There is a great deal of interest in the link between diet, lifestyle and cancer. Evidence is strengthening that body weight, physical activity and dietary factors influence the personal risk of developing some cancers, with dietary factors estimated to account for around 30% of all cancers in industrialized countries(1). Diet and lifestyle factors are now considered to be second only to smoking as a cause of cancer.
Cancer in New Zealand
Cancer accounts for around one in three deaths in New Zealand and is a major cause of hospitalisation. There are about 17,000 new registrations of cancer each year, and 7,500 deaths, with the highest rates in the middle and older age groups(2). The aging population, along with increases in obesity, physical inactivity, and inadequate intakes of fruits and vegetables, are all likely to have an impact.
Key lifestyle and dietary recommendations for reducing cancer risk
In New Zealand, one in four deaths from cancer is tobacco-related(3). Complete avoidance of smoking is strongly recommended as the most important cancer-prevention measure.
Ultra Violet Radiation
Skin cancer is the most common cancer in New Zealand, with New Zealanders at high risk of developing a skin cancer during their lifetime. Our skin cancer rates are among the highest in the world. Melanoma incidence rates in Australia and New Zealand are around four times as high as those found in Canada, the USA and the United Kingdom(1).
New Zealand’s high rates of skin cancer are due to a number of factors including the strength of the UVR that reaches New Zealand during the daylight savings months, low ozone values, our outdoor lifestyle, our tendency to ‘seek the sun’, and the high proportion of people with fair skin in our population. Light skin type, large numbers of moles and excessive sun exposure (particularly intermittent episodes of sunburn), especially in childhood and adolescence, are the major predictors of melanoma risk.
Yet, skin cancer is readily preventable. Over 90% of all skin cancer cases in high UVR environments like New Zealand are attributed to excess exposure to UVR(2).
Advice is to:
- SLIP into a shirt – and slip into some shade, especially between 11am and 4pm (between October and March inclusive) when the ultraviolet rays are most fierce.
- SLOP on some sunscreen before going outdoors.
- SLAP on a hat with a brim or a cap with flaps. More people get burned on the face and neck than any other part of the body, so a good hat is important.
- WRAP on a pair of sunglasses. Choose close fitting, wrap-around glasses that meet the Australian / New Zealand Standard AS1067.
Diet and Cancer
A comprehensive report published by the World Cancer Research Fund and American Institute for Cancer Research(4) provides a review of the current literature on diet and cancer and gives some key recommendations on how to reduce risk of cancer by modifying our diet and lifestyle.
Obesity and body weight
Obesity has been associated with an increased risk of a number of cancers; for example cancer of the colorectum, endometrium, breast (post-menopausal) and pancreas.
The mechanisms by which obesity increases cancer risk are becoming increasingly understood. Obesity influences the levels of a number of hormones and growth factors. For example, high levels of insulin increase the risk of cancer of the colon and endometrium, and possibly of the pancreas and kidney. Increased levels of the hormone leptin are associated with prostate and colorectal cancer. Also, increased levels of sex-steroid hormones are strongly associated with endometrial and post-menopausal breast cancer, and may have an impact on cancer of the colon and other cancers.
Maintaining a healthy body weight throughout life is important not only to reduce risk of certain cancers, but also to reduce risk of other chronic diseases such as heart disease, stroke and diabetes.
All forms of activity protect against obesity by increasing energy expenditure. Sustained physical activity may also, independently of body fatness, protect against colon cancer and female hormone-related cancers.
It is recommended that we are all physically active, taking exercise equivalent to brisk walking for at least 30 minutes every day. As fitness improves, the aim should be to increase this to at least 60 minutes a day.
High-fat and high-sugar foods
Foods high in fat and foods and drinks high in sugar can be high in energy and, if over-consumed, can contribute to weight gain and obesity.
It is recommended that we limit consumption of high-energy foods and drinks.
