Diet and cancer: the evidence
This page explains how we know that diet can cause cancer. There is an overview of the scientific evidence and further links so you can find out more if you wish.
On this page
- A healthy diet can reduce the risk of cancer
- But diet is difficult to study...
- Fruit and vegetables may reduce the risk of many cancers
- Fruits and vegetables contain nutrients that could help protect against cancer
- Fruit and vegetables are a good source of vitamins, minerals, and fibre
- Fruit and vegetables have wide health benefits
- Eating lots of red or processed meat can increase the risk of cancer
- Eating lots of fish may lower the risk of bowel cancer
- Eating lots of fibre can reduce the risk of bowel cancer
- Eating lots of salt can increase the risk of stomach cancer
- Eating lots of saturated fat may increase the risk of breast cancer
People with less healthy diets are more likely to develop cancer. Many studies have been conducted looking at the association between diet and cancer, and experts agree the food we eat can affect our risk of cancer.
Scientists have estimated that less healthy diets cause nearly one in ten (9%) cancer cases in the UK.
Very few specific foods or drinks have been convincingly shown to increase or reduce the risk of cancer.This is because our diets include many different foods, and those foods consist of many different nutrients and chemicals that could affect the risk of cancer. It is very difficult to design studies that can accurately look at the effect of a single food item.
The way food intake is measured can be problematic too. Many studies use ‘food-frequency questionnaires’ which ask participants how often and how much they have eaten particular foods over a period of time. But this relies on participants accurately remembering their past food intake. Better-quality studies use ‘daily food diaries’ for participants to record what they’ve eaten each day. But, sometimes food frequency questionnaires are the most practical approach – it’s a lot to ask people in a study to fill in a food diary every day for a long period of time.
Also, it is very difficult to design diet studies that can accurately take account of other lifestyle factors which are very important in cancer risk, such as smoking tobacco and drinking alcohol. For example, people who drink or smoke heavily usually also have lower intake of fruit and vegetables. So when we see higher cancer risks in those people, it might be hard to disentangle the effects of one from the others. We need to take this possibility into account when considering the evidence on diet and cancer.
This page tells you about aspects of our diet that are linked to cancer by the current scientific evidence. Only foods which are supported by a body of good-quality evidence are included here. For the many other foods which have been studied, the current evidence is not good enough to say for definite whether there is a link.
Research has suggested that eating lots of fruit and vegetables could reduce the risk of mouth, oesophageal, bowel, throat, lung and some types of stomach cancers.
Some studies have found that people who eat the most fruit and vegetables can lower their risk of cancer by around 10% compared to those who eat the least 1. Eating one portion of fruit and/or veg each day can cut the risk of mouth cancer by half - and eating more portions cuts the risk by even more.
A recent study suggested around one in 20 cancers in the UK may be linked to people eating fewer than five portions a day of fruit and vegetables. More than half of all mouth cancer cases, and almost half of oesophageal and laryngeal cancers, are linked to diets low in fruit and vegetables.
In the UK, most of us do not meet recommended levels of fruit and vegetables in the diet .
Fruits and vegetables contain a wide variety of different nutrients with properties that could make it more difficult for cancer to develop. These nutrients include carotenoids, folate, vitamin C, vitamin E, selenium, flavonoids and various other phytochemicals (chemicals found in plants).
Some of their properties include the following:
- Carotenoids act as antioxidants. Antioxidants block other chemicals known as free radicals. Free radicals are highly reactive and have the potential to cause damage to cells, including damage that may lead to cancer.
- Folate plays a vital role in DNA repair.
- Vitamin C and E act as antioxidants, protect DNA from damage and stimulate the immune system.
- Small levels of selenium play an essential role as part of certain proteins which have antioxidant and anti-inflammatory properties, as well as protecting against DNA damage.
- Flavonoids could also have antioxidant properties and reduce inflammation.
Fruit and vegetables contain a wide range of nutrients. Researchers are still trying to work out which of these might reduce cancer risk.
But it may be that you need these nutrients in balanced combinations to reduce the risk of cancer effectively. Differently coloured fruit and vegetables often contain different nutrients so it’s a good idea to eat a wide range of colours .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 .
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.
Fruit and vegetables are also a very good source of natural fibre, and there is strong evidence that high levels of fibre reduce risk of bowel cancer.
People have been advised to increase their consumption of fruit and vegetables since the 1990s. Since then, many expert reports on diet and cancer prevention have supported eating five portions of fruit and vegetables a day. In 2005 the Department of Health made a concerted effort to promote their 5-a-day programme.
