Food controversies

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.

Explore some of the stories you may have read or heard about, and find out our take on the evidence.

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 artificial sweeteners. Some studies in the 1980s 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 involving 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 to 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.

Pesticides are widely used in agriculture and there are concerns that they could increase the risk of cancer.

Some studies have suggested that pesticides could increase the risk of leukaemia, lymphoma, brain tumours, breast cancer or prostate cancer. But for now, the evidence is not strong enough to show a definite link.

Organic fruit and vegetables

High doses of some pesticides can cause cancer in animals, but the levels found in foods are tightly regulated to make sure they are well below this dose. The Food Standards Agency is responsible for food safety and standards in the UK . The Agency considers that current levels of pesticide residues in the UK food supply do not present a significant concern for human health.

Fruit and vegetables sometimes contain very small amounts of pesticides, but there is no evidence that these small amounts increase the risk of cancer in people who eat them. There is also evidence that eating organic food will not affect your cancer risk.

In fact, fruit and vegetables are an important part of a balanced diet, providing vitamins, minerals and fibre - and people who eat plenty of fruit and vegetables may have a slightly lower cancer risk.

Agricultural workers and farmer

People exposed to higher levels of pesticides as part of their job – for example in industry or through farming - may be at slightly higher risk of cancer..

The International Agency for Research into Cancer (IARC) hasooked at the evidence and said that regularly spraying pesticides as part of your job “probably” slightly increases the risk of cancer. But for most individual pesticides, the evidence was either too weak to come to a conclusion, or only strong enough to suggest a “possible” effect.

To protect workers, and also the public, pesticide use is monitored and regulated on a global, European and UK level, by the World Health Organisation, the European Food Safety Agency and the Health and Safety Executive. These websites have information on how workers can make sure they are using these products as safely as possible. Pesticides which were found to be potentially dangerous, such as DDT and lindane, have now been banned by these regulatory agencies.

Problems with the evidence

The scientific evidence on pesticides and cancer is still uncertain with studies producing conflicting results and more research is needed in this area. So far, the studies that have been done share common problems. Some only involve a small number of people. This makes it more likely that their results are down to chance. Working out the amount of pesticides that people were actually exposed to is difficult and some studies estimate these exposures based only on a person’s job. Others ask people with cancer to remember whether and how they used pesticides in the past.There are also a wide variety of different pesticides and it’s extremely difficult to separate out the effects of each of these in research.

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, soy milk 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.

The term ‘superfood’ is used to describe foods with apparently special health-related powers. These include blueberries, broccoli, garlic, raspberries, green tea and many more. Typically, such foods are hailed as having the power to prevent or even cure many diseases, including cancer.

But the term ‘superfood’ is really just a marketing tool, with little scientific basis. It’s certainly true that a healthy, balanced and varied diet can help to reduce the risk of cancer but it is unlikely that any single food will make a major difference on its own.

Many so-called ‘superfoods’ contain natural chemicals that have been shown to have positive health effects in laboratory studies. These include antioxidants, vitamins and minerals. It is true that some of these ingredients can affect cancer cells in a laboratory setting, including killing them and stopping them from growing.  However, foods contain many chemicals and laboratory studies are usually carried out using a purified ingredient from a particular food. So if researchers want to test the effect of an antioxidant contained in blueberries, they will use a purified version of that chemical rather than fresh blueberries.

Our diets are made up of hundreds of different types of food with thousands of different nutrients. An isolated chemical may behave very differently in a test tube than when it is eaten as part of food.

For example, the dose may be different. Often, scientists have to use very large doses of these purified compounds to see any effects in their studies. Typically these doses are much higher than what we would actually get in our diet. So even eating very large portions of a ‘superfood’ might not provide enough of a specific ingredient to have any effect on our health.

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.

More information about the vitamin supplements trials results.

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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