Vitamin D and Vitamin C for Immune Support: What Each Does

Vitamin D and vitamin C do different jobs, but both show up inside the cells that make up your immune system.*

Your immune system is a network of cell types. Neutrophils are white blood cells that engulf germs. T cells and B cells act more specifically.

Reviews show that vitamin D and vitamin C are each used by these cells directly. That is why both are discussed under immune support, and why one does not stand in for the other.*

A 2011 review in the Journal of Investigative Medicine describes B cells, T cells and antigen-presenting cells (cells that show germ fragments to other immune cells). All of them carry the vitamin D receptor, a docking site for the active form of the vitamin. They can also make that active form themselves.*

The review also looks at macrophages, which are large cells that engulf germs. In them, transcription (switching on) of the gene for cathelicidin, a germ-fighting protein, is described as "absolutely dependent on sufficient 25 D," the storage form measured in a blood test.*

For another germ-fighting protein, beta defensin 4, the review says the vitamin D receptor complex binds the gene. It adds that a second factor, NFkB, is also required.*

Who took part: this is a review of lab and clinical research, not a single trial.

The catch: much of the work describes how cells behave in the lab. It shows the mechanism, not how many days of illness a supplement changes.

A 2017 review in Nutrients^[2] reports that neutrophils collect vitamin C to 50 to 100 times the level in blood plasma. Inside the cell it supports movement toward germs, engulfing them and the oxidative burst, the cell's built-in chemical response.*

The same review says vitamin C also helps clear spent immune cells and is linked to T-cell and B-cell growth in lab studies. Vitamin C is also an antioxidant, meaning it helps protect cells from oxidative stress.*

Who took part: healthy volunteers with low vitamin C status, in one study the review describes.

What changed: neutrophil movement and oxidative burst were about 20% higher after roughly 250 mg a day from a food source.*

The catch: the authors say adults who already have adequate status may not see the same effect. One author works for a multivitamin maker and the other receives funding from it.

MedlinePlus lists the vitamin D RDA, the daily amount that meets the needs of most healthy people, as 600 IU (15 mcg) for ages 1 to 70^[4] and 800 IU (20 mcg) after 70.

Skin makes vitamin D in sunlight. Few foods have much of it: fatty fish is among the best sources, with beef liver, cheese and egg yolks smaller.

For vitamin C, the RDA is 90 mg for men and 75 mg for women^[5]. Smokers need 35 mg more. Citrus, kiwi, strawberries, broccoli and peppers are good food sources.

Vitamin C is water-soluble and the body does not store it, so it has to come in regularly. Cooking, long storage and steaming reduce the amount in food, and raw produce keeps the most.*

For reference, our essentials bundle puts four formulas together: magnesium, vitamin D3 + K2, vitamin C and omega-3.

A 2020 analysis of NHANES^[3], the national US health and nutrition survey, looked at intakes of immune-related nutrients.

Who took part: 26,282 adults over age 19, surveyed from 2005 to 2016.

What it found: 46% had vitamin C intake below the Estimated Average Requirement (the level that meets the needs of about half of healthy people) from food alone. For vitamin D the food-only figure was 95%. With supplements included, those figures were 33% and 65%.

The catch: intake was self-reported from two 24-hour recalls, and four authors work for a supplement company. Intake also differs from blood levels, since sunlight adds vitamin D.

The pattern still stands out. Even with supplements counted, a sizable share of adults stayed below the benchmark.

The two vitamins fill different gaps. Vitamin D is hard to get from food and depends on sun exposure, which varies by season and by how much time you spend indoors. Vitamin C is easy to find in produce but is not stored, so each day starts fresh.*

Immune cells use both, and survey data show many adults miss the benchmark for each. Covering both every day, from food first and a supplement where food falls short, is a consistent way to support normal immune function.*

A single capsule or spray does not replace a varied diet. It is meant to fill the distance between what you eat and what the RDA lists.

Both vitamins have upper limits, and more is not better. Vitamin D toxicity almost always comes from supplements and can raise blood calcium, with kidney stones and kidney damage as possible results, according to MedlinePlus^[4]. The best measure of vitamin D status is a 25-hydroxyvitamin D blood test.

For vitamin C, amounts above the upper limit can cause stomach upset and diarrhea^[5] and, rarely, kidney stones.

Check with your doctor first if you:

• Take blood thinners or any other medication, since vitamin K and vitamin D supplements can interact
• Have a history of kidney stones, kidney disease or stomach problems
• Are pregnant or breastfeeding
• Are choosing a supplement for a child
• Have a health condition or a shellfish allergy (the krill oil softgel in the bundle contains gelatin and shellfish-derived oil)

The NIH-listed upper limit is 4,000 IU (100 mcg) a day of vitamin D for adults, and 2,000 mg a day of vitamin C.