Red Meat Again Linked to Cancer

 
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Last month we discussed a series of review articles published in the Annals of Internal Medicine back in November 2019 in which a research team led by Dr. Johnston showed that while there was a statistically significant association between the intake of red and processed meats with cancer, cardiovascular disease (CVD) and total mortality, it was too weak to justify current recommendations to advise Americans to limit their intake of red and processed meats below current levels of consumption. Drawing perhaps the most ire from other researchers and public health experts was the accompanying editorial by Dr. Johnston and his group which concluded: “The panel suggests that adults continue current unprocessed red meat consumption. Similarly, the panel suggests adults continue current processed meat consumption.” Dr. Johnston's editorial stated: “In terms of how to interpret our weak recommendation, it indicates that the panel believed that for the majority of individuals, the desirable effects (a potential lowered risk for cancer and cardiometabolic outcomes) associated with reducing meat consumption probably do not outweigh the undesirable effects (impact on quality of life, burden of modifying cultural and personal meal preparation and eating habits). The weak recommendation reflects the panel's awareness that values and preferences differ widely, and that as a result, a minority of fully informed individuals will choose to reduce meat consumption.” Also no doubt disturbing to nutrition epidemiologists was Dr. Johnston’s argument that more population studies were unlikely to provide credible enough evidence to deprive people of foods they enjoyed eating (1).

In the past we saw population studies show a strong association between smoking and lung cancer, emphysema, increased CVD and a significant reduction in life expectancy. This was possible because millions of Americans started smoking as teenagers and smoked the rest of their lives, while millions of other Americans never smoked at all. But what if nearly all Americans had smoked and researchers had to find the association between the amount of cigarettes smoked and efforts to assess number of cigarettes smoked was as inaccurate as data collected about how much red and processed meats people eat over their lifetime? It is likely if that was the case that the epidemiological evidence linking smoking with cancer, CVD and total mortality would have appeared far weaker than it really is. This is the problem we have with data on red meat intake and cancer.

To get a more reliable estimate of the risk of consuming red and processed meats what is needed is large group of people who rarely or never eat red and processed meats over their entire lifetimes, but in most other ways consume a fairly typical modern diet but in other ways have lifestyles very similar to many other Americans. This would make it possible to compare this group who eat little or no red and processed meats to comparable age-matched Americans with similar diets and lifestyles but who do consume red and processed meats throughout their lives. Is there such a population? Turns out about half of Seventh Day Adventists (SDA) are vegetarians (mostly lacto-ovo), but even those who consume other animal foods mostly consume poultry and/or fish and most never or only rarely consume red and processed meats. Therefore, a comparison of SDA and otherwise comparable Americans who do eat red and processed meats may provide a more reliable estimate of whether or not the regular consumption of red and processed meats over a lifetime is contributing to the risk of cancer and other serious diseases.

SDA vs Comparable Americans and Cancer Study

Previous research has shown SDA in California live longer than do others. On average, SDA women live 4.4 years longer and SDA men live 7.3 years longer than the general population (2). Since 2002, Dr. Fraser at Loma Linda University has been investigating the effects of diet and other lifestyle factors on the risk of developing cancer in SDA living in North America. This ongoing study is called the Adventist Health Study 2 (AHS-2) and has previously shown that vegetarian SDA have a lower BMI, less metabolic syndrome, diabetes, hypertension, and all-cause mortality than SDA who are not vegetarian (3). This new study directly compared all-cause mortality and cancer incidence rates between the 86,610 AHS-2 participants and the 383,600 subjects in the National Longitudinal Mortality Study (NLMS) of the US population. Because SDA are less likely to smoke and drink alcohol than other Americans, the data was adjusted for the effects of smoking, education level, age, region of residence, sex, and race when appropriate. After nearly 8 years of follow up, the principal findings included that both all-cause mortality and the incidence of cancer were 33% and 30% lower among the SDA subjects than among comparable NLMS subjects (4).

The results of Dr. Fraser’s study comparing SDA with the general US population suggests a diet with relatively little or no red and processed meats likely does lead to a significant reduction in total mortality and the incidence and death rates from cancer (especially colo-rectal, breast, lung, and prostate) that are likely significantly greater than previous observational studies. Dr. Fraser’s SDA study suggests that the recent editorial by Dr. Johnston’s group may well be significantly underestimating the adverse health effects of red and processed meats. If so, then this new study suggests Dr. Johnston’s editorial’s conclusions that current guidelines to limit red and processed meats are based on too small a health benefit to justify current public health dietary guidelines to limit red and processed meats are more dubious.

Another recent study of 29,682 US adults pooled from 6 prior prospective cohort studies also found the intake of processed meat, unprocessed red meat, or poultry were all significantly associated with incident cardiovascular disease, but fish intake was not. Intake of processed meat or unprocessed red meat was significantly associated with all-cause mortality, but the intake of poultry or fish were not (5).

Bottom Line: Despite the controversial suggestions made in Dr. Johnston’s editorial that public health dietary guidelines should no longer advise people to limit their intake of red or processed meats, the preponderance of credible research continues to suggest that people who consume less meat and instead consume a more plant-based diet with a modest amount of fish, nuts and legumes, in place of red and processed meats are likely to live longer and have a lower risk of developing or dying from cancer.

By James J. Kenney PhD

References:

  1. Ann Intern Med. 2019;171:756-764. doi:10.7326/M19-1621.
  2. Fraser GE, Shavlick DJ. Ten years of life: is it a matter of choice? Arch Intern Med. 2001;161:1645-52.
  3. Orlich MJ, Singh PN, Sabate J, et al. Vegetarian dietary patterns and mortality in AHS-2. JAMA Intern Med. 2013;173:1230-8.
  4. https://acsjournals.onlinelibrary.wiley.com/doi/abs/10.1002/cncr.32571.
  5. doi:10.1001/jamainternmed.2019.6969 or https://media.jamanetwork.com/news-item/examining-consumption-of-processed-meat-unprocessed-red-meat-poultry-or-fish-with-risk-of-cvd-death/.
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