In November 2007 a major report was published that reviewed existing studies on the relative risk of different types of cancer through lifestyle choices. Drawing together high quality research and international scientific expertise, this report, published jointly by the World Cancer Research Fund (WCRF) and American Institute for Cancer Research (AICR) is the state of the art on the relationship between food, body composition, physical activity and different types of cancer.
Some cancers may be preventable
Although DNA (genetic material) damage promotes cancer, only 5-10% of cancers are directly inherited, and someone with an inherited cancer-promoting gene will not necessarily develop cancer (but is at increased risk compared to the general population). Exposure to environmental factors that promote gene damage is by far the most important determinant of whether we develop cancer or not. Although not all environmental carcinogens (cancer producing substances) such as tobacco smoke, radiation and infection, can be avoided, there are many aspects of our daily life that can be modified to prevent damage to our DNA, such as a healthy diet and lifestyle choices.
Background to the report
Since the first WCRF report ten years ago, research into cancer prevention has increased dramatically and new improved electronic methods of analysing and assessing evidence have been developed. The need for an updated report was clear. During a five year process, over 20 of the world’s leading scientists examined a series of specially commissioned systematic reviews of the world’s scientific literature to examine how modifiable lifestyle factors affect cancer risk (WCRF/AICR 2007). Based on the quality and strength of the evidence, judgements were made about whether causal or protective relationships between foods, nutrients, body composition and physical activity were convincing, probable or limited. When the cumulative weight of epidemiological evidence, experimental and other biological findings, is consistent, unbiaised, strong, graded, coherent, repeated, and plausible, a causal relationship is more likely. Individually, none of those factors is likely to be sufficient to infer a causal relationship. This article highlights relevant convincing and probable judgements on causal relationships.
Although humans are designed to be regularly physically active, in recent years, especially in high-income countries, physical activity levels have continued to decline. Occupations have become more sedentary, most journeys are made in motorised vehicles, machines do most of the household chores and active recreation has been replaced by TV and computer games. This lack of activity is likely to be an important factor in overweight and obesity, which themselves increase the risk of some cancers. The report supports the general theory that humans are evolved and adapted to be physically active throughout life and that sedentary ways of life can be unhealthy.
There is convincing evidence that physical activity is protective against colon cancer, and probably protective against endometrial and breast cancer in postmenopausal women. The evidence suggests that all types and degrees of physical activity may be protective and there are limited data on specific physical activities.
In response to recent circumstances of plenty, a new and imperative health concern has emerged; overweight and obesity on a pandemic scale. The mechanisms by which body fatness increases the risk of cancer are not well understood. However, evidence does suggest:
- Obesity, especially abdominal fatness, causes higher than normal levels of hormones and growth factors that promote the growth of cancer cells. For example, high insulin production increases the risk of colon and endometrial cancers and possibly cancers of the pancreas and kidney, whereas excess leptin in the blood is linked to colorectal and prostate cancers.
- Obesity is characterised by a low-grade chronic inflammatory state. Inflammation is a physiological response to infection or trauma, which in the acute phase can be helpful. However chronic inflammation can result in DNA damage and cancer promotion.
The causal relationship between greater body fatness and cancer is now more robust that in the 1990. Specifically there is convincing evidence that there is a causal relationship between greater body fatness and oesophageal, pancreas, colon, endometrial, kidney and breast cancers (postmenopausal women), and probably also gallbladder cancer both directly and indirectly through the formation of gall stones. By contrast greater body fatness probably protects against breast cancer in premenopausal women but no mechanism for this effect has been identified.
Historically, the main focus of attention on breast-feeding was related to the benefits of human milk for the growing infant, however more recently research has highlighted the benefits to the child in later life and to the mother. Recent studies report that the longer a woman breast-feeds, the greater the protection against breast cancer. It is thought that the change in hormones related to a reduction in menstrual cycles is responsible for the benefit.
There is convincing evidence that breast-feeding protects the mother against breast cancer at all ages (including after the menopause).
Dietary fibre is mostly found in cereals, roots, tubers, pulses, fruits and vegetables. Although there is no clear link between particular starchy foods and cancer, there is extensive evidence that fibre is protective against bowel cancer. Fibre increases stool weight and speeds the passage of waste from the gut, maybe also speeding the passage of carcinogens from the system, but fibre is also fermented by the gut bacteria to produce short chain fatty acids, which help keep gut cells healthy.
Foods containing dietary fibre probably protect against bowel cancer.
Fruits and vegetables
Fruits and vegetables are a major source of vitamins, minerals and phytonutrients in the diet. In general the report found it difficult to ascribe an association between particular plant foods and cancer because of the complex mixture of nutrients they each contain, all of which might be contributing to any protective effect. Studies that have assessed key nutrients like carotene, lycopene (found in tomatoes), vitamin C, B vitamins and selenium have been more informative. These active components protect DNA against oxidative damage and/or may inhibit the activation of carcinogens in the body or by inhibiting the growth, or causing death of, cancer cells.
