Recently, research has come to light that links the eating of white bread with an increased risk of cancer. The study, published in the International Journal of Cancer, found that individuals eating the most white bread (equivalent to five slices of bread a day) were almost twice as likely to develop cancer of the kidney compared to those eating the least (equivalent to about one-and-a-half slices a daily).
Research of this nature is what is known as "epidemiological" in nature. This means researchers are essentially looking for associations between, say, lifestyle factors and disease. However, it's always important to remember that an association between two things does not prove that one causes the other. For example, owning a car is associated with an increased risk of heart disease. We need to be wary though, about then concluding that cars cause heart disease. In reality, it's probably not owning a car that is the true risk factor that explains this association – it's more likely to be something else that usually comes with car ownership (such as the more sedentary way of life) that is really to blame.
So, when an epidemiological study like the one linking white bread to kidney cancer comes to light, the next step is usually to at least try to explain what might explain the association. Well, one thing we know about white bread is that it offers very little in the way of nutritional value – it's not really food, more fodder . Eating a lot of white bread may simply squeeze out of the diet more nutritious foods that might have cancer-protective properties.
Another potential explanation for the link between white bread and cancer though, is that white bread does actually cause cancer. White bread releases sugar relatively briskly into the bloodstream (it is what is known as a high glycaemic index (GI) food). This will tend to get the body secreting a lot of the hormone insulin (the chief hormone responsible for lowering sugar levels in the bloodstream). There is some evidence that the biochemical changes this process induces may have cancer-promoting effects within the body.
High GI foods will tend to stimulate the secretion of not just insulin but can also raise levels of a related compound known as insulin-like growth factor (IGF). This substance is believed to have the capacity to initiate cancerous change in the body's cells. Also, the activity of IGF is to some degree modulated by other related substances known as IGF binding proteins (IGFBPs). In a study published in the American Journal of Clinical Nutrition , the consumption of a high GI meal, compared to slower sugar-releasing food, was found to lead to changes in the levels of two types of IGFBP[1]. It has been suggested that such changes promote the activity of IGF, and in so doing raise cancer risk.
Support for the idea that such biochemical changes might actually increase cancer risk has come from several studies which have found a link between the eating of high GI foods and cancer. For instance, a study published earlier this year in the Annals of Oncology assessed the diets of more than 5,000 women for their intake of sweet foods such as biscuits, cakes, sugar and chocolate[2]. This study found that compared to women with relatively low levels of consumption of sweet foods, those with the highest intake had a 19 per cent increased risk of breast cancer. The authors of this study concluded that they had found a direct association between breast cancer risk and the consumption of sweet fare. Also, a study published last year[3] found a link between the eating of high GI foods and breast cancer in post-menopausal women, while another found a similar link with prostate cancer[4].
There are lots of reasons why eating refined grains such as white bread as a staple food is not to be advised. The evidence already links the eating of such foods with obesity and diabetes. I suspect that evidence will eventually mount up to show that it boosts our risk of cancer too. Other foods of relatively high GI to avoid include wholemeal bread (its GI is actually as high as white bread), potatoes and white rice. Slower sugar-releasing foods to base the diet on include meat, fish, green vegetables, beans, lentils and nuts.
References:
1. Brand-Miller JC, et al. The glycemic index of foods influences postprandial insulin-like growth factor–binding protein responses in lean young subjects. Am. J. Clinical Nutrition 2005;82: 350 – 354
2. Tavani A, et al. Consumption of sweet foods and breast cancer risk in Italy Ann Oncol. 2006 Feb;17(2):341-5
3. Silvera SA, et al. Dietary carbohydrates and breast cancer risk: a prospective study of the roles of overall glycemic index and glycemic load. Int J Cancer. 2005 114(4):653-8
4. Augustin LS, et al. Glycemic index, glycemic load and risk of prostate cancer. Int J Cancer. 2004 10;112(3):446-50
Dr John Briffa is a London–based doctor, author and health writer with a special interest in nutrition and natural medicine. Practical advice about all aspects of health and well-being can be found at www.drbriffa.com
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