The Mail Online reports that "red meat IS bad for you" saying that eating pork or beef twice a week raises the risk of heart disease by 7%. The Sun meanwhile advises that eating chicken twice a week raises your risk.
The health risks from a diet high in processed meats, like sausages or bacon, seemed to be well established. The World Health Organization states that such meats are cancer-causing, while unprocessed red meat (such as beef) is classed as probably causing cancer. Similarly, diets high in salt and saturated fat have been linked with cardiovascular disease (CVD), and people with high red and processed meat intake may often fall into this group, too.
However, some of these findings were cast into doubt – at least in some sections of the media – after the 2019 publication of a controversial review (which we covered at the time) that reported that "red and processed meat is probably not harmful to our health".
Now a new study has pooled together the findings from 6 US studies. The studies included nearly 30,000 middle-aged adults who completed a food questionnaire and then were followed up for around 20 years. People who ate 2 servings of processed meat a week had 7% increased risk of developing CVD compared with those who ate none. People who ate 2 servings of red meat or poultry a week had a 3% to 4% increase in risk.
In absolute terms, however, eating 2 servings a week would increase any individual's baseline risk of CVD by about 1% to 2%. These are very small risk increases for individuals, but they could be having an impact at a population level due to the popularity of these types of food.
No link was found between consumption of fish and risk of CVD.
The links need further study, but the findings further support our understanding around a healthy balanced diet that minimises processed meat consumption and is low in salt and saturated fat.
Where did the story come from?
The study was conducted by researchers from Cornell University, New York, and Northwestern University Feinberg School of Medicine in Chicago, US. One of the researchers received funding through a fellowship from the American Heart Association Strategically Focused Research Networks. The Lifetime Risk Pooling Project, which provided data to the study, was funded by the NIH/NHLBI and by the Northwestern University Feinberg School of Medicine. The study was published in the journal JAMA Internal Medicine.
The UK media coverage was generally accurate and many sources did discuss the earlier 2019 review. Though at the time, many of the same sources did take the findings of the 2019 review at face value along with enthusiastic headlines such as the Daily Mirror's "Scientists rule that bacon is safe to eat every other day – and sausage and steak too."
What kind of research was this?
The current study pooled data from 6 cohort studies that had followed up US adults to assess their risk of CVD. Observational cohorts are the classic way of looking at associations between exposures such as diet and later health outcomes like CVD. However, they cannot prove definite cause and effect as other health and lifestyle factors may be involved.
What did the research involve?
The 6 cohort studies pooled in this analysis were the:
- ARIC (Atherosclerosis Risk in Communities) study
- CARDIA (Coronary Artery Risk Development in Young Adults) study
- CHS (Cardiovascular Health Study)
- FHS (Framingham Heart Study)
- FOS (Framingham Offspring Study)
- MESA (Multi-Ethnic Study of Atherosclerosis)
The study used data collected at participant recruitment, which was the late 80s for the first 4 studies, early 90s for FOS and 2000 to 2002 for the MESA study.
The analysis involved 29,682 adults who were free from CVD at recruitment and completed information on their dietary intake. They were of average age 54 years, 44% male, and 30% non-white.
Dietary intake was assessed using a standard food frequency questionnaire. This assessed intake of processed meat, unprocessed red meat, poultry and fish. One serving was estimated to be a 4oz (113g) piece of unprocessed red meat or poultry, 3oz (85g) piece of fish, and for processed meat, 2 slices of bacon, 2 small links of sausage, or 1 hot dog.
Participants were followed up for 19 years on average, and researchers looked for the development of cardiovascular disease, which could include fatal or non-fatal heart disease, heart failure or stroke. They also looked at the rate of death from any cause.
The researchers looked at the link between dietary intake and cardiovascular disease, accounting for possible confounding factors of:
- age, gender, ethnicity
- educational level
- alcohol intake
- physical activity levels
- overall dietary quality (for example, intake of fruit and vegetables, whole or refined grains, high- or low-fat dairy etc)
What were the basic results?
Participants consumed a weekly average of 1.5 servings of processed meat, 3 for unprocessed, 2 for poultry and 1.6 servings of fish. People with higher intake were generally more likely to be smokers, drink more alcohol, have a higher body mass index (BMI) and a lower overall diet quality.
There were 6,963 new diagnoses of cardiovascular "events" recorded during follow-up.
Taking account of confounders, consuming 2 servings of processed meat a week (vs none) was linked with 7% increased risk of cardiovascular disease (hazard ratio 1.07, 95% CI 1.04 to 1.11). Two servings of red meat was linked with 3% increased risk (HR 1.03, 95% CI 1.01 to 1.06), and 2 servings of poultry with 4% increased risk (HR 1.04, 95% CI 1.01 to 1.06). Fish intake was not linked with risk of CVD.
In terms of the actual difference eating 2 servings a week would make to an individual's baseline risk of CVD, the researchers calculated:
- processed meat: you would increase your risk by 0.4% over 10 years, 1.02% over 20 years and 1.74% over 30 years
- red meat: 0.17% over 10 years, 0.41% over 20 years and 0.62% over 30 years
- poultry: 0.20% over 10 years, 0.54% over 20 years and 1.03% over 30 years
Looking at risk of death from any cause, 2 servings of processed meat was similarly linked with 3% relative risk increase (0.9% absolute risk increase over 30 years) and red meat also with 3% relative risk increase (0.76% absolute difference over 30 years).
How did the researchers interpret the results?
The researchers conclude that their findings suggest that, among US adults, higher intake of processed meat, red meat or poultry was linked with a small increased risk of developing CVD, and processed meat and red meat with a