The report found that overall the evidence that fruits and vegetables protect against cancer is less conclusive than previously believed. It is probable that non-starchy vegetables and fruits protect against cancers of the mouth, throat, oesophagus, lung and stomach. Foods that probably protect against certain cancers are:
- chopped garlic against stomach cancer (the chopping releases an enzyme which promotes the formation of beneficial sulphur compounds)
- carotenoids against cancer of the mouth, throat and lung
- lycopene (found in tomatoes, especially if processed, such as tomato sauces, soups and ketchup) against prostate cancer
- vitamin C against oesophageal cancer
Alcohol has been consumed in most societies since Paleolithic times or earlier. Beers, wines and spirits are popular drinks and although prolonged high consumption of alcohol is a well-known cause of liver cirrhosis, knowledge of other ill effects is more recent. Ethanol is classified as a human carcinogen and the cancer is caused by the ethanol irrespective of the type of drink. No safe level of alcohol consumption (where there was no increase in cancer risk) was observed, and generally it was found that the more alcohol consumed, the greater the potential to develop cancer.
The evidence was judged to be stronger now than previously. There is convincing evidence of a causal relationship between alcoholic drinks and mouth, throat, oesophagus, colorectum (in men), and breast cancer (in women). There is a probable causal relationship between liver and bowel cancer in women and the consumption of alcoholic drinks.
Meat, poultry, fish and eggs
It is generally believed that humans evolved as omnivores and that healthy diets include both plant foods and those of animal origin – such as meat, poultry, fish and eggs. These foods are good sources of high quality protein and many vital micronutrients. However, when red meat (eg. beef, lamb, and pork) is consumed, the level of N-nitroso compounds in the body increases, which is thought to be partly due to its high haem-iron content. Also, many processed meats like ham, bacon, pastrami, sausages, bratwurst and ‘hot dogs’ have nitrates, nitrites and other preservatives added during curing. Dietary nitrates and nitrites are probable human carcinogens because they are converted in the body to N-nitroso compounds.
There is convincing evidence that consistent consumption of large quantities of red and processed meats can increase bowel cancer risk.
Milk and dairy products
Milk and products made from milk, such as cheese, butter, ghee and yoghurt, have been consumed ever since suitable ruminant animals were domesticated. Dietary calcium can be taken as a marker for dairy intake in Westernised areas like Europe. Dietary calcium is thought to protect against cancer as it directly influences cell growth and renewal and may bind to bile acids and fats in the gut, thereby preventing them from damaging the intestinal lining. Milk also contains bioactive substances, which may also play a protective role. On the other hand, a high calcium intake can increase cell proliferation in the prostate.
The evidence on cancer and dairy products points in different directions. Milk probably protects against colorectal cancer and there is limited evidence suggesting that milk protects against bladder cancer. There is a probable causal relationship between diets high in calcium and the risk of prostate cancer.
Fats, oils, sugars and salt
Fats and oils are the most energy dense constituent of the diet. Sugars are sweet-tasting carbohydrates. Fats and sugars are components of many energy-dense foods and drinks, which may be a contributory factor to the weight gain and overweight, which can increase the risk of cancer. However there is no convincing or probable evidence that fat, oils or sugars specifically cause any type of cancer. Salt (sodium chloride) is essential for normal body function, and in the past was a highly prized commodity. Nowadays salt is abundant and found mainly in salt-preserved foods eg salted meats, fish, olives, many processed foods eg. sausages, ready meals, gravy granules, and more obviously on crisps, nuts and other savoury snacks. There is evidence that high salt intakes damage the lining of the stomach, increase production of N-nitroso compounds in the body, and promote stomach carcinogens.
High intakes of salt and salt-preserved foods is a probable contributor to stomach cancer risk.
Non communicable diseases, including cancer, are major public health burdens world wide. The present report, reviewing existing evidence, offers a significant contribution towards our knowledge base about cancers, by helping us determine the extent to which food, nutrition, physical activity, and body composition modify the risk of cancer, and highlighting which factors are most important. This information will be used best in combination with recommendations, issued by national governments, to promote healthy lifestyles.
Making smart food and lifestyle choices early in life and through adulthood will help reduce the risk of certain conditions such as obesity, heart disease, hypertension, diabetes, and certain cancers. A healthy diet can be achieved through a balanced food intake, consuming certain foods in moderation and including a wide variety of different foods.
- WCRF/AICR (1997). Food, Nutrition and the Prevention of Cancer – a Global Perspective. Washington D.C.
- WCRF/AICR (2007). Food, Nutrition, Physical Activity and the Prevention of Cancer – a Global Perspective. Washington D.C. Available from www.dietandcancerreport.org