Fruits, vegetables, pulses and wholegrains
Consuming plenty of fruits and vegetables is a key aspect of cancer-prevention.
Current recommendations are that we should all eat at least five portions of fruits and vegetables each day (in New Zealand, only around two thirds of us currently eat enough(5)). It is also recommended that relatively unprocessed cereals and/or pulses are eaten with every meal.
Fruits, vegetables, pulses, nuts and seeds provide a wide variety of micronutrients and other bioactive compounds that might reduce risk of cancer (for example carotenoids, lycopene, vitamin C and selenium). However, foods contain complex mixtures of different constituents so it is not possible at the present time to ascribe specific compounds in foods to a causal or protective effect.
Meat and meat products
High consumption of red or processed meat has been associated with an increased risk of colorectal cancer.
However, lean meat provides an important source of nutrients, in particular protein, iron, zinc and vitamin B12 and moderate intakes (of about 500g cooked meat per week) can be part of a healthy diet, alongside plant-based foods.
Alcoholic drinks are a cause of cancer at a number of sites, including the mouth, pharynx and larynx, oesophagus, colorectum and breast and probably liver.
Reactive metabolites of alcohol such as acetaldehyde may be carcinogenic. Further, alcohol may act as a solvent, enhancing the penetration of carcinogens into cells. Also, the effects of alcohol may be mediated through the production of prostaglandins, lipid peroxidation and the generation of free radicals. Finally, very high alcohol consumers may have a poor intake of essential nutrients that may be protective against cancer.
It is recommended that alcoholic drinks (if consumed at all) should be limited to two drinks daily for men and one for women. Pregnant women should not consume any alcohol.
Processed foods and salty foods
Foods high in salt have been linked to stomach cancer; there is some evidence that salt may damage the lining of the stomach and salt has also been shown to increase N-nitroso compound formation. In addition, salt may have a synergistic interaction with gastric carcinogens. It is recommended that intake of salty foods is limited.
High-dose nutrition supplements
The best source of nourishment is food and drinks and for most healthy people a nutrient-dense diet rather than supplements is the optimal approach to ensuring nutritional adequacy. It is recommended that high-dose vitamin or mineral supplements are avoided as a way of preventing cancer.
Breastfeeding has been found to be protective for breast cancer, principally thorough hormonal mechanisms. It is recommended that mothers breastfeed exclusively for up to six months and then add other liquids and foods to the baby’s diet. Ideally, breastfeeding should continue alongside complementary foods until at least one year of age or beyond(6).
Dietary and lifestyle advice to reduce risk of cancer are also consistent with reducing risk of other chronic diseases. Overall, recommendations are to maintain a healthy body weight, engage in regular physical activity and eat a balanced diet with plenty of wholegrain cereals and fruits and vegetables, limit alcohol intake, be safe in the sun, and don’t smoke.
1. New Zealand Cancer Society http://www.cancernz.org.nz/HealthPromotion/PhysicalActivityNutrition/Cancer/ (Accessed July 2008)
2. Ministry of Health www.moh.govt.nz/cancercontrol (Accessed July 2008)
3 Ministry of Health (2003) The New Zealand Cancer Control Strategy. Wellington: Ministry of Health and the New Zealand Cancer Control Trust.
4 World Cancer Research Fund / American Institute for Cancer Research (2007). Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. Washington DC: AICR. (www.wcrf.org)
5 Ministry of Health (2008). A Portrait of Health. Key Results of the 2006/7 New Zealand Health Survey. Wellington. Ministry of Health.
6. Ministry of Health (2008) Food and Nutrition Guidelines for Healthy Infants and Toddlers (Aged 0-2): A background paper (4th Ed). Wellington. Ministry of Health.
This Science Byte was peer reviewed by Jim Mann (Professor in Human Nutrition and Medicine at the University of Otago) and Dr Jan Pearson (Health Promotion Manager, Cancer Society National Office).