Colorectal cancer and processed meat

July 2009

The Scientific Advisory Committee on Nutrition (SACN) has recently released a report on “Iron and Health”.  The World Cancer Research Fund (WCRF) advocates that the intake of red meat should be restricted to 500g per week and that very little if any, should be processed.  This advice is based on an evaluation of the scientific evidence which was published in the Expert Report “Food Nutrition and Physical Activity and the Prevention of Cancer: a Global perspective in 2007”.

However the SACN report has done a separate evaluation of the evidence and is rather more cautious in its conclusions. In particular it draws attention to limitations and weaknesses in the various studies on this subject.  Key points made include the following:

There are a number of methodological inconsistencies between the different studies which make comparisons difficult. These include: adequacy of the dietary assessment methods to obtain reliable estimates of red and processed meat intake; lack of consistency in the categorisation of red and processed meat; and variability in the reporting of quantities of red and processed meat intake.

In most studies red meat and processed meat intake is based on a single dietary assessment at the start of the study. This does not take account of changes in dietary patterns over a number of years and could therefore be an unreliable estimate of intake over the specified follow-up period. The relevant period between dietary intake and development of cancer is also uncertain and dietary intakes estimated at baseline may not be the relevant period for dietary assessment in relation to cancer risk.

There are also considerable inconsistencies between studies in categorisation and definition of red and processed meat. Some studies have collected detailed information of the foods included under the red and processed meat categories, while others have used very broad classifications (e.g., beef, pork, lamb). In addition, some studies have separated red and processed meat categories and have only included fresh or untreated red meat in the red meat category while other studies have also included processed meat under the red meat category.

Another difficulty is the variability in reporting quantities of red and processed meat intake. While most studies compared highest versus lowest red and processed meat intake in grams per day/week/month, some studies reported intake as g/1000 kcal, frequency of intake per week/month or servings/day. There are also large differences in the quantiles of intake between different studies so that the amounts in lowest quantiles described in some studies are higher than the top quantiles in other studies.

Although results from prospective studies of dietary fibre and colorectal cancer are inconsistent, it has been suggested that higher intakes of foods containing fibre may protect against colorectal cancer risk. However only 8 out of the 21 studies on red meat intake and colorectal cancer risk and 5 out of the 14 studies on processed meat intake and colorectal cancer risk adjusted for fibre intake.

There are also a number of other factors that have been associated with colorectal cancer risk. These include genetic predisposition, high total fat intake, low fruit and vegetable intake, low fibre intake, low physical activity, and meat preparation and cooking methods. Studies have varied in the adjustments made for all these factors, so the effects of confounding cannot be excluded.

Taking all these factors into consideration it is very difficult to extrapolate form these findings a firm recommendation on the amount of red meat or processed meat intake that is associated with the risk of colorectal cancer

The SACN report reaches a rather more tentative conclusion on the consumption of red and processed meat than the WCRF. SACN states that

‘’Meat, particularly red meat, is the main source of haem iron. Epidemiological studies suggest an increased risk for colorectal cancer in association with increasing intakes of red and processed meat. Although the increased risk is not statistically significant in most studies, this may be due to lack of statistical power to detect an association. Overall, the available evidence suggests that red and processed meat intake is probably associated with increased colorectal cancer risk. However, since the evidence is based on prospective observational studies, the effects of confounding by other factors associated with increased colorectal cancer risk cannot be excluded. It is not possible to identify if there is a dose response or a threshold level of red or processed meat associated with increased colorectal cancer risk because of a number of limitations in the available data, including lack of consistency in categorisation and in reporting quantities of red and processed meat intake.’’

It concludes that lower consumption of red and processed meat would probably reduce the risk of colorectal cancer, and that although the evidence is not conclusive, as a precaution, it may be advisable for intakes of red and processed meat not to increase above the current average (70 g/d) and for high consumers of red and processed meat (100 g/d or more) to reduce their intakes.