Cancer-causing dietary supplements: what you need to know

basics Oct 12, 2018

After writing last week’s post about diet and cancer, I was a little alarmed at the idea that vitamin and mineral supplements may actually increase cancer risk, and so I decided to dig a little deeper on this issue.

You can listen to the audio version here:

Before I start, let me just say this: it’s complex, it’s not clear-cut, and it’s a bit of a minefield, but here’s what I’ve come up with… 

The current state-of-play

Many cancer patients take one or more supplements every day because they believe them to be “anti-cancer” or “anti-toxicity”, yet they perhaps don’t realise that “anti-toxicity” may be a bad thing because it might actually interfere with their treatment.

In a study involving breast cancer patients [1], nearly half of the patients interviewed were taking multivitamins, a fifth were taking vitamins C, D and Omega-3 oils; 15% were taking vitamin E, B6 and folic acid, and just over a third were taking calcium supplements. Only a third were advised by their clinicians to start taking a supplement during their treatment, 10% were advised to stop taking one, and 7% were advised to stop taking ALL supplements, except a multivitamin.

Interestingly, over half of patients didn’t receive any advice one way or the other.

What does the science say about the most common supplements taken?

Multivitamin and mineral supplements

These are by far the most popular supplements and are taken by half of all cancer patients [2]. Unfortunately, there are no randomized trials assessing the effects of multivitamins on toxicity or survival, however it looks likely that they are not going to do any harm. Some research in colorectal and breast cancer patients showed that multivitamins were neither beneficial, nor harmful, in terms of toxicity or survival [3],[4]. Other studies reported a reduction in cancer incidence among men taking a daily multivitamin compared with men taking a placebo. On balance, though, there just isn’t enough evidence for recommending the use of multivitamins and minerals in cancer patients.

Antioxidant vitamins and minerals

There is a lot of interest around the use of antioxidants during cancer treatments, and with good reason. We know that oxidative stress is one of the causes of cancer, so naturally we could be forgiven for thinking that taking an antioxidant supplement would be beneficial. Indeed, there is some evidence that antioxidants can also reduce the amount of oxidative damage done to the body’s healthy tissues from chemotherapy and radiotherapy, which may enable a patient to take a higher-than-normal dose of treatment.

On the flip side, however, it may also reduce the effectiveness of treatment against the cancer cells themselves, since radiotherapy and many chemotherapy drugs (including alkylating agents, anthracyclines and platinum complexes) work by producing reactive oxygen species (ROS) which causes the cancer cells to commit suicide – a phenomenon known as apoptosis. Antioxidant supplements, therefore, might actually reduce the effectiveness of these oxidative stress-inducing drugs and therapies.

However, there are some potentially serious things to be aware of. A number of large-scale, randomized antioxidant cancer prevention trials have taken place and are summarized here [5]. Although the results were largely negative, there were a few notable exceptions which raised concerns:

  1. Beta-carotene is naturally found in carrots and is what gives them their characteristic orange color. However, beta-carotene supplements have been shown to increase the risk of lung cancer and stomach cancer.
  2. Vitamin E is found in sunflower seeds, some nuts, and leafy greens like spinach, and is widely accepted as having protective effects against heart disease and cancer, but vitamin E supplements may actually increase the risk of prostate cancer and colorectal cancer.
  3. One of the more complex findings was regarding selenium supplements, which have been shown to reduce lung cancer in people who had lower levels of selenium already, but increased lung cancer incidence in those with higher levels of selenium. On a more positive note, selenium supplements also reduced gastric cancer occurrence.
  4. Beyond cancer risk, antioxidant supplements, specifically beta-carotene, vitamin E and higher doses of vitamin A, have also been shown to increase cardiovascular disease, diabetes and overall mortality in the general population.

On balance, all the available evidence does not support the use of antioxidant supplements by cancer patients. 

Other antioxidant compounds commonly taken as supplements

Vitamin C and coenzyme Q10 are also popular among cancer patients because of their reported effects in protecting healthy cells from the toxic effects of chemotherapy, BUT, as for other antioxidant supplements, they can also reduce the effectiveness of chemotherapy. For example, vitamin C reduces chemotherapy effectiveness by 30-70% [6].

Turmeric, also known as curcumin, has made headlines in the past because of its reported anti-cancer and antioxidant properties, but conflicting studies have shown it can either decrease or increase [7] the effects of chemotherapy. In the case of increasing chemo’s toxicity, this can, for obvious reasons, be very dangerous.

Some patients take acidophilus or other probiotic supplements to repopulate their gut flora after taking antibiotics, but in actual fact it can promote infection if taken during chemotherapy because of the immunosuppressive effects of chemo. 

Fish oil is widely known for its anti-cancer benefits (amongst others), but can be dangerous for patients with a low platelet count because it can induce bleeding, and without sufficient platelets, blood clotting is impaired and the potential for excessive blood loss is a real danger. It can also interfere with chemotherapy.

Ginger root is another supplement many patients take because of the reported effects of inhibiting cancer stem cell growth [8], but it’s important to note these findings were from an extract of ginger root, not the wholefood itself, and ginger can also lead to bleeding, so care must be taken, especially amongst patients with a low platelet count.

Patients also need to be careful when it comes to eating grapefruit or drinking grapefruit juice because it can block an enzyme found in the intestine that helps the body metabolize certain chemo drugs [9].

Vitamin D

Vitamin D is a fat-soluble vitamin that is mainly absorbed from UVB rays through the skin, but it can also be gained from the diet in foods such as oily fish, fish liver oils, beef, liver, cheese, egg yolks, mushrooms and fortified foods such as tofu and soy milk. 

