Colon Cancer and Red Meat
Colon Cancer and Red Meat
By Dr Janice Harland, Independent Nutritionist
There has been increasing awareness of the role that a good diet can play in promoting health over the last two or three decades. However when it comes to assessing the role of individual foods the evidence has been far from conclusive. The major causes of ill health and premature death - heart disease, stroke and cancer - are all multifactoral in nature and identifying the effect of a single factor in these diseases (which may develop over 20 or 30 years) is difficult. With cancer this is particularly true, as there is not a recognised risk factor or non-invasive marker for the disease such as blood pressure or blood cholesterol.
As appropriate clinical study methodology has not yet been developed, that measures cause and effect in the area of diet and cancer development, researchers are left looking at associations between extremes of food intake and the development of cancer over a number of years. Such associative studies follow specially selected population groups or cohort, assessing food intake at the beginning of the study and at regular intervals of few years thereafter. Usually food intake over a few days is used to estimate the type of diet adopted for a lifetime. Information relating to the number within the cohort who die from or develop cancer is also collected. It is recognised that such studies only measure an association between one particular type of diet and the disease risk and do not measure specific cause. In these studies a number of variables, such as age , sex, educational status and other dietary factors may be adjusted to make the comparison groups more similar, but this is far from an exact science.
Around the world and most particularly in the USA there are a number of well-established cohorts that have been followed for in excess of ten years and these provide the data that is used to understand the association between red meat and colorectal cancer.
Numerous groups of experts have reviewed evidence provided by these cohorts in publications that have spanned 20 years. The actual methodology used and the conclusions of the various expert groups are not consistent. Latterly media attention has focused on the findings from World Cancer Research Fund/American Institute of Cancer Research (WCRF/AICR) report, “Food, Nutrition, Physical Activity and the Prevention of Cancer: A Global Perspective", which concluded that The evidence that red meat is a cause of colorectal cancer is convincing,’ and made the recommendation that "population average consumption of red meat to be no more than 300g (11 oz) a week, very little if any of which is processed".
Prior to this a review conducted by the UK's Committee on Medical Aspects of Food (COMA) stated there was moderate evidence that lower red meat and processed meat consumption would reduce the risk of colorectal cancer. Their recommendation was that an adult individual’s consumption of red and processed meat should not rise above current levels. Recently the Government's SACN (Scientific Advisory Committee on Nutrition) concluded that evidence suggests that red and processed meat intake is probably associated with increased colorectal cancer risk, but also pointed to the limitations of the evidence as it is based on associative studies and the effects other factors cannot be excluded. In addition there is no proven dose-response or baseline effect of red and processed meat consumption. In the last decade at least a further four pooled analyses of the data have been published in peer-reviewed scientific journals and failed to find identify a strong association and practically meaningful relationship between red meat and cancer. Other Authorities such as International Agency for Research on Cancer (IARC) and respected individual scientists such Prof Stewart Truswell have questioned the WCRF/AIRC report's conclusions particularly the designation of the evidence as convincing and pointed to weaknesses in the methodology applied. It should also be recognised that current red meat consumption is highly variable and, with UK population average intakes of about 69g per day in women to 91g per day for men (National Diet and Nutrition Survey data, 2010), is in line with WCRF/AICR recommendations.
In view of the inconsistencies in the data, a reliance on associative evidence that includes a wide range of confounding factors, limited clinical study data and no detailed mechanisms of action, can such evidence really be considered convincing?