Diet and cancer - acrylamide, artificial sweeteners, green tea, soy, tomatoes and vitamin supplements
There are often stories in the media about certain foods or nutrients that are supposed to increase or decrease the risk of cancer. On the balance of evidence, it's unlikely that specific 'superfoods', on their own, could directly affect the risk of cancer, but some of the stories are still controversial.
Here, you can read about some of the stories you may have read or heard about, and find out our take on the evidence.
For information about other controversial topics, like mobile phones, power lines, stress and much more, go to our Cancer Controversies section.
In 2002, Sweden’s National Food Agency reported that many foods contain high levels of a chemical called acrylamide. International public concern followed since acrylamide is known to cause cancer by damaging DNA. This substance is produced when starchy foods are heated to high temperatures, and is found in foods such as chips, crisps and some bread.
Later studies in people found that the levels of acrylamide in most food are far too low to cause cancer. Many studies found that acrylamide has no impact on the risk of several different cancers, including breast, bowel, lung, brain and testicular cancers. Even food industry workers, who are exposed to twice as much acrylamide as other people, do not have higher rates of cancer. There have been some indications that acrylamide could be linked to higher risks of ovarian and womb cancers, but the evidence for this is limited and inconsistent, so we can’t be sure if this link is real.
In 2014, a draft report was released by the European Food Safety Authority, which said that although there is not enough evidence to show acrylamide could cause cancer in people, the levels of acrylamide people may get from food could still be a cause for concern. They called for more research to better understand the levels of acrylamide in people’s foods and how this might affect health, and they will be finalising their recommendations in 2015. But at the moment, there is still little concrete evidence linking acrylamide and cancer in people.
Artificial sweeteners are used in a wide variety of foods and drinks. Almost everyone in developed countries consumes them, whether they know it or not. Because of this, any potential cancer risks would be very far-reaching. But overall, studies on artificial sweeteners have found that they do not increase the risk of cancer.
Saccharin is one of the best studied of the artificial sweeteners. Some studies in the 80s found that it could cause bladder cancer in rats. Because of this, the Canadian government banned saccharin, and the American government warned that it could cause cancer. We now know that these effects were specific to rats and not relevant to humans. Bladder cancer risks are the same even in diabetics, who use sweeteners more frequently, and in people who lived through WWII, when saccharin use was high.
Aspartame, another type of sweetener, was also the subject of a cancer scare. This was because of an article linking it to rising brain tumour rates. This article had very little scientific basis and many later studies showed that aspartame, like saccharin, was safe for humans.
Another study in rats raised the alarm again but the European Food Safety Authority (EFSA) found that the study had some major flaws. The EFSA’s report concluded that aspartame does not increase the risk of cancer below the daily recommended level. Even people who consume a lot of sweetened foods take in well below this amount.
Large studies looking at actual people have now provided strong evidence that artificial sweeteners are safe for humans. For example, one study looked at almost half a million people and found that aspartame does not increase the risk of leukaemia, lymphoma or brain tumours. New generation sweeteners seem t be safe, however we will continue to monitor studies.
Some studies have found that green tea could reduce the risk of many cancers including cancers of the breast, prostate, mouth, foodpipe, stomach and bowel. The most promising studies were done in Asian countries. Studies in Western countries have mostly found no effect on cancer risk. These differences may be because Asians drink large amounts of green tea while people in Western countries drink mainly black tea.
Green tea contains high levels of a group of chemicals called catechins. Because of the way it is prepared, green tea contains 3-10 times more catechins than black tea. Laboratory studies on cells have shown that catechins could block the growth of cancers. They prevent DNA damage by mopping up free radicals, blocking the growth of tumour cells and stopping the activation of cancer-causing chemicals.
We still need more evidence from large-scale studies and clinical trials to prove that green tea could help to prevent some cancers.
The soybean is a staple part of East Asian diets and the evidence around the benefits or harms of soy is mixed. Some studies have linked eating soy products, such as tofu, soymilk or miso, to reduced risks of breast, prostate and bowel cancer, while others show no reduction in risk.
The studies that have shown a significant reduction in risk of cancer have looked at Asian populations, where the diet is high in soy. In 2014 a large study across Japan showed that soy didn't affect the risk of womb (endometrial) cancer. The much smaller amounts eaten by Western populations are unlikely to have any benefits.
Soy contains a group of chemicals called isoflavones. In our bodies, these act like mild versions of the hormone oestrogen. Many human cancers, such as breast cancer, are linked to high levels of this hormone. So some scientists believe that by taking the place of our own oestrogen, soy isoflavones can reduce the risk of hormonal cancers. Others are concerned that for the same reasons, isoflavones could actually increase the risk of some cancers.
Clinical trials are needed to say for sure if soy reduces or increases the risk of cancer.
Tomatoes contain a chemical called lycopene. This is found in all forms of tomatoes and tomato products including fresh, tinned, paste, juice and ketchup. Lycopene is a powerful antioxidant and mops up free radicals that could damage DNA.
It is unclear if lycopene could actually reduce the risk of prostate cancer. A large study of 47,000 men found that eating 2-4 weekly servings of tomatoes reduced prostate cancer risk by a quarter. And the EPIC study found that people who have the highest levels of lycopene in their blood have lower risk of advanced types of prostate cancer, but no decreased risk of developing prostate cancer overall.
However, not all studies agree and there are questions still to answer. For example, we still don't know how large a dose of lycopene you would need to reduce the risk of cancer. Even so, eating lots of tomatoes can help you get your recommended five portions of fruit and vegetables a day. They are also an excellent source of vitamins A, C and E.
Vitamin supplements do not have the same benefits as getting naturally-occurring vitamins in your food. It is thought that in fruit and vegetables, vitamins and nutrients interact with other chemicals to produce positive effects. On their own, they could be much less beneficial.
Several clinical trials have looked at the effects of vitamin supplements on cancer risk. Some of these have found that very high doses could actually increase the risk of cancer. An organisation called the Cochrane Collaboration first carried out a review of the evidence in 2008, which was updated in 2012 and now includes the results of 78 clinical trials of vitamin supplements. It found that these supplements, far from prolonging a person's life, either have neutral or harmful effects.
The best way to get your full range of vitamins and minerals is to eat a healthy, balanced diet, with a variety of fruit and vegetables. Supplements do not substitute for a healthy diet, although some people may be advised to take them at certain times in their lives. For example, doctors may advise women who are planning to have a baby to take a daily 400-microgram supplement of folic acid. And dark-skinned or elderly people may need to take vitamin D supplements since they need more sun exposure than other people to make enough vitamin D.
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