Vitamin C, also called ascorbic acid, is a popular choice to boost immune function–especially during cold and flu season.
And now, during the coronavirus pandemic, plenty of people are turning to vitamin C to reduce their risk of infection or speed their recovery.
On the other hand, skeptics and science news headlines sometimes claim it doesn’t help immunity.
Are the doubters right, or is there value in taking vitamin C supplements to lower infection risk and severity?
In this article, you’ll learn what research really says about vitamin C and immunity, how it works in your body, the 3 best ways to prevent vitamin C deficiency on the keto diet, and other practical tips.
Is Vitamin C the Immunity Nutrient?
Vitamin C (ascorbic acid) isn’t the only nutrient you need for immune function, nor is immunity the only role vitamin C plays in your body.
However, its reputation for helping immunity is well-deserved.
As an essential micronutrient that your body can’t produce by itself, ascorbic acid contributes to both innate and adaptive immunity[*].
In other words, your body requires dietary vitamin C for the immediate defensive response that occurs as soon as you’re exposed to germs, as well as the long-lasting, highly-specific response that allows you to develop immunity to certain infections over time.
And vitamin C deficiency impairs immunity and increases infection risk[*].
But as a 2013 scientific review found, taking vitamin C supplements can shorten the duration of upper respiratory infections and may reduce the risk of these infections by about 50% for some people[*].
Additionally, some researchers have speculated that taking vitamin C may help treat viruses in the coronavirus family going back as far as 2003[*].
Also, in February, Chinese researchers at Wuhan University fast-tracked a clinical trial to learn whether intravenous vitamin C can help patients with COVID-19 (novel coronavirus disease)[*].
Although the IV study results aren’t in yet, there’s plenty of reason to be hopeful.
And according to a recent position statement, an expert health care provider panel from Shanghai recommends 50-100 mg/kg per day of vitamin C for hospitalized coronavirus patients[*].
Keep reading to learn the scientific details of how vitamin C works in your body, plus what happens when you’re deficient.
How Vitamin C Works in Your Body
Vitamin C (ascorbic acid) helps your body deal with cellular stress, and it’s also a key player in the creation of over a dozen neurotransmitters, hormones, amino acids, and proteins[*].
In other words, adequate vitamin C levels are incredibly important for the proper functioning of your body.
But some organisms, including humans, lost the ability to make ascorbic acid at some point in our evolutionary history[*][*].
Therefore, we have to obtain it from our diet.
And if you don’t eat enough of the right foods, a severe vitamin C deficiency leads to a condition called scurvy. Because ascorbic acid helps your body make collagen, insufficient levels halt the production of collagen and cause swollen, bleeding gums, incomplete wound healing, and fragile bones[*].
However, even if you don’t get scurvy, a mild deficiency of vitamin C can still decrease your resistance to infections[*].
Additionally, if you’re under lots of stress, your body may require more vitamin C to prevent a deficiency[*].
Essentially, athletes and even regular people who are under a lot of stress–for whatever reason–might require more vitamin C than usual.
To understand why that could be the case, let’s dive deeper into how vitamin C works in your body and regulates your immune response.
Vitamin C, Oxidative Stress, and Immune Function
Inside your cells, vitamin C acts as a powerful antioxidant or electron donor, meaning it helps control oxidative stress[*].
Oxidative stress occurs when compounds called radical oxidative species (ROS) come into contact with other molecules.
When that happens, the unstable free radicals “steal” electrons from cells, proteins, lipids, or other important molecules in your body.
Basically, oxidative stress is a form of damage that occurs when molecules lose an electron due to ROS. It’s linked to serious conditions including heart disease and cancer[*].
But vitamin C can save the day by “donating” an electron during this process, which prevents the damage from occurring at the cellular level[*].
However, during times of excessive cellular stress, your body’s antioxidant defenses may become overwhelmed, leading to a shortage of vitamin C[*].
And not only is that a problem for the cells under stress, but it also affects your immune function.
Typically, ascorbic acid assists with the following essential roles of your immune system[*]:
- Antimicrobial activity
- Natural killer cell activities
- Lymphocyte proliferation
- Chemotaxis (moving immune cells where they’re needed)
- Protecting healthy cells against free radicals generated by immune activity
The last bullet point is especially fascinating. Remember how vitamin C helps protect cells against oxidative stress?
It turns out your immune system also uses free radicals to kill invading germs and unhealthy cells, and adequate vitamin C levels are crucial for protecting healthy cells from being damaged by your own immune system [*][*].
