Can some foods cause cancer?
Many simple foods are rumoured to trigger cancer. But is there any scientific evidence to support the claims, asks Sasha Gonzales
So much has been written about what to eat to prevent cancer that most of us could doubtless name some of these foods. From green tea to cabbage, the anti-cancer foods issue has had so much hype that it is more or less accepted as the truth.
It's the same story with foods that are thought to cause or increase our risk of the disease. Certain foods have been demonised in the media, causing paranoia among the public and, indeed, among cancer patients.
But is there any truth to these beliefs or are our fears largely unfounded?
Clinical oncologist Dr Victor Hsue says that much of what is reported in the media is merely speculation or hype - and not rooted in scientific evidence.
How do we separate fact from fiction? Hsue advises to look out for claims that are difficult to verify and not backed up with scientific proof.
These eight foods have come under the spotlight in recent years for their supposed links with cancer. There is no evidence to suggest that they can cause the disease yet it is widely believed that they should not be consumed for this reason.
Here, we debase the most common cancer food myths:
We have been told that sugar feeds cancer cells and should be avoided, and many cancer patients believe this claim. In 2008, the international chapter of the World Cancer Research Fund (WCRF) funded a study on food avoidance behaviour among cancer patients and found that sugar is one of the most common foods boycotted.
But according to Patricia Chiu, manager of health information and education at WCRF Hong Kong, sugar intake has not been shown to increase the progression of cancer.
Of course, all sugary items - honey, candy and fizzy drinks - should be consumed in moderation because they can lead to weight gain. And being overweight or obese can increase one's risk of several cancers. A cancer patient undergoing treatment may have special nutritional requirements, so Chiu advises speaking to a doctor or dietitian before cutting out sugar entirely.
Charring or cooking meat over an open flame changes its molecular structure, forming heterocyclic amines and polycyclic hydrocarbons that are thought to increase one's risk of stomach cancer.
Chiu says that, to date, there is limited and inconsistent evidence that barbecued animal meat directly causes stomach cancer.
Soya bean products
Many claims have been made about soya bean-based foods, says Hsue. While some people claim these foods can reduce one's risk of breast, prostate, ovarian and uterine cancer, they have also been associated with an increase in the growth of breast tumours. So far, there is no strong evidence to support either claim, says Chiu.
Soya foods contain isoflavones (a class of phytoestrogens), which may act like the female sex hormone, oestrogen. But they also have anti-oestrogen properties. As part of a healthy diet, a moderate intake (one to two servings a day) of soya foods, such as soya milk and tofu, is fine.
However, soya or phytoestrogen supplements should be avoided if you are a survivor of oestrogen receptor-positive breast cancer, adds Chiu, because it is still not known whether soya foods affect cancer risk and survival.
Chiu says experts usually advise women who have had this type of breast cancer against consuming excessive amounts of soya. And particularly against supplements, because very little is yet known about their effects.
Breast cancer survivors should consult their oncologist if they have any concerns.
Foods that contain hormones
Chicken is widely thought to contain high levels of growth hormones. But this is not the case because the use of hormones was banned in chicken farms in Hong Kong in the 1980s, says Chiu. According to a report by the Centre for Food Safety, no hormone residues were detected from chicken samples obtained from local markets.
Chiu says most hormones are fat-soluble, so if you are still worried about residue, trim the fat off chicken pieces before cooking them.
Cows' milk is another food that some people question because it contains recombinant bovine growth hormone (rBGH), a synthetic hormone used to boost milk production. But Hsue says the amount of rBGH in milk is reportedly very low and is unlikely to have any effect if ingested.
Protein-rich bird's nest is said to increase the chance of a cancer recurring. It is believed to "nourish" cancer cells, thus promoting cell growth. Chiu says that very limited research exists on this subject and that the findings are inconclusive. More research is needed before any firm conclusions can be made.
According to Hsue, it is not just bird's nest but other nutrient-rich foods, such as fish maw and sea cucumber, that people avoid, as they are thought to "feed" cancer cells.
If you are a cancer patient, Hsue warns against depriving yourself of healthy foods. This can lower your immunity against cancer and also slow down your recovery from surgery, radiotherapy or chemotherapy.
According to WCRF's Second Expert Report, smoked foods - meats, in particular - may contain cancer-causing polycyclic aromatic hydrocarbons, depending on the fuel used to produce the smoke.
Smoked meats are often salted or cured, and some think they are likely to lead to the formation of carcinogenic compounds in the stomach.
Still, Chiu says, there has been limited evidence that smoked foods do indeed cause stomach cancer. It is, therefore, acceptable to consume smoked foods in small amounts.
Reports have suggested that drinking coffee can cause bladder and pancreatic cancer, but Chiu says that, once again, the evidence is still inconsistent.
In other studies, researchers have found that coffee can actually reduce the risk of some cancers. According to the latest findings in the WCRF Continuous Update Project report on endometrial cancer, drinking the brew can lower the risk of uterus-related cancer.
However, there is still insufficient information to recommend drinking coffee as a preventative measure. A study by the American Cancer Society last year found a link between drinking coffee and a reduced risk of oral cancer.
Fermented and preserved foods
Certainly, fresh food is always healthier and more preferable to preserved or fermented goods, but Hsue says that the occasional meal of ham, pickles, sausages, bread and cheese is unlikely to do any harm.
Chiu, however, warns that, because such processed foods are high in sodium, they should be consumed in moderation.
According to a WCRF Hong Kong survey last year on salt and sodium-heavy condiments, diets containing large amounts of salted foods could be linked to one's risk of stomach cancer.