilä H, Chalker E. “Vitamin C for preventing and treating the common cold.” Cochrane Database Syst Rev. 2013. https://doi.org/10.1002/14651858.CD000980.pub4
4. Padayatty SJ, Katz A, Wang Y, Eck P, Kwon O, Lee JH, Chen S, Corpe C, Dutta A, Dutta SK, Levine M. “Vitamin C as an antioxidant: evaluation of its role in disease prevention.” J Am Coll Nutr. 2003;22(1):18–35. doi:10.1080/07315724.2003.10719272
5. NIH Office of Dietary Supplements. “Vitamin C Fact Sheet.” https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/
6. NIH Office of Dietary Supplements. “Zinc Fact Sheet.” https://ods.od.nih.gov/factsheets/Zinc-Consumer/
7. Hemilä H. “Zinc lozenges and the common cold.” Cochrane Database Syst Rev. 2013.
8. Turkmen, N., Sari, F., & Velioglu, Y. S. (2005). “The effect of cooking methods on total phenolics and antioxidant activity of selected green vegetables.” Food Chemistry, 93(4), 713–718.

Acknowledgment: 
This blog was prepared with assistance from ChatGPT by OpenAI. The featured image was also created with the help of AI tools.

Note:
This content is for informational purposes only and is not intended as medical advice. Always consult your healthcare provider before starting any supplement.

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