When weighing up the risk of cancer from food, it's not only what you put into your body that matters, although the type and quantity of food certainly contribute to cancer risk. However there's another side to the cancer and food equation and that is physical activity. It may not be intuitively obvious but there is evidence that physical activity itself protects against cancer.
For colon cancer, the evidence is strong and convincing. It's less convincing, but nonetheless possible that physical activity protects against breast cancer, lung cancer and reduces overall cancer rates.
In September 1997, the World Cancer Research Fund presented a comprehensive review of the evidence linking food, nutrition and the prevention of cancer. From the review, I discovered the current state of knowledge about the link between physical activity and cancer.
For optimal health, adults need to achieve a balance between the energy taken into the body via food and the energy spent by the body via metabolism and physical activity. An energy-dense diet full of fatty foods combined with a sedentary lifestyle tips the energy balance out of equilibrium. When energy intake exceeds energy output over a long period of time, the result is an increase in many chronic diseases, including cancer.
Very active people have a risk of colon cancer that is only about 60% of that of sedentary people. The risk rises as the amount of activity drops. It's not certain from the evidence whether lifelong activity is necessary to achieve this protection although it seems likely.
The evidence is convincing because the results are so consistent. Of the 20 studies which look at the link between colon cancer and physical activity, all but 3 show that activity protects and they all show roughly the same level of protection.
Each study has limitations which should be recognised before applying the results to other populations. Most of the studies are about men and most are done in the USA. In any epidemiological study, it is hard to pinpoint one factor such as physical activity and claim that it is the factor responsible for the drop in cancer risk. In affluent societies such as the USA, people who are most physically active tend to be more health conscious, smoking less and eating a healthier diet. Separating out these other important factors is not an easy task. Most studies have not investigated the effect on cancer risk of both energy intake and physical activity. The few studies that did look at both energy intake and activity found that they interact. The people with the highest cancer risk had the greater body mass and were also the least physically active.
Various theories have been proposed to explain how physical activity might affect colon cancer. One suggestion is that physical activity speeds up gut activity so that carcinogens in food or carcinogens produced during digestion spend less time damaging cells in the colon. Another theory is that physical activity has a positive effect on the immune system and on the levels of growth factors that inhibit the growth of cancer cells.
Physical activity possibly reduces the risk of breast cancer, according to evidence from epidemiological studies. In one American study, the breast cancer rate of university graduates was observed years after graduation. Those who had not been athletic at university had an 86% greater risk of breast cancer than the women who had been athletes at university. Another study estimated physical activity levels from the occupation listed on the death certificate. Those women who had been in physically demanding jobs had a lower rate of breast cancer.
There is also corroborative evidence from animal studies where exercise protects against the growth of mammary tumours in experimental animals.
The evidence is not as convincing as for colon cancer. There are only 5 epidemiological studies. Nevertheless, all of these, except one, found evidence of a protective effect of physical activity.
The way that physical activity affects breast cancer is probably through its effect on oestrogen metabolism. It is well-established that natural oestrogens, the oestrogens produced in the body during a woman's menstrual cycle, increase the risk of breast cancer. The risk is greater the more one is exposed to high levels of oestrogen, which is why early age at menarche, late menopause, late age at first pregnancy, and no pregnancies are associated with a higher risk of breast cancer. Physical activity decreases the body's production of oestrogen and lowers both the age of menarche and the age of menopause.
The lifestyle of most people in the developed world and increasingly of those in urban areas of developing countries creates an energy imbalance with energy intake often greater than energy output. One way to attain energy balance is to change to food with a lower energy content, eating a diet based on bulky plant-foods, mainly starchy staples, fruit and vegetables. Another way is to eat less food. A third way is to step up the energy output by increasing the amount of physical activity. All of these will reduce the risk of cancer.
Food, Nutrition and the Prevention of Cancer: a global perspective, World Cancer Research Fund and the American Institute for Cancer Research, 1997.Word Count: 868 Words