But to sum up, during times of stress or infection, your requirements for ascorbic acid may increase[*].
And without enough vitamin C, the cellular damage from infections as well as your immune system goes up.
However, because your body can’t make ascorbic acid on its own, you can only obtain it from your diet or from supplements.
Why People Think Vitamin C + Immunity is a Myth
All of the statements from this article so far as based on peer-reviewed science, and they’re not controversial.
The facts of how vitamin C functions in your body, and its importance for your immune system, are well-established.
But some people debate whether there’s any benefit to taking ascorbic acid supplements, especially if you eat a healthy diet.
Do a little digging and you’ll find headlines like this one: “Why vitamin C won’t ‘boost’ your immune system against the coronavirus.”
First of all, as the author of that article correctly points out, there’s no evidence that vitamin C has any effect against coronavirus–at least not yet.
However, a lack of evidence certainly doesn’t prove that vitamin C won’t boost your immune system against SARS-CoV-2 virus, either.
And the same author admits that “supplementing vitamin C can shorten the duration of a cold by about one day” and quotes an expert who states that “If there’s going to be an advantage [for COVID-19], it’s going to be very modest.”
As you can see, the title of the skeptical article doesn’t really fit with the statements included in the article itself. You can draw your own conclusions about why that may be.
In reality, the effectiveness of vitamin C against COVID-19 is a completely open question. Fortunately, we’ll have better data soon.
But in the meantime, let’s take a closer look at the best arguments for why vitamin C supplements might not be the answer.
Dietary Intake May Be Sufficient
In the United States, the dietary reference intake (DRI) for adults is 75-90 mg of vitamin C per day[*].
For perspective, a medium-sized orange contains about 63 milligrams of vitamin C[*].
Therefore, it’s no wonder that some people think taking 200-6000 milligrams or more of vitamin C each day in supplement form is overkill.
However, keep in mind that stress and illness can deplete your ascorbic acid stores, which is likely why studies often find a benefit to supplementing above typical dietary levels for sick people[*][*].
Bottom line: there’s virtually no downside, and at least a small proven benefit, to taking extra vitamin C if you’re under stress or sick.
Oral Vitamin C Absorbs Poorly
The poor oral absorption of vitamin C is probably a legitimate criticism against vitamin C dietary supplements.
For example, as the authors of a 2018 scientific paper point out, “once its oral intake exceeds 200 mg daily, it becomes difficult to increase the plasma concentrations of vitamin C”[*].
And the reason for the poor absorption appears to be because the vitamin C transporters found in human intestines can only handle so much ascorbic acid at once[*].
Although some evidence suggests that brain and muscle transport increases during high-dose ascorbic acid usage, there’s not enough data yet to say whether the transport of vitamin C across the intestinal barrier behaves similarly[*].
Additionally, a 2004 paper found that a 1.25-gram oral dose resulted in approximately 6-fold lower levels compared to a comparable intravenous dose of ascorbic acid, and the difference gets worse with higher doses[*].
However, other evidence shows that more-frequent oral dosing can result in higher peak plasma levels of vitamin C as well as higher overall levels, which may also translate to higher cellular uptake of vitamin C when you need it–like during times of stress[*].
Basically, while oral absorption is a limiting factor in vitamin C usage, you can partially overcome it by taking more-frequent doses.
In the next section, we’ll cover the 3 best ways to take vitamin C, including practical advice for how to maximize absorption.
The 3 Best Ways to Get More Vitamin C
#1: Eat Low-Carb Foods High in Vitamin C
If you’re healthy and aren’t stressed out, you can get by with eating a healthy diet that’s high in vitamin C.
And while oranges and orange juice get most of the credit for dietary sources of ascorbic acid, there are better options that are also keto-friendly.
A medium orange has 63 mg of vitamin C and 16 grams of carbs, while an 8-ounce glass of OJ has 72 mg of vitamin C and 26 grams of carbs[*][*].
Now, compare those options to our top low-carb picks to ensure you get adequate extra vitamin C and stay in ketosis:
- A medium red bell pepper has 152 mg of vitamin C and 4.7 grams of net carbs[*]
- A medium green bell pepper has 95 mg of vitamin C and 3.5 g net carbs[*]
- 100 grams of raw kale offers 93 mg vitamin C and just 0.3 g net carbs[*]
- 100 grams of broccoli has 91 mg vitamin C and 1.4 g net carbs[*]
- 100 grams of watercress has 43 mg vitamin C and 0.7 g carbs[*]
- 100 grams of strawberries contain 7g net carbs and 42 mg vitamin C[*]
Or, if you want more whole food options rich in ascorbic acid and other micronutrients, you can also choose from other low-carb veggies and keto-friendly fruits.