Eating plenty of fruit and vegetables can help you maintain a healthy body weight. Keeping a healthy weight can help you reduce the risk of bowel, breast (post menopausal), kidney, womb, oesophageal, pancreatic and gall bladder cancers. And getting enough fruit and vegetables can also reduce the risk of other diseases including heart disease. 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.
Eating lots of red or processed meat increases the risk of bowel cancer. Red meat includes all fresh, minced and frozen beef, pork and lamb. Processed meat includes ham, bacon, salami and sausages.
Around a quarter of bowel cancer cases in men, and around a sixth in women, are linked to eating red or processed meat. Bowel cancer risk increases by more than a quarter (28%) for every 120g of red meat eaten per day, and by almost a tenth (9%) for every 30g of processed meat eaten per day. Processed meat is more strongly linked to cancer risk than red meat.
There is growing evidence that links red meat to pancreatic cancer and stomach cancer. 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. Another very large study found that people who eat the most red or processed meat have 40-50% higher risk of pancreatic cancer .
There is no strong evidence that eating white meat, such as chicken, can increase cancer risk.
In the UK, we get a fairly high proportion of our energy intake from red and processed meats. The Government advises that people who eat more than 90g (cooked weight) of red and processed meat a day should cut down to 70g or less .
There are a few different ways red or processed meat could increase the risk of cancer. The biological reasons for the link between red or processed meat and cancer are still unclear, but it is likely that chemicals found in red and processed meat play a part.
Red and processed meat contains chemicals that could cause cancer:
Red and processed meat contains a red pigment called haem. Haem could 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 . There is some evidence that the effects of haem could be countered by chlorophyll, found in green vegetables.
Haem could stimulate the bacteria in our guts to produce chemicals called N-nitroso compounds, or NOCs45. 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.
Nitrites, nitrates and N-nitroso compounds
Nitrites and nitrates are used to preserve processed meat and may explain why some studies find that processed meat increases the risk of cancer to a greater extent than red meat. In the bowel, nitrites are converted into N-nitroso compounds, which could cause cancer. 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 .
Heterocyclic amines & polycyclic aromatic hydrocarbons
Cooking meat at high temperatures can produce harmful chemicals such as heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs) , many of which can cause cancer. The presence of these chemicals may explain why some studies find that meat cooked at high temperatures such as grilling or barbequing might increase the risk of bowel cancer more than meats cooked at lower temperatures such as boiling or braising.
The EPIC study recently reported that people who ate 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, 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 .
In the UK, our fish consumption is well below Government recommended levels of at least two portions of fish a week.
A recent study found more than one in ten (12%) bowel cancers are linked to a low fibre diet. A review of all studies on the topic has shown eating 10g of fibre per day can reduce the risk of bowel cancer by around 10%. Cereal fibre and whole grains seem to have the most effect on reducing bowel cancer risk.
In the UK, our average fibre intake is below the recommended level. Fibre triggers the production of helpful chemicals, and increases the frequency of bowel movements.
Bacteria in the bowel interact with fibre to produce several chemicals including butyrate. 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.
Fibre dilutes the contents of stools, and increases their bulk and the frequency of bowel movements. All of this reduces the contact time between the bowel and chemicals in the stools and could reduce the amount of cancer-causing chemicals that are absorbed through the lining of the gut.
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. But most evidence comes from countries with higher salt consumption than the UK, like Japan.
In the UK it's thought that nearly a quarter (25%) of stomach cancer cases are linked to eating more than 6g of salt each day. A review which combined the results of all relevant studies showed that people who regularly eat high amounts of salt each day have two-thirds higher risk of stomach cancer compared with those who eat low amounts.
Too much salt can increase blood pressure and the risk of heart disease and stroke.
Salt could affect the risk of stomach cancer by damaging the lining of the stomach and causing inflammation, or by making the stomach lining more sensitive to carcinogens such as nitrates. Salt could also interact with a stomach bug called Helicobacter pylori that cause both stomach ulcers and stomach cancer .
In the UK, we typically eat much more than the recommended 6g of salt per day.
Most studies on fat intake and the risk of breast cancer have suggested either no link or a small increased risk of the disease. Most studies that have found a small increased risk of breast cancer showed links with the intake of total fat or saturated fat. An analysis of four UK studies found no association between fat intake and breast cancer risk in middle-aged women.
Altogether, it is still not clear whether fat intake affects the risk of breast cancer. But if there is an effect, it is probably because fat in our diets increases the levels of oestrogen and other hormones in our blood, which fuels the development of cancer.
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