Diet and cancer: the evidence

Scientists have estimated that unhealthy diets cause from 10 to 30% of cancer deaths in developed countries. 1,2

Very few specific foods or drinks have been convincingly shown to increase or reduce the risk of cancer. 3 This is because person’s diet consists of many different foods, nutrients and chemicals that affect their risk of cancer. It is very difficult to design studies that can accurately look at the effect of a single food item. This page will tell you about those aspects of our diet that are reasonably linked to cancer by the current scientific evidence.

Cancer Research UK currently funds the UK arm of a massive international study called EPIC, which is studying the links between diet and cancer. EPIC scientists recently found that eating foods high in fibre can reduce the risk of bowel cancer. 4 People who ate the most fibre had 25-40% lower risk of bowel cancer compared to people who ate the least. Some other large studies have supported these findings. 5,6,7,8

Not all studies agree. Some have found that fibre does not affect bowel cancer risk. 9,10,11 But these studies only investigated populations from single countries, and may have looked at ranges of fibre that were too low. For example, Americans eat very little fibre on average. So a large study that focused on Americans would not be able to see the benefits of the high levels of fibre that, for example, an Italian person would eat.

The EPIC study covers 10 European countries, with a wide range of fibre intake and can detect connections that other studies may have missed. 12

Fibre could protect against bowel cancer in a few different ways

Bacteria in the bowel interact with fibre to produce several chemicals including butyrate. 13 Butyrate changes the conditions in the bowel, so that tumours are less likely to develop. Lab experiments have shown that butyrate can also stop the growth of cancer cells and cause them to die off. 14

Fibre also increases the weight of the stools and the frequency of bowel movements. This reduces the contact time between the bowel and chemicals in the stools. 13

 The EPIC study has found that eating lots of fruit and vegetables could reduce the risk of mouth, oesophageal and lung cancers, 15,16 as well as some types of stomach cancer. 17 EPIC has also found that fruit and vegetables are unlikely to reduce the risk of breast, prostate, ovarian or kidney cancers. 18,19,20

Some studies have found that people who eat the most fruit and vegetables can lower their risk of cancer by about a quarter compared to those who eat the least. 21,22,23 Specifically, eating lots of fruit and vegetables could reduce the risk of mouth, oesophageal and laryngeal cancers by at least a third, 15,24 and the risk of lung cancer by at least a quarter. 25

Fruit and vegetables contain a wide range of nutrients. Researchers are still trying to work out which of these might reduce cancer risk. Studies are looking at specific nutrients such as vitamin E, 26 and folate. 27

But it may be that you need these nutrients in balanced combinations to reduce the risk of cancer effectively. 28 Differently coloured fruit and vegetables contain different nutrients so it is important to eat a wide range of colours. 29 One study found that people who eat the widest range of fruit and vegetables have 22% lower risk of mouth cancer than those who eat the narrowest range. 30

There is strong evidence that the nutrients in fruit and vegetables do not reduce the risk of cancer when they are taken as supplements. High doses of supplements could even have harmful effects. 31

Fruit and vegetables are also a very good source of natural fibre 4, and there is strong evidence that high levels of fibre reduce risk of bowel cancer.

People were first recommended to eat five portions of fruit and vegetables a day in 1991 based on the scientific evidence at the time. Since then, many expert reports on diet and cancer prevention have supported the 5-a-day message 21,32,33,34,35.

Eating five daily portions of fruit and vegetables can help you maintain a healthy body weight. 21 Doing this can help you reduce the risk of bowel, breast, kidney, womb and oesophageal cancers. And getting enough fruit and vegetables can also reduce the risk of many other diseases including heart disease and diabetes. 21,36 The EPIC study found that people who ate the most fruit and vegetables reduced their risk of dying from chronic diseases like heart diseases, cancer and diabetes by a quarter. 37

 Many studies have shown that eating lots of red or processed meat increases the risk of bowel cancer. 38,39,40,41 Red meat includes all fresh, minced and frozen beef, pork and lamb. Processed meat includes ham, bacon, salami and sausages. The EPIC study found that people who ate two daily 80g portions of these meats increased their bowel cancer risk by a third, compared to those who ate just 20g a day. 40 Processed meat increased bowel cancer risk more than red meat. Studies that review all the available scientific evidence agree with these results. 39,42

There is growing evidence that links red meat to pancreatic cancer, 43,44 and stomach cancer. 45 The EPIC study found that eating lots of meat, particularly red and processed meat could also increase the risk of stomach cancer. People eating over 100g of meat a day had over 3 times the risk of getting stomach cancer. 46 Another very large study found that people who eat the most red or processed meat have 40-50% higher risk of pancreatic cancer. 47

There is no strong evidence that eating white meat, such as chicken, can increase cancer risk. 40

Red and processed meat contains a red pigment called haem. Haem could stimulate the bacteria in our guts to produce chemicals called N-nitroso compounds, or NOCs. 48 Many of these are known to cause cancer. Almost all red and processed meats contain more haem than white meats. This may explain why red and processed meats increase bowel cancer risk while white meats do not. 49

Haem could also irritate or damage the cells in the bowel. The cells divide much more than normal to compensate for this damage. This increases the chance that one of these cells could acquire changes that set it down the road to cancer. 48 There is some evidence that the effects of haem could be countered by chlorophyll, found in green vegetables. 50,51

The way meat is cooked can also affect the risk of cancer. Cooking meat at high temperatures, such as frying or barbecuing, produces chemicals called heterocyclic amines. 52 These can damage DNA and increase the risk of cancer. 53,54

Some processed meat contains chemicals called nitrites. In the bowel, nitrites are converted into NOCs, which could cause cancer. 50 One group of scientists analysed over 60 studies and found that nitrites, and foods rich in them, are linked to higher risks of stomach cancer. 55

The EPIC study recently reported that people who an 80g portion of fish a day reduced their bowel cancer risk by a third compared to people who ate less than that in a week. Some other studies have shown similar results, 56 but the evidence is still inconsistent.

It is not clear how eating fish could reduce the risk of cancer. Fish oils are especially rich in polyunsaturated omega-3 fatty acids (O3FAs), but there is no strong evidence that these can reduce the risk of cancer. 57

The link between fat and breast cancer is very controversial. Scientists have disagreed over whether the amount of fat you eat affects your risk of breast cancer.

One of the problems lies in the way fat is measured. Some studies use questionnaires to calculate the fat content in the food people eat. But these are not very accurate. Other studies ask people to fill in food diaries, which give a more precise record of what people eat.

Two of the largest studies so far, including EPIC, have used these diaries to show that women who eat the most saturated fat have higher risk of breast cancer. 58,59 Fat in our diets probably affects the risk of breast cancer by increasing the levels of oestrogen and other hormones in our blood. 60

There is some evidence that eating too much salty food, or food that has been preserved with salt, could increase the risk of stomach cancer. Stomach cancer is especially common in countries like Japan where people tend to eat lots of salty and salt-preserved foods. 61

Salt could affect the risk of stomach cancer by damaging the lining of the stomach and causing inflammation. Salt could also interact with a stomach bug called Helicobacter pylori that cause both stomach ulcers and stomach cancer. 61