#2: Take Liposomal Vitamin C
Liposomal vitamin C is a unique supplement that encapsulates ascorbic acid inside very tiny fatty particles.
As a result, some research shows that a single 36-gram oral dose of liposomal vitamin C can result in double the peak plasma levels of regular ascorbic acid, and may also penetrate cell walls better[*].
Although liposomal vitamin C is more expensive than regular vitamin C supplements, it’s probably worth considering as an alternative to IV vitamin C for some sick people.
#3: Take High-Dose Ascorbic Acid
Earlier, we covered that oral absorption of vitamin C is a limiting factor.
And that’s why hospitals use IV vitamin C to treat sepsis (a potentially fatal response to infection), and why IV vitamin C is the form scientists are currently studying for COVID-19 treatment[*][*].
However, some research demonstrates that more-frequent doses of vitamin C may partially overcome its poor oral absorption[*].
For example, a 1999 study found that taking a gram of vitamin C every waking hour for the first three days after symptoms appeared, then taking a gram three times daily until symptoms went away, decreased cold and flu symptoms by a whopping 85%[*].
And as the authors of a 2017 review concluded, “prevention of infection requires dietary vitamin C intakes that provide at least adequate, if not saturating plasma levels (i.e., 100-200 mg/day), which optimize cell and tissue levels. In contrast, treatment of established infections requires significantly higher (gram) doses of the vitamin to compensate for the increased inflammatory response and metabolic demand”[*].
The bottom line: if you aren’t sick, your immune system will function well enough with relatively low doses of 100-200 milligrams per day of vitamin C, which you can obtain from your diet.
But if you notice viral symptoms, taking up to a gram every hour for at least three days might be a game-changer.
Who Should Take Vitamin C?
Most people can get plenty of ascorbic acid from their diets–if they eat keto-friendly foods rich in vitamin C, like bell peppers, broccoli, and strawberries.
And taking supplements to make up for poor nutrition is not ideal, so if you suspect you may be deficient in vitamin C, your first priority should be to fix your diet.
However, as we discussed earlier in this article, higher doses of vitamin C may be beneficial for people who are stressed, sick, or have weak immune systems.
Fortunately, most studies show that high-dose vitamin C is safe and well-tolerated, even in unwell people.
For example, a 2013 study in cancer patients found that ultra-high intravenous doses of up to 110 grams were safe and well-tolerated[*].
And because oral vitamin C doses absorb six to ten times less than IV doses, it’s probably impossible to overdose on oral ascorbic acid[*].
In fact, there’s no scientific evidence of any serious adverse effects from vitamin C in humans[*].
However, if you do take too much, you’ll probably experience gastrointestinal issues like diarrhea[*]. If that happens, you can simply reduce your dosage until you don’t experience unpleasant side effects.
Additionally, some evidence shows that high doses of vitamin C may worsen copper deficiency[*]. Therefore, you probably shouldn’t take high doses of vitamin C long-term, or if you do, check your copper levels and consider taking copper supplements.
And finally, taking megadoses of vitamin C long-term may increase your risk of kidney stones by increasing oxalate formation[*].
Speaking to your doctor before starting a new supplement is always wise, especially if you have a medical condition, take prescription drugs, are a senior citizen, or are pregnant or nursing.
The Takeaway: Vitamin C for Immunity
The coming months and years should provide better insight into the effects of vitamin C on immunity and infection resistance, including COVID-19.
However, current research shows at least a modest benefit to high, frequent doses of oral ascorbic acid for people with viral infections, with no serious adverse effects.
If you’re not sick, you should be able to get adequate vitamin C by eating a healthy diet of whole foods.
But if you notice symptoms, consider taking a gram of vitamin C every waking hour for three days or more, then a gram of vitamin C three times per day until symptoms subside.
And if you notice diarrhea or an upset stomach, reduce your dosage until the side effects go away.
Also, keep in mind that vitamin C is not the only nutrient your body requires for proper immune function. To learn about other important micronutrients, and the best keto-friendly dietary sources for them, don’t miss Keto Micronutrients: How to Avoid Vitamin & Mineral Deficiencies in Ketosis.
And for more info about the effects of keto on your immune system, read www.perfectketo.com/carbohydrates-coronavirus.