Vitamin C fruits and veggies

Vitamin C

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A Course In Nutrition:  Learning Your A, B, C’s

Welcome back to part 9 of our multiple-part series on vitamins.  This one is all about vitamin C!

In the last blog of this series, we talked about Vitamin B12, Cobalamin.

We highlighted:

  • What forms to look for in B12 supplementation
  • Dementia and B12 deficiency
  • Physiological reasons for B12 deficiency
  • Optimal levels and recommended intakes
  • A delicious, B12-rich clam chowder recipe

Next, we’ll dive deep into the final water-soluble vitamin in the ABC series: Vitamin C.

Forms of Vitamin C

Although most animals can synthesize this vitamin from glucose in their livers, humans are not one of them. We must obtain this vitamin from diet and/or supplements. Here’s a breakdown of the types you are likely to come across in supplements and whole foods, and how they interact in the body.

  • Ascorbic acid is the natural and synthetic form of vitamin C. It comes in two forms, known as enantiomers, which are mirror-images of each other. They are d-ascorbic acid, (d for “dextro”, or rightward turning) and l-ascorbic acid (l for “levo”, or leftward turning). L-ascorbic acid is the form found in nature, and is also the most bioavailable. It is absorbed from diet via sodium-dependent transporters in the small intestine.

Ascorbic acid

  • Mineral ascorbates are salts formed with a mineral (usually sodium or calcium, but sometimes potassium, magnesium, zinc, or others) and ascorbic acid. This form of vitamin C is known as “buffered”, as the mineral salt is less acidic, which may be better tolerated in those who experience gastrointestinal upset when consuming ascorbic acid.
  • Vitamin C with bioflavonoids is a common supplemental form. In nature, vitamin C is paired up with other vitamins and minerals, fiber, and phytochemicals, such as bioflavonoids. It is common to find vitamin C supplements with plant bioflavonoids to mimic the vitamin C found in nature because they may enhance absorption of vitamin C. Bioflavonoids, sometimes referred to as vitamin P, are antioxidants themselves and therefore may protect vitamin C from oxidation and thus increase its bioavailability.
  • Dehydroascorbic acid is the oxidized form of vitamin C. Cells are able to recycle dehydroascorbic acid to its reduced form, ascorbate, via reactions catalyzed by enzymes. This may happen several times before the oxidized form is finally broken down and the molecule is excreted. The oxidized form of vitamin C can also be absorbed in the small intestine via glucose transporters. Once absorbed, it can be reduced to its active form, ascorbate.

Vitamin C has many roles in the body. Here’s a few highlights of its importance for human physiology.

Vitamin C is an Antioxidant

The most well-known function of vitamin C is as an antioxidant, meaning it can reduce free radicals in the body to protect from their damage. Free radicals, also known as reactive oxygen species, are molecules with an unpaired electron. These unpaired electrons make the molecule extremely unstable, so they react with protein and DNA to the cell’s detriment.

Vitamin C’s antioxidant ability has important implications for immune function, as the regular functioning of immune cells generates reactive oxygen species to destroy pathogens. Through other mechanisms, vitamin C enhances immune cell function and modulates the inflammatory response via cytokine production. In essence, vitamin C helps the immune system to mount an adequate attack against pathogens without excessive collateral damage.

In addition to being able to donate electrons to neutralize free radicals, vitamin C can recharge other antioxidants, such as vitamin E.

Vitamin C is a Cofactor in the Synthesis of Biomolecules

Collagen is a substance that requires vitamin C for its synthesis, stabilization, and strength. Vitamin C’s role in collagen development is as a cofactor for two enzymes: prolyl hydroxylase and lysyl hydroxylase. These enzymes give collagen, a key component of skin and connective tissue, stability, elasticity, and strength by enabling cross-linking between molecules. The implication of collagen’s importance include skin health (an important element of the immune system as the first barrier from the environment), wound healing, and joint mobility.

Another biomolecule that requires vitamin C for its synthesis is L-carnitine. L-carnitine boosts metabolism by enabling the oxidation of fatty acids for energy. In other words, it helps your body use stored fat for fuel.

Vitamin C is also important for the synthesis of neurotransmitters, other proteins, and blood vessel formation.

Vitamin C and Fertility

For men, vitamin C may enhance fertility by protecting sperm from oxidative damage and enhancing their mobility. In women, vitamin C also assists ovarian function via the antioxidant mechanism, as well as its ability to improve iron absorption. Adequate iron intake is necessary for the development of a mature egg.


Optimal Levels

Typical plasma concentrations of vitamin C are about 50-70 µM, while other cells and tissues of the body are much more concentrated, such as in immune cells. The highest concentrations of vitamin C are in the brain (2-10 mM), adrenal glands (4-10 mM), and the liver (~1 mM). Interestingly, the brain alters its vitamin C content based on its status in the body, sequestering this vitamin for its own function during times of scarcity.

Peak plasma concentration of vitamin C from foods or oral supplementation is about 220 µmol/L, which may be achieved from doses of about 3 grams at 4 hour intervals. Maximum absorption of vitamin C occurs at intakes around 200 milligrams. Above that, absorption decreases because of saturation of the sodium-dependent transporters that uptake vitamin C in the small intestine. Higher plasma concentrations of vitamin C can be achieved intravenously, and this type of intervention may be beneficial in the treatment of some cancers or other health conditions.

Deficiency of Vitamin C

Scurvy is the condition that comes to mind when considering a severely deficient vitamin C intake. Symptoms of scurvy initially include weakness, fatigue, loss of appetite, and joint pain, and progress to loose teeth, bleeding gums, scaly skin, and slow-healing wounds with prolonged deficiency.

Dietary intake of only 10 mg/day can prevent the onset of scurvy, though dietary deficiency is not limited to sailors of yester-year. Many modern people are at risk for deficiency of vitamin C due to highly processed diets, environmental exposure to chemicals and pollutants, and high stress lifestyles. It is important to regularly consume vitamin C in the diet, as it is a water-soluble vitamin and therefore is not stored in cells and tissues for an appreciable amount of time.

Recommended Intake

Recommended dietary intakes for vitamin C are 75 mg/day for women and 90 mg/day for men, which are likely to sustain plasma concentrations of about 50 µM. Though these intakes may be sufficient for healthy people, lifestyle factors and health conditions may necessitate a higher intake of vitamin C. Factors that may affect vitamin C requirements include:

  • Genetic polymorphisms
  • Smoking (which increases oxidative stress and impairs recycling of oxidized vitamin C to its reduced state)
  • Alcohol use
  • Pregnancy
  • Conditions of ill health

The upper limit, which is the highest level of intake that is unlikely to cause negative effects, is set at 2,000 mg/day. Symptoms of high vitamin C intake may include gastrointestinal upset, though for many people, intakes above this level may not produce any side effects at all.

Food Sources

Vitamin C is abundant in fresh fruit and vegetables. This vitamin is easily destroyed by heat, light, and oxygen from the air, so foods rich in vitamin C are best consumed fresh and raw. Luckily, many of the fruits and vegetables on this list are delicious raw or lightly steamed.

Foods rich in vitamin C include:

  • Papaya
  • Guava
  • Bell peppers
  • Broccoli
  • Brussels sprouts
  • Strawberries
  • Pineapple
  • Oranges
  • Grapefruit
  • Kiwi
  • Cranberries

The list goes on and on – nearly all fruits and vegetables will contain some amount of vitamin C. Generally speaking, if you consume about 2-3 servings of fresh fruit and 3-5 servings of vegetables a day, you are likely getting adequate vitamin C.

Recipe – Green Pea Hummus

The following recipe is a delicious alternative to hummus, as it is prepared from green peas rather than chickpeas. Green peas are another great source of vitamin C, with about 20 mg per cup, as well as vitamins K, B1, and E, as well as numerous antioxidant bioflavonoids. Serve this dip alongside a cup of sliced bell peppers for an additional 115 mg of vitamin C and you’ve exceeded the recommended daily intake with one delicious snack!

green pea hummus in a clear bowl on a plate with sliced red and yellow bell pepper


  • 1 cup frozen peas, thawed and rinsed
  • 2 tbsp tahini
  • 2 tbsp extra-virgin olive oil
  • 1 clove garlic, minced
  • 2 tbsp lemon juice
  • Salt and pepper to taste
  • More water, as necessary for desired consistency

Puree all ingredients in a food processor or with an immersion blender until creamy.

Recipe adapted from Rocco’s Healthy + Delicious, by Rocco Dispirito.


About the author

Karyn Lane is working towards her holistic nutrition certification in NTI’s Nutrition Therapist Master Program. She finds her chemistry degree a useful tool in her study of holistic nutrition and loves to treat herself as a laboratory for new recipes and cooking techniques. You can follow her on Instagram @feel.alive.nourishment.

About Nutrition Therapy Institute’s Holistic Nutrition Certification

Nutrition Therapy Institute (NTI) is a leader in holistic nutrition education. Since 1999, NTI has provided students with the highest quality in nutrition training by offering comprehensive holistic nutrition courses online and in-person to help students achieve thriving careers as holistic nutrition therapists in the field of holistic nutrition counseling and wellness. Interested in starting our holistic nutrition courses and earning your holistic nutrition certification? Attend an informational webinar to learn more by signing up HERE.  


Image by pasja1000 is free for use by Pixabay

Image by DENker is licensed under CC BY-SA 3.0

Image by Alexas_Fotos is free for use by Pixabay

Recipe Image by Karyn Lane

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