We sure do get a lot of warnings about what not to eat.
On Tuesday, March 24, the Vancouver Sun reported out on a recent observational study that linked eating red meat with a higher risk of cancer. This recent study from the National Cancer Institute in the US concludes: "Red and processed meat intakes were associated with modest increases in total mortality, cancer mortality, and cardiovascular disease mortality."
I've included the journal abstract from pubmed below.
As you read this, keep in mind this is an observational study, not a controlled experiment, and an important consideration is the fact that correlation does not indicate causation.
Something else to keep in mind is that the researchers based their results on subjects' self-reported lifetime dietary intake based on a questionnaire.
While both of these are significant defects of this study, the third one is easily the most insidious - particularly health-conscious individuals may have deliberately limited their consumption of red meat out of the belief that this will be healthier. These individuals are more likely to consume fewer processed foods, instead favouring fresh, simple foods such as fruits and especially vegetables. They are also more likely to undertake other healthy-lifestyle pursuits, such as sleeping well, avoiding the overconsumption of alcohol and sugary foods, and remaining active. They are demographically different from people who are less health-conscious - Census figures show us that health indices increase with affluence - people with better educations and better-paying jobs live longer. They take more and longer vacations, are more likely to seek medical and health advice from professionals, and live nearer urban centres - a factor also known to influence overall risk of mortality.
The research team did take steps to control for some such issues - matching age, education, marital status, family history of cancer, race, body mass index, smoking history, physical activity, energy intake, alcohol intake, vitamin supplement use, fruit consumption, vegetable consumption, and menopausal hormone therapy among women.
I'll add as an aside the failure of the body mass index with regard to strength athletes such as powerlifers, bodybuilders and Olympic lifters, whose extra bodyweight does not impact upon health parameters in the same way as obesity. A vastly superior measure here would have been percentage bodyfat, but accurate estimates of this measure are expensive - a full-body DEXA (dual emission x-ray absorptionometry) is the gold standard for this measure, and can cost upwards of a hundred dollars a pop. Getting back to the study, the problem remains that this is an observational study based on self-reports of lifetime dietary intake - not only does memory fail, but people are known to... aaahhh, shall we say exaggerate? Additionally, you have to question what it was that was different about individuals who deliberately chose to eschew red meat and processed meat products.
After controlling for all these factors, the research only found a "modest" increased risk, which was reported in something referred to in the article as a "hazard ratio". Similar to (and depending on the article, the same as) an odds ratio, this statistic is used when predicting binary outcomes from multiple factors - in this case, the outcome was "lived" or "died" during the 10-year follow up of the subjects. A similar, but much simpler concept is the relative risk:
From wikipedia: the ratio of the probability of the event occurring in the exposed group versus a non-exposed group.Here, since the risk of dying changes by the minute over a ten year period of time, the model is more complicated, but the interpretation is the same:
For example, if the probability of developing lung cancer among smokers was 20% and among non-smokers 1%, then the relative risk of cancer associated with smoking would be 20. Smokers would be twenty times as likely as non-smokers to develop lung cancer.
Simply put, a hazard ratio of 1.31 implies that there is a 31% greater risk of death between two comparison groups - in this case, this involved comparing death-rates among red-meat-eaters from the highest with the lowest-consumption groups. We don't know the actual death rates for this group, but the overall death rate over the ten year period for the reference population was 14%: 500,000 people aged 50 to 71 filled in a questionnaire. Ten years later, 14% (47,976 men and 23,276 women) of them had died from all causes. If 16% of the high-consumption and 12% of the low-consumption red-meat eaters had died, the relative risk would be 1.33.
A final note is that we don't know what these people ate or how they took care of themselves over those ten years. Beware of observational studies - at best, they provide food for thought.
Meat intake and mortality: a prospective study of over half a million people.
Sinha R, Cross AJ, Graubard BI, Leitzmann MF, Schatzkin A.
Division of Cancer Epidemiology and Genetics, National Cancer Institute-Nutritional Epidemiology Branch, Rockville, MD 20852, USA. firstname.lastname@example.org
BACKGROUND: High intakes of red or processed meat may increase the risk of mortality. Our objective was to determine the relations of red, white, and processed meat intakes to risk for total and cause-specific mortality.
METHODS: The study population included the National Institutes of Health-AARP (formerly known as the American Association of Retired Persons) Diet and Health Study cohort of half a million people aged 50 to 71 years at baseline. Meat intake was estimated from a food frequency questionnaire administered at baseline. Cox proportional hazards regression models estimated hazard ratios (HRs) and 95% confidence intervals (CIs) within quintiles of meat intake. The covariates included in the models were age, education, marital status, family history of cancer (yes/no) (cancer mortality only), race, body mass index, 31-level smoking history, physical activity, energy intake, alcohol intake, vitamin supplement use, fruit consumption, vegetable consumption, and menopausal hormone therapy among women. Main outcome measures included total mortality and deaths due to cancer, cardiovascular disease, injuries and sudden deaths, and all other causes.
RESULTS: There were 47 976 male deaths and 23 276 female deaths during 10 years of follow-up. Men and women in the highest vs lowest quintile of red (HR, 1.31 [95% CI, 1.27-1.35], and HR, 1.36 [95% CI, 1.30-1.43], respectively) and processed meat (HR, 1.16 [95% CI, 1.12-1.20], and HR, 1.25 [95% CI, 1.20-1.31], respectively) intakes had elevated risks for overall mortality. Regarding cause-specific mortality, men and women had elevated risks for cancer mortality for red (HR, 1.22 [95% CI, 1.16-1.29], and HR, 1.20 [95% CI, 1.12-1.30], respectively) and processed meat (HR, 1.12 [95% CI, 1.06-1.19], and HR, 1.11 [95% CI 1.04-1.19], respectively) intakes. Furthermore, cardiovascular disease risk was elevated for men and women in the highest quintile of red (HR, 1.27 [95% CI, 1.20-1.35], and HR, 1.50 [95% CI, 1.37-1.65], respectively) and processed meat (HR, 1.09 [95% CI, 1.03-1.15], and HR, 1.38 [95% CI, 1.26-1.51], respectively) intakes. When comparing the highest with the lowest quintile of white meat intake, there was an inverse association for total mortality and cancer mortality, as well as all other deaths for both men and women.
CONCLUSION: Red and processed meat intakes were associated with modest increases in total mortality, cancer mortality, and cardiovascular disease mortality.