The Power of Vitamin C with D3: What You Need to Know

  • By Performance Lab
  • 7 minute read
The Power of Vitamin C with D3: What You Need to Know

Sick and tired of the sniffles or catching a cold whenever someone coughs around you?

Vitamin C and D3 are powerful immune boosters (among other things) that can help to enhance immune function and maintain your health—no matter the season.

During the change of seasons, colds, flu, and viruses seem to travel quickly—and even if it’s not cool weather, one sneeze and you’re down for the count.

While there’s no surefire way to dodge sickness, bolstering your immune system is an excellent place to start.

When you give your immune system the vitamins, minerals, and other nutrients it needs to thrive, you can protect your body from the billions of germs that come your way.

Scour the internet, and there are tons of “immune boosters” available, but if you’re looking to maximize immune function, there are two simple ones you need to know about: vitamin C and D3. If you’re unfamiliar with their roles in immunity, keep reading to learn the details!

Vitamin C

You’ve likely heard about vitamin C—a water-soluble nutrient found in high concentrations in citrus fruits that’s a shining star for the immune system.

Also known as ascorbic acid, vitamin C is an essential nutrient that cannot be produced in the body and must come through diet or supplementation.

And it also happens to be one of the most well-known and best-studied antioxidants: a substance that readily donates electrons to unstable molecules to stabilize them and prevent damage from “free radicals.”

Free radicals are a normal byproduct of metabolism, and if not neutralized, they can damage DNA, proteins, and other key cell components 1.

As one of the primary antioxidants in the body, vitamin C helps protect your cells against free-radical-induced damage and toxins that can enter through food, water, and air.

But as easy as vitamin C may be to get, most Americans aren’t getting enough. It’s estimated that a whopping 38.9% of Americans aren’t meeting their daily needs for vitamin C 2. Is it any wonder why sickness travels so fast?

Apart from being a powerful antioxidant, vitamin C has other roles.

  1. Brain health: Many enzymes in the brain require vitamin C to function, which is evident when you look at where vitamin C is concentrated; the brain houses the highest levels of vitamin C than any other organ in the body. Not only does it act as an antioxidant to protect the brain from oxidative damage, but it also acts as a co-fact for dopamine beta-hydroxylase in the conversion of dopamine to noradrenaline, is involved in modulating dopaminergic and glutamatergic neurotransmission and regulating catecholamine and acetylcholine release from synaptic vesicles 3.
  2. Collagen production: Collagen is one of the most abundant proteins in the human body and is essential for the strength of body tissues like tendons, ligaments, bone, skin, connective tissue, and blood vessels—and vitamin C is required for its production 4. Essentially, collagen is the “glue” that holds out bodies together.
  3. Immune function: As we said before, the body’s immune system relies on sufficient vitamin C intake to function optimally. It supports various cellular functions of both the innate and adaptive immune system, and low intake can lead to impaired immune defenses and increases susceptibility to illness and infection 5.
  4. Adrenal gland support: As the name suggests, the adrenals are pyramid-shaped glands that sit atop each kidney. Aside from the brain, they contain the highest concentration of vitamin C and are responsible for producing several essential hormones, including cortisol, that regulate the body’s stress response. Many enzymes involved in steroid and stress hormone synthesis rely on vitamin C 6.

How Much Vitamin C Do You Need?

The dosage is big with any supplement. Too little, and it’s not effective; too much could be harmful. So, what’s the tipping point with vitamin C?

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Luckily, as a water-soluble nutrient, high-dose vitamin C isn’t likely to be toxic. Still, it can cause unpleasant gastrointestinal side effects, mainly via bowel movements.

But here’s the kicker: the RDI for vitamin C is just 75 mg per day for women and 90 mg per day for men 7.

Studies show that the bioavailability of vitamin C is relatively good, with 70-90% being absorbed with dietary intakes below 1,000 mg—above that, and the absorption decreases to about 50%.

So, while maxing out with 5,000 mg of vitamin C isn’t likely to do you any favors, you do have to keep in mind that vitamin C is quickly depleted.

Low vitamin C levels can be the result of 8:

  • Smoking
  • Alcoholism
  • Excessive stress
  • Illness or infection
  • Dietary restrictions
  • Type 1 diabetes
  • GI disorders

Vitamin D

The other half of the equation is vitamin D, also known as cholecalciferol or the “sunshine vitamin.” It is a fat-soluble vitamin produced in the body via skin exposure to UVB rays from sunlight.