(As a little side note, did you know that cats can’t absorb vitamin D from the sun because of their fur? Instead, it gets absorbed in the oils on their fur, which they then consume when grooming themselves.)

Vitamin D deficiency is one of the most prevalent deficiencies in the general population as a whole, and cancer patients are no different, with nearly a third of patients having greatly reduced serum levels of the vitamin D metabolite [10]. Some studies have shown that patients with low serum vitamin D have a poorer prognosis, and for this reason, some oncologists will recommend vitamin D supplementation for certain patients.

However, there are limited studies looking at the effects of vitamin D supplementation on prognosis, and findings are sometimes conflicting. For example, promising results in advanced prostate cancer patients receiving docetaxel chemotherapy were totally contradicted by a follow-up study which had to be halted part way through when the interim results were showing an increased mortality rate in patients receiving the supplement compared with patients receiving a placebo (36% vs 29%) [11].

The main point to note here is that every patient is different, and every cancer is different, and there is unlikely to be a universal, one-size fits all benefit from vitamin D supplementation on survival among cancer patients. Responses are also influenced by a patient’s baseline vitamin D status, any genetic polymorphisms they may have in their vitamin D receptor gene, which in turn determines the biologic activity of vitamin D, and whether the tumor itself is expressing the receptor too.

Omega-3 oil supplements

Docosahexanoic acid (DHA) and eicoso pentanoic acid (EPA) are polyunsaturated fats found in Omega-3 oil supplements that can increase the production of reactive oxygen species (ROS) in cancer cells, which in turn can trigger apoptosis (cell suicide). For this reason, they are currently being tested as potential adjuncts to chemotherapy (an adjunct is a treatment given alongside chemotherapy). The hope is that this may maximize the sensitivity of the tumor cells to chemotherapy whilst at the same time reducing the drug sensitivity of non-cancerous cells.

It’s very early days, but initial findings suggest that DHA supplementation in patients with metastatic breast cancer who are also receiving an anthrycycline-based chemotherapy are surviving longer without disease recurrence [12]. Trials are still ongoing in this area [13].

The bottom line: some general guiding principles

Despite the general advice being to avoid taking supplements, many patients still take a multivitamin or dietary supplement of some kind every day. What they perhaps don’t realise, is that a better course of action would be to focus on getting all their nutrients from a varied and balanced diet full of fresh fruit and veg and wholegrains. Unless a patient has a nutritional deficiency, which is usually determined by blood test, oncologists generally advise patients to avoid supplements.

Ironically, many patients are more interested in taking a supplement than pursuing other positive diet and lifestyle changes, which has more potential for improving their prognosis. For example, half of the breast cancer patients in a particular study cohort were taking multivitamins, but 70% of them were overweight or obese, and 13% of them were smokers [1].

The big take-home message from this post is to take care when advising patients on dietary supplements. Really, this will all come down to a patient’s individual circumstances – genetic makeup, baseline serum levels of particular vitamins and minerals, what kind of cancer they have, and of course, what treatment(s) they are receiving.

In short, patients with an adequate vitamin status will receive no benefit from taking vitamin supplements, and it’s worth repeating, should only take supplements after consulting with their oncologist to guard against potential harmful effects or detrimental interactions with their treatments.

Those with vitamin and mineral deficiencies will see most benefit from supplementation, again under consultation with a qualified health professional.

In the future, we may see targeted nutritional therapies being tailored to patients in respect of their dietary habits, their genetic makeup, their tumor histology, and of course, their treatment plan. In this move towards personalized treatment, clearer benefits will begin to emerge.

The most appropriate advice you can give your patient will involve living a healthy lifestyle, including weight control, eating a diet that is low in saturated fat and refined carbohydrates, high in fiber, and with moderate alcohol consumption. Nutritional support in the form of supplements should be reserved for those with a deficiency. Most patients believe that supplements can’t do any harm because they are “natural”, and are sceptical of the negative reports from clinical trials because they believe them to be biased towards a medical model, and this is why it’s so important to have open and honest conversations with your patients about the best choices they can make to live as healthily as possible.

If you’d like to find out more about helping your patients decide for themselves to make positive lifestyle changes, download this free guide where I break down three things that you can use to help your patients live well with cancer. You can download your copy of the guide here.

You may not be able to promise them a cure, but you CAN give them hope to live as happy and fulfilling a life as possible whilst they still can. You can help them choose hope over fear. And by helping your patients adopt healthy behaviors, such as being more active and eating more healthily, you are in fact helping reduce their risk of secondary metastases. What patient wouldn’t want that?!

One more thing!

Once again, a wee reminder that October is Breast Cancer Awareness month, and I would love it if you could share this post on social media and tag me (@essentialcancereducation on Facebook and Instagram, @mhairimorris on Twitter). The more we can spread the word about making the best possible choices in terms of our own health, the more we can hope to reduce cancer incidence in the future. Thanks so much – I appreciate you :-)

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[1] Zirpoli et al., 2013

[2] Fortmann et al., 2013

[3] Ng et al., 2010

[4] Kwan et al., 2011

[5] Dolara et al., 2012

[6] Heaney et al., 2008

[7] Shakibaei et al., 2013

[8] Ray et al., 2015

[9] Valenzuela et al., 2011

[10] Teleni et al., 2013

[11] Buttigliero et al., 2011

[12] Bougnoux et al., 2009

[13] Harvie, 2014

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