Last week, a prestigious medical journal questioned the recommendations made by WHO and other international agencies on the consumption of red meat and processed meat (hamburgers, sausages and sausages). Next, different international scientific organizations (the World Cancer Research Fund and the Harvard School of Public Health among others) and national (the Carlos III Health Institute, the Spanish Society of Epidemiology and the research network in Epidemiology and Public Health CIBERESP) included in their web pages a critical response to this new publication.
Why are contradictory messages sent? Did international organizations like WHO, the World Cancer Research Fund (WCRF) and others fool the population with their recommendations? Are they subjective opinions based on beliefs or poorly designed or poorly executed studies? Definitely not. Nutritional epidemiology is a very relevant but complex field of research: whoever answered a dietary questionnaire will immediately realize how difficult it is to measure the diet. Studies that investigate lifestyles without modifying exposure – so-called observational studies – have limitations. In theory, there could always be unknown or unrecognized factors that are associated with the diet and that would explain all or part of the results obtained. A single study is not enough. For this reason, information from many studies is often combined to investigate consistency between them and gain a global perspective on the relationship between an exposure – a food in our case – and a disease.
The authors of the new publications present analyzes of this type: reviews of all observational studies considered to be of better quality. Interestingly, its results are similar to previous reviews: increased risk of cardiovascular disease, diabetes and certain types of cancer in people with greater meat consumption Red and / or processed. However, they choose not to give value to their own results considering that, in any case, the effect is modest and the result is based on observational studies, with the limitations mentioned. From their perspective, we would need “intervention” studies, that is, studies where participants' exposure is randomly assigned, with a group that is asked to eat a lot of red meat – or a lot of processed meat – and another in which consumption would be eliminated or minimized. These types of nutritional intervention studies are complex, have ethical problems if we suspect undesirable effects and often require long follow-ups. In the end, participants get tired and abandon the proposed diet. In summary, the authors who question the recommendations in force consider that the limited effect found and the absence of intervention studies would justify their withdrawal.
How should nutritional recommendations be made? Who or who are in a position to formulate them? It is very instructive to read the considerations that the International Agency for Research on Cancer (IARC, belonging to WHO) or the WCRF make when evaluate the evidence. The IARC, for example, takes into account all existing studies, including animal experiments. In its judgment on red meat, for example, the IARC considers that the most convincing evidence comes in this case from laboratory studies. It would be reasonable to ask researchers who question these recommendations to also review these types of studies. In research, all fields have limitations. Animals are not exactly the same as humans, cells from tissues do not faithfully reproduce their functioning in living beings and human studies have, as we have seen, their own limitations. None is perfect but all add up. The most complete vision is only possible taking into account the different parts of the puzzle.
Even ignoring laboratory evidence, it is important to consider some more considerations:
1) The “small effect” on health could be greater: measuring the diet poorly in these studies underestimates the real effect
2) Small effects on various relatively frequent chronic diseases result in not negligible numbers in the population as a whole
3) The consumption of red and processed meat is associated with obesity, a fully established risk factor for these diseases
4) Intervention studies such as PREDIMED show a protective effect of the Mediterranean diet against some of these diseases. PREDIMED does not directly investigate red and processed meat, but the Mediterranean diet implies less consumption of meat and saturated fat.
In short, it seems sensible to follow the current recommendations, based on the continuous review of the evidence carried out by groups of experts in nutritional research. These recommendations do not propose to eliminate meat from our diet, but to moderate the consumption of red meat and processed meat (two or three times a week). In Spain there has been an increase in meat consumption and a progressive abandonment of the Mediterranean diet, traditionally implemented in large areas of the country. Obesity has become the epidemic of the 21st century, also in our environment. We move less and eat more. Many chronic diseases find in this change of habits the best breeding ground. Our health is also in our hands. Exercising, avoiding obesity, moderating caloric and alcohol consumption while maintaining a healthy diet are reasonable recommendations. A diet that emphasizes the consumption of vegetables and fruits, fish, legumes, cereals and olive oil.
Marina Pollan She is the director of the National Epidemiology Center (Carlos III Health Institute)
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