Can You Absorb More Than 100% Vitamin C?

October 14, 2025 •

4 min read

According to the National Institutes of Health, when consuming moderate amounts of vitamin C (30–180 mg), the body absorbs approximately 70–90% of it. This fact alone suggests the impossibility of absorbing more than 100% vitamin C, but the full story of how your body processes this essential nutrient is even more complex and important to understand.

Quick Summary

The body regulates vitamin C absorption based on intake levels. At moderate doses, absorption is high, but as oral intake increases, absorption efficiency decreases significantly, and the excess is excreted. Oral intake cannot exceed a certain plasma concentration, but high-dose intravenous administration can bypass this limitation. High oral doses often cause digestive discomfort.

Key Points

Absorption is Saturable: The body's ability to absorb vitamin C is limited by intestinal transport systems that become saturated at high doses.
Efficiency Decreases with Dose: As oral intake increases beyond a certain threshold (around 180-200 mg), the percentage of vitamin C absorbed decreases significantly.
Excess is Excreted: Any unabsorbed vitamin C from high oral doses is quickly excreted through the urine and can cause gastrointestinal discomfort.
Oral vs. IV Delivery: Intravenous administration bypasses the digestive system, allowing for much higher plasma concentrations than are possible with oral intake.
Risks of Megadosing: Excessive oral intake can lead to side effects like diarrhea, stomach cramps, and an increased risk of kidney stones in susceptible individuals.
Optimal Daily Intake: For most healthy adults, an intake of around 200 mg is sufficient to achieve optimal plasma and tissue saturation.

The Dose-Dependent Reality of Oral Vitamin C Absorption

Your body does not operate on a simple 'more is better' principle when it comes to absorbing nutrients. The absorption of water-soluble vitamins like vitamin C is a carefully regulated process governed by specialized transport systems in the small intestine. For moderate dietary intakes, these active transport mechanisms are highly efficient. However, they are also saturable, meaning they can only handle a finite amount of the vitamin at a time.

What happens with increasing oral doses?

When you take high-dose supplements, you overwhelm the intestinal transporters, and the rate of absorption decreases dramatically. Research shows that while your body absorbs 70–90% of a 30–180 mg dose, this figure drops to less than 50% for doses above 1 gram (1,000 mg) per day. The unabsorbed vitamin C then travels to the large intestine, where it can cause gastrointestinal issues like diarrhea and bloating, and is eventually excreted in the urine.

Essentially, pouring more water into a filled cup will not make the cup hold more; it will simply spill over. This is the same for oral vitamin C absorption. Your body reaches a saturation point, and any amount beyond that is simply wasted.

The concept of optimal intake

Many health professionals suggest that an optimal daily oral intake is around 200 mg, a level that allows for near-maximal plasma and tissue concentrations. This can often be achieved through a balanced diet rich in fruits and vegetables, without the need for high-dose supplements.

Oral vs. Intravenous (IV) Vitamin C: A Comparative Look

Oral and intravenous administration of vitamin C are fundamentally different in how the body processes them. This difference is key to understanding why megadoses behave so differently depending on the delivery method.


Feature	Oral Vitamin C	Intravenous (IV) Vitamin C
Delivery Method	Passes through the digestive system.	Delivered directly into the bloodstream.
Absorption Rate	Highly dependent on dosage, decreases with higher intake (e.g., <50% for >1g).	Approaches 100% because it bypasses the digestive tract.
Peak Plasma Concentration	Tightly controlled and limited by intestinal absorption (max ~220 µmol/L).	Can achieve very high, pharmacological concentrations (up to 26,000 µmol/L).
Potential Uses	Daily dietary intake and supplementation for general health.	Used in controlled clinical settings, often for supportive therapy in serious conditions like cancer.
Side Effects	Gastrointestinal distress (diarrhea, cramps) at high doses.	Potential risks include blood clots, infection, and kidney stone formation, especially in susceptible individuals.
Cost	Generally inexpensive.	Significantly more expensive due to medical supervision.

The Risks and Limitations of Excess Vitamin C

Beyond the limited absorption, excessive intake of vitamin C through supplements carries certain risks. The Tolerable Upper Intake Level (UL) for adults is 2,000 mg per day. Consistently exceeding this level can lead to various issues, primarily because the body tries to excrete the surplus.

Gastrointestinal Distress: The most common symptom of excessive oral intake is diarrhea, as the unabsorbed vitamin C can irritate the gastrointestinal tract.
Kidney Stones: High-dose vitamin C can increase the excretion of oxalate, a component of kidney stones. Individuals with a history of kidney stones should be particularly cautious.
Iron Overload: Vitamin C enhances the absorption of nonheme iron. For people with hemochromatosis (a condition of iron overload), excessive vitamin C intake can be harmful and lead to tissue damage.
Nutrient Imbalances: Very high doses may interfere with the body's processing of other nutrients, such as copper and vitamin B-12.

How to optimize your intake

For most people, the best way to get enough vitamin C is through a diet rich in fruits and vegetables. For those who choose to supplement, sticking to moderate doses below 1,000 mg and spacing them out throughout the day can maximize absorption and minimize side effects.

Conclusion: The Myth of Unlimited Absorption

In conclusion, the idea that you can absorb more than 100% vitamin C is a physiological impossibility for oral intake. The body has a built-in regulatory system to prevent this, which involves dose-dependent intestinal transporters and renal excretion. While megadoses are sold on the premise of being beneficial, the efficiency of absorption decreases as the dose increases, with much of the excess simply being eliminated from the body. For everyday health, a balanced diet is the most effective way to maintain optimal vitamin C levels. For higher, therapeutic concentrations, intravenous administration is the only proven method, and it must be done under strict medical supervision due to associated risks. The evidence consistently shows that for oral intake, more is not necessarily better; in fact, it can be less efficient and potentially cause adverse effects.

Further reading on vitamin C can be found on the Office of Dietary Supplements website: Vitamin C Fact Sheet for Health Professionals.

Frequently Asked Questions

No, if you take a 1000 mg oral supplement, your body will absorb less than 50% of it because the intestinal absorption mechanisms become saturated at higher doses. The excess is simply excreted.

For oral intake, the maximum absorption per dose is around 200-400 mg. Absorption efficiency drops dramatically above this level as the transport systems in the gut become saturated.

Excess vitamin C that isn't absorbed in the small intestine proceeds to the large intestine. This unabsorbed vitamin can irritate the gastrointestinal tract, leading to symptoms like diarrhea, bloating, and stomach cramps.

Yes, IV vitamin C bypasses the digestive system and achieves nearly 100% absorption, resulting in significantly higher blood plasma concentrations. This is why IV administration is used in specific, medically supervised therapeutic scenarios.

In some individuals, high-dose vitamin C supplementation can increase the excretion of oxalate, which can lead to the formation of kidney stones. This risk is higher for people with a history of the condition.

To maximize absorption from oral supplements, take moderate doses (e.g., 200-500 mg) and split them throughout the day instead of taking a single, large megadose. Taking it with food can also aid absorption.

The RDA for most adults is 90 mg daily for men and 75 mg for women, with higher recommendations for smokers. This intake level is easily achievable through a healthy diet.