When UVB light hits the skin, it’s absorbed by the double bonds in 7-dehydrocholesterol (the precursor molecule to vitamin D), leading to the opening of the B ring to form pre-vitamin D3, which undergoes rapid rearrangement to form vitamin D3 9.

Most D3 production happens in the epidermis, diffuses into the dermal capillary bed, and is transported to the liver.

Although the primary action of vitamin D3 is in calcium and phosphorus metabolism, vitamin D receptors are located throughout the body, so vitamin D elicits actions in nearly every body system. Some of the other functions include 10-12:

  • Inhibits parathyroid hormone (PTH) secretion
  • Adaptive and innate immune responses
  • Insulin secretion
  • Cellular proliferation and differentiation
  • Thyroid health
  • Gut health and integrity of the gut mucosal lining
  • Mental health

Getting Enough Vitamin D

For the average person, meeting the intake requirements of all essential nutrients is challenging, but for vitamin D, that challenge can further increase in colder months.

For people who rely on adequate intake from the Sun, November to March can be challenging to fulfill needs, hence why “seasonal affective disorder” has been linked to low vitamin D levels.

With fewer hours of sun and lower-intensity sunlight, you’re not getting what you did in the summer. Combine that with a plant-based diet lacking animal-sourced vitamin D, and it’s a recipe for deficiency.

This could also be why cold and flus are much more common during winter. Because of vitamin D’s role in the immune system, insufficient levels could lead to weakened immunity and higher susceptibility to illness and infection 13.

Vitamin D Deficiency

Vitamin D deficiency has reached epidemic status, with over 50% of the population having vitamin D insufficiency 14. Despite how easy it is to obtain, most of the population doesn’t consume enough, and the consequences can be colossal.

Although research is conflicting, many studies point out the association between low vitamin D status and chronic diseases like cancer, cardiovascular disease, diabetes, autoimmune diseases, and depression.

Low levels of vitamin D can lead to plenty of symptoms, including:

  • Weakened immunity and frequent illness or infection
  • Fatigue
  • Back pain or bone pain
  • Hair loss
  • Slow wound healing
  • Mood changes
  • Bone loss
  • Muscle pain
  • Weight gain

If you’re experiencing any of these symptoms, it might be time to get yourself to a doctor and have your vitamin D levels tested. If you are indeed low, consider a vitamin D3 supplement.

The recommended vitamin D intake isn’t extreme and currently sits at just 400-800 IU or 10-20 mcg 1.

However, those amounts may not be sufficient if you have a darker skin tone or aren’t exposed to the sun daily.

Because you have to account for factors like BMI, sun exposure, skin color, and more, a daily intake of between 1,000 and 4,000 IU (25-100 mcg) is generally enough to maintain optimal blood levels.

Why You Should Take Vitamin C And D Together

So, what’s the deal with vitamin C and D3? Why should you take them together? Although each vitamin has its role in maintaining health and optimal performance, there is some overlap.

Here’s why you should take them together:

  1. Healthier bones: Bone health is a big one, especially as you age, and the vitamin C/D3 duo can help to maintain bone strength regardless of age. Vitamin C is required to produce collagen, which helps bone stability and resiliency to stress and fractures. Vitamin D3 is needed to metabolize calcium and phosphorus, and getting enough helps to keep calcium in the bone where it belongs and maintain bone density to prevent osteoporosis.
  2. Stronger immune system: Immune function is the other primary reason we need vitamin C and D3. Both nutrients are heavily involved in supporting various aspects of innate and adaptive immunity. A deficiency of either has been shown to impair immune defenses and increase the risk of illness and infection.

Whatever way you slice it, vitamin C and D3 are non-negotiable nutrients that must come through diet or supplementation to maintain optimal levels.

If you struggle to consume foods rich in either nutrient, consider supplementation.

Need some guidance on where to start? Check out this super stack!

  1. Performance Lab NutriGenesis Multi: A state-of-the-art multivitamin stocked with 100% RDI or more of 17+ essential vitamins and minerals in the most bioavailable form for optimal absorption and bioactivities.
  2. Performance Lab PL-Immune: A powerful blend of advanced immune-supportive nutrients designed to restore frontline defenses, bolster immune cell function, and maintain healthy and balanced immune responses.
  3. Performance Lab D3+K2: Featuring NutriGenesis vitamins, D3+K2 provides optimal doses of vitamin D3 and K2 to support heart, muscle, bone, and immune system health.


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