Antioxidant vs. Anticancer – Are antioxidants safe during radiotherapy?

You don’t know what you don’t know.

One area of our practice that often comes as a surprise to people undergoing cancer treatment is the recommendation to avoid high doses of antioxidants during radiotherapy and some types of chemotherapy.

But aren’t they good for you?

Antioxidants are components of certain foods that help to neutralise free radicals that cause cell damage. Nutrients that have antioxidant properties include vitamins A, C and E, and minerals copper, zinc and selenium. Pytochemicals, such as isothiocyanates and lycopenes (read more at Don’t fight yo nutrients!’) also have an antioxidant effect.

With their ability to stabilise free radicals and reduce oxidative stress, antioxidants are wonderful at repairing cell damage. This is normally a huge benefit for our long term health, wellbeing and anti-aging. Radiotherapy (RT) treatment uses an oxidative process to damage cancer cells (read more at In the same way antioxidants protect normal cells, there’s a theoretical risk that they may also protect cancer cells, potentially minimising the efficacy of this treatment. This highlights how powerful antioxidants are and the impact they can have on our health outcomes.

Some studies have shown that high doses of antioxidants can reduce side effects of RT. We love this impact, but it may also be counterintuitive. Reducing side effects may be a sign that antioxidants are protecting cells. The antioxidants can’t distinguish between healthy and cancer cells. For a short period of time, we want cell damage. We want to control or kill malignant cells. Radiation oncologists use very precise treatment fields to minimise ‘collateral damage’ to surrounding healthy tissue. So whilst we can use the power of antioxidants to help repair our patient’s healthy cells after RT has finished, we first need to give the anticancer therapy a good shot at doing its job.


To avoid this possible protection, we recommend avoiding high doses of antioxidants in the form of supplements during RT. This includes supplementing with vitamins A (beta carotene), C, E, selenium and powders such as spirulina, green powders (e.g. Vital Greens) and even too much whey protein (which has been shown to enhance our body’s natural antioxidant profile).


Although the risk may be theoretical, we want to give cancer treatment that we know works, the best chance of working. Erring on the side of caution is to avoid any supplements, powders or concentrated forms of antioxidants during RT; then include rich food sources of antioxidants from one week after finishing RT to help the healthy cells repair. Be aware that some anticancer therapies may remain active in the body for weeks-months after treatment has finished, so it is important that we take this into consideration and discuss with the treatment team when guiding supplement intake.

What about food?

Can you still recommend spinach leaves in salads and berries in smoothies? The answer is yes! Including food sources of antioxidants is safe within reasonable levels. We want to meet our patients’ recommended dietary intake of micronutrients, without going overboard. A quarter cup of berries for brekky is different to 1kg juiced carrots each day. As APDs, it’s our role to guide and support these patients with safe dietary practices through their treatment. You don’t know what you don’t know. And now you know.


For specific advice or guidance for you or your patients, reach out to us today.


Tailor your plate with OnCore – Oncology and Preventative Nutrition

Lauren Atkins

Senior Oncology Dietitian  |  Accredited Practising Dietitian

Co-founder – OnCore Nutrition

[email protected]


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