Clone of Bacon Sandwiches and Cancer - the issues

As of the 23rd May 2022 this website is archived and will receive no further updates. was produced by the Winton programme for the public understanding of risk based in the Statistical Laboratory in the University of Cambridge. The aim was to help improve the way that uncertainty and risk are discussed in society, and show how probability and statistics can be both useful and entertaining.

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What evidence is being used?

The report itself is a review of all the available evidence on the influences of various foods and activities on the development of cancer, as judged by a panel of internationally distinguished scientists.

The news reports have focused on the consumption of red and processed meat. The panel concluded that there is convincing evidence that consuming red and processed meat contributes to the development of colorectal cancer, and that there is limited evidence that they contribute to the development of some other cancers (oesophagus, lung, pancreas and endometrium). In this context, 'red meat' means beef, goat, lamb and pork. Red meat from other species such as buffalo or deer have not been considered. Processed meat is not easily defined, and the report notes that the term has been used inconsistently in epidemiological studies. However, the report gives this guideline:

In the broad sense of the word, most meat is processed — cooking is a process. But as commonly used, the term ‘processed meat’ refers to meats (usually red meats) preserved by smoking, curing, or salting, or by the addition of preservatives. Meats preserved only by refrigeration, however they are cooked, are usually not classified as ‘processed meat’.

(WCRF 2007: 117)

It is important to notice here that, contrary to the implied reading in some of the news coverage, this report is not discussing new evidence, but instead is a meta-analysis of already available evidence. Furthermore, it is not the first such analysis, as the WCRF published a previous report in 1997, with the earlier analysis coming to a similar conclusion – although the report adds that the evidence has by now become more conclusive (WCRF 2007: 128).

Has there been a systematic review?

Yes, in this case the report itself is a systematic review.

What judgments are being made, and whose?

The judgment being made in the report fits into two categories, the judgment on the strength of the evidence and the associated risk of the various foods considered in the report, and, extrapolating from that, the public recommendations that the writers and the WCRF have made as guidelines for the consumer.

The technical judgments on the evidence in the report are made by a panel of international experts in the field: recruitment procedures, as well as the methodology, are explained in the appendix. A list of the experts, peer reviewers, and other people involved in the report is available here. As for the second type of judgment, it is not made clear who in particular made them, and with which guidelines in mind. However, the report provides this explanation:

Judgements of ‘convincing’ or ‘probable’ generally justify goals and recommendations. These are proposed as the basis for public policies and for personal choices that, if effectively implemented, will be expected to reduce the incidence of cancer for people, families, and communities.

(WCRF 2007: 366)

They also admit that these recommendations do not follow immediately from the evidence, and that their wording has been the subject of much discussion among the panel:

Reliable judgements are carefully derived from good evidence. But specific public health and personal goals and recommendations do not automatically follow from the evidence, however strong and consistent. The process of moving from evidence to judgements and to recommendations has been one of the Panel’s main responsibilities, and has involved much discussion and debate until final agreement has been reached. The goals and recommendations here have been unanimously agreed

. (WCRF 2007: 366)

What is the peer opinion of the expert judgment provided?

In the case of the report itself, peer opinion is not provided. In the news reports, the opinions sought from other parties range from none (the Sun's original report on the story), to soliciting David Spiegelhalter's opinion in a follow-up piece the week after. Another comentator was Karol Sikora from the World Health Organisation, who iwas quoted as “dismissing” the report's advice. Because he does not think that the evidence provided by the report supports the conclusion that a small amount of red and processed meat is harmful, he is reported as arguing that the report's health advice is scaremongering and therefore “the worst type of public health advice”.
In essence, the competing expert opinion on the report does not focus on the evidence provided in the report, but on the judgment of what level of public advice this evidence warrants.

Is there a numerical assessment of the probability of the outcomes?

Yes, in the relevant sections of the report, following mostly on the numbers reported in the individual studies the report analyses. However, to communicate the final conclusions on the risks, it grades the evidence into “convincing”, “probable” and “limited” (chapter 3).

If no formal analysis, is there an appeal to additional principles, such as the ‘precautionary principle’?

Although there is a formal analysis, the actual conclusions drawn for the purposes of communicating the risks are unquantified. It is interesting to look at the precise wording of the advice, and to compare with how it has been interpreted with respect to the precautionary principle:

There is "convincing” evidence that processed meats, including ham and bacon, increase the risk of colorectal cancer. People who consume them are advised to do so sparingly.

(press release, original emphasis)

Which has been interpreted as much more restrictive in several news reports:

Consumers were told to curb drinking, avoid processed meats - including bacon, ham and sausages - and cut their intake of red meat and salt. (Daily Mail ).

Interestingly even the WHO expert this news report consulted for his opinion, Karol Sikora, was quoted as saying:

Alcohol, red meat and bacon in moderation will do us no harm and to suggest they will is wrong. (ibid)

which on the face of it does not seem to be a direct contradiction to the advice from the press release (or, indeed, the report itself), which advises moderation rather than abstinence.

On the other hand, the advice coming from the WCRF is slightly more confusing: in the “media” section of their website (which also includes the press release), they have put up a number of quotations from some of the experts involved in the report for the media to use. One of these, the quotation relating to the meat chapter is by Dr Greg Martin, the head of Science and Research for WCRF:

We recommend that people should avoid eating processed meats altogether, and that if they do eat it then they should only do so very occasionally.

This quotation itself stands in stark contrast to the advice as it is presented both in the report and in the press release. It must be pointed out that this is the opinion of Dr Martin, rather than the official position of the WCRF. However, it can be seen that this confusion, so apparent in the national press, about what the precise advice should be about eating processed meat, apparently started at source.

How are the conclusions communicated?

The report was published in full on the WCRF website, together with some supporting material. The recommendations have been distilled into 10 points, which have been put onto a separate webpage. The main findings of the report have been summarised in a press release, with ready made quotations by experts involved in the study, all in the “media” section of the site.