Millions of us take vitamin tablets every day in the hope that they will keep us healthy and better equipped to prevent serious illnesses such as cancer.
In 2022, UK consumers spent more than £500 million on supplements – most commonly on vitamins C, D and B12, as well as the minerals iron and calcium.
An estimated one in three people take supplements, believing they will improve their well-being and protect against disease.
But could some vitamin pills actually do the opposite?
A recent study, published in August in the Journal of Clinical Investigation, reported that when mice with lung tumors received regular high doses of vitamins C and E, their cancers grew and spread more quickly.
In humans, these two nutrients play an essential role: vitamin C is important for wound healing and the health of skin, blood vessels, bones and cartilage; and vitamin E for eye health and immunity.
They are found in abundance in everyday foods, but an estimated one in four people regularly take vitamin C supplements, for example, either individually or as part of a daily multivitamin regimen.
Many people also seem to believe that high doses of vitamin C, in particular, can prevent winter colds and flu.
However, worrying new findings by scientists at the Karolinska Institute in Stockholm, Sweden, suggest that consuming more than the body needs of these two vitamins helps lung cancer grow by stimulating the growth of new blood vessels, thus providing more oxygen-rich blood to improve its spread.
In this study, researchers injected mice with lung cancer cells before giving them increasing doses of vitamins C and E in their drinking water.
They then performed regular ultrasound scans to monitor the formation of blood vessels that help cancer cells form a solid tumor.
The scientists concluded that while there is no evidence to suggest that excessive levels of vitamins C and E actually cause lung cancer, their research indicated that it could fuel rapid cancer growth in patients with previously undiagnosed tumors.
And although the study involved mice, the results raised concerns that humans consuming high doses of vitamin supplements might face similar risks.
These findings were unexpected because vitamins C and E are antioxidants, meaning they normally protect the body’s cells from damage caused by harmful free radicals (molecules produced by environmental factors such as pollution and metabolic processes). such as breathing).
As Professor Martin Bergo, an expert in biosciences and nutrition who led the research at the Karolinska Institute, explains: “These antioxidants activate a mechanism that causes cancerous tumors to form new blood vessels.
“This is surprising, because antioxidants were previously thought to have a protective effect.”
The researchers emphasized that there is no evidence that consuming vitamins and minerals in daily foods, where they are found in low concentrations, is harmful.
However, this study is not the first to hint at a link between excessive supplement consumption and tumors. A 2019 study in the journal Annals of Internal Medicine, which followed 27,000 American adults over 12 years, found that taking high-dose calcium supplements – more than 1,000 mg per day – doubled the risk of dying from any type of cancer.
The NHS says adults need around 700 mg of calcium a day to maintain strong bones and teeth, regulate heart rate and ensure blood clots normally, and that “you should be able to get all the calcium you need.” you need in your daily diet.” This includes foods such as cheese, milk, almonds, spinach, beans and oranges.
Calcium supplements are recommended by the NHS only for patients with osteoporosis, a disease that weakens bones, who may need to increase their calcium intake to help protect their bones.
Calcium supplements available over the counter come in different doses, including 800 mg, consumers are generally advised to take one or two per day.
Consuming more than 1,500 mg a day, according to the NHS, can cause stomach pain and diarrhea, but the US study by researchers at Tufts University in Massachusetts found that side effects at Long term high calcium intake could be much more serious for people with calcium. some people.
Importantly, the study also found that eating larger amounts of calcium-rich foods did not increase the risk of dying from cancer; risks were limited to high consumption in the form of supplements. These results were controversial because previous studies had shown that taking calcium supplements actually reduced tumor risk, particularly bowel cancer.
And the Tufts researchers emphasized that there is no concrete evidence that calcium pills actually cause cancer — and no obvious mechanism by which they do so.
Meanwhile, another popular supplement, beta-carotene, has been shown to increase the risk of lung cancer in smokers.
Beta-carotene is what gives yellow and orange fruits and vegetables their distinctive color: mangoes, apricots, carrots and red peppers are good sources.
The body converts beta-carotene into vitamin A, which is essential for healthy eyesight, a strong immune system and fertility.
However, several studies have found a link between beta-carotene supplements and lung cancer, with the most recent being published in the journal Nutrition Reviews in August.
This study, carried out by researchers at Tsinghua University in Beijing, pooled data from 18 previous studies on this topic. The study concludes that even at low doses, beta-carotene supplements increase the risk of lung cancer in smokers by about 16 percent.
The Department of Health and Social Care warns against taking more than 7 mg of beta-carotene per day unless advised by a doctor (some supplements contain 15 mg per tablet), and smokers should avoid completely due to risk of lung cancer.
Furthermore, the World Cancer Research Fund states that “there is strong evidence that some high-dose supplements may be harmful, such as high-dose beta-carotene supplements, which may increase the risk of lung cancer.” among smokers and non-smokers.
Experts agree that there is little or no risk in regularly taking individual multivitamins or supplements within the recommended daily allowance.
“There is insufficient evidence to show that vitamin C and E supplements can increase the risk of cancer,” Dr Claire Knight, senior health information officer at Cancer Research UK, told Good Health . “Most of the research (on these two supplements) to date has been done on animals, so additional studies are needed.”
The Health and Supplement Information Service (HSIS), which represents health food manufacturers, points out that the doses of vitamins C and E used in the Karolinska mouse study are “difficult to translate to humans.” ‘man “.
Spokesperson Dr Nisa Aslam says: “This study should not be used to suggest that humans avoid taking multivitamin and multimineral supplements in appropriate doses.
“In fact, such a supplement will help ensure that people fill gaps in their diet and achieve recommended daily levels.”
The World Cancer Research Fund recommends that consumers check supplement labels to see if their contents contain 100% – and no more – of what is called the nutritional reference value (NRV).
This replaced the term recommended daily amount (RDA), the amount the body needs daily to prevent deficiencies.
Some high-dose vitamin C tablets, for example, contain well over 1,000 percent of what the body actually needs.
However, certain groups of people clearly benefit from taking supplements. Vegans, for example, may need to take vitamin B12 (found in meat, eggs and dairy), which helps protect nerve cells.
Older people and those with limited access to sunlight may need a vitamin D supplement, especially in winter, says Professor Tom Sanders, a nutrition and dietetics expert at King’s College London, who takes 10 mg of vitamin D. per day but no other supplement.
In fact, the NHS advises everyone to take a daily supplement of 10 mg of vitamin D during autumn and winter. People who are dark-skinned, live in nursing homes, or wear clothing that covers most of their skin outdoors should consider doing this year-round.
“In general, dietary supplements that provide approximately the recommended daily amounts pose no risk and may have some benefit,” says Professor Sanders.
“However, dietary vitamin deficiencies are relatively rare in the UK, with a few exceptions (such as vitamin D).”
Mike Lean, professor of human nutrition at the University of Glasgow, says the human body is not equipped to process excessive amounts of vitamins.
“We evolved to require only very small amounts of vitamins, found in a variety of natural foods,” he says. “They are necessary for the functioning of different cells and organs.”
“But taking large amounts of vitamins, in supplement form, has no benefit and can become toxic.”
“Humans do not have the biological mechanisms to quickly eliminate excessive amounts of vitamins, because we have never encountered such high amounts during evolution.”
Professor Lean says he does not take any vitamin or mineral supplements, but instead follows a healthy diet which he says meets all his nutritional needs.
“My advice,” he says, “would be to enjoy a wide range of natural foods; Avoid manufactured foods, snacks and drinks that are deficient in vitamins – and don’t take vitamin pills.
