Fri Mar 13

Should I take vitamin C to ward off colds, lower blood pressure or reduce cancer risk?

Written by: Nial Wheate, Professor, School of Natural Sciences, Macquarie University

Vitamin C is one of the most iconic nutrients in popular health culture, often credited with preventing colds, boosting immunity and even fighting serious diseases.

But while it’s essential for our bodies to function, its benefits are often misunderstood or overstated. Before you stock up on supplements, here’s what to consider.

What is vitamin C and why does my body need it?

Vitamin C, also known as ascorbic acid, plays several essential roles in the body.

It is a powerful antioxidant ^[1], protecting cells from damage. Vitamin C supports the immune functions of the body, aids in absorption of iron, and is involved in wound healing.

Vitamin C also helps in the synthesis of collagen, which holds together tissues and is a structural component of gums and skin. A severe deficiency of vitamin C leads to a health condition called scurvy ^[2], where the body produces insufficient collagen and can’t hold tissue together. Eventually, the gums cannot hold onto teeth and they fall out, and blood vessels break down, causing internal bleeding.

Humans cannot synthesise vitamin C. We must take it in through our diet ^[3]. Most of our vitamin C comes from vegetables (about 40%), fruits (19%) and from vegetable or fruit juices (29%).

Chemically, the vitamin C in supplements is identical to the vitamin C in food. Your body cannot tell the difference.

What is missing in supplement forms of vitamin C is the fibre, flavonoids, other vitamins, minerals and phytochemicals that come with food, and that may work together with vitamin C.

These other compounds help with absorption, provide complementary antioxidants, and together with vitamin C, provide health benefits that the vitamin by itself does not.

Historically, sailors ^[5] often had a very limited diet and were often struck down with scurvy. But if you have a balanced diet, you don’t need vitamin C supplements.

What does vitamin C treat and not treat?

Common cold

Vitamin C has been promoted as a way to boost the immune system. It’s widely considered as a way to prevent and treat the common cold and flu.

However, results from a review of all the evidence ^[6] has shown regular supplementation of 200 mg or more vitamin C does not reduce the incidence of the common cold.

Regular vitamin C supplement does reduce the duration, and at doses greater than 1,000 mg or more, could reduce the severity ^[7] of common cold symptoms.

When vitamin C is used for treating common colds and only taken at the start of cold symptoms, it does not affect the duration and severity ^[8]. Some studies have a shown very limited benefit when taken daily before getting sick, but the benefit was very small. Overall, the authors concluded routine supplementation with vitamin C is not worthwhile.

Heart disease and stroke

Research ^[9] has shown vitamin C supplementation does not change the risk of a range of cardiovascular diseases including heart attack (myocardial infraction), stroke or angina.

One study ^[10] found vitamin C supplementation at more than 200 mg daily may lower systolic blood pressure (the top number in a reading) by around 4 mmHg and diastolic blood pressure (the lower number) by around 2 mmHg. These are very small changes.

These effects are comparable to regular aerobic exercise and may not be clinically meaningful compared to treatment with conventional medicine ^[11], which generally lowers systolic blood pressure by at least 12 mmHg.

Cancer

There are consistent results from multiple studies that show vitamin C supplementation is unable to prevent cancer, including for gastrointestinal ^[12], lung ^[13], breast, prostate and colorectal ^[14] cancers.

Is it possible to take too much vitamin C?

The Recommended Daily Intake (RDI) of vitamin C for adults is 45 mg per day ^[15]. You can get that from a single small glass of orange juice ^[16].

The tolerable upper intake level ^[17] of vitamin C is 2,000 mg daily for adults.

Vitamin C is water-soluble and gets excreted in urine, so the body cannot store it. This means mega-dosing does not provide any benefit, and may in fact cause health problems.

At high doses (above 2,000 mg daily), vitamin C may cause mild to serious side effects ^[18]. Too much is known to cause diarrhoea, nausea and abdominal cramps. It can also contribute to the formation of kidney stones ^[19] in men, but not women.

For people who have chronic kidney disease ^[20], vitamin C can be especially problematic because vitamin C is flushed from the body by the kidneys. But when the kidneys don’t work properly, it can build up and cause kidney stones.

Should you take a vitamin C supplement?

For most people, a vitamin C tablet is unnecessary. You will get enough from a good balanced diet, from foods such as citrus fruits, berries, tomatoes, capsicum, broccoli and kale.

The evidence doesn’t support claims that vitamin C supplements prevent colds, heart disease or cancer. In fact, the risks may outweigh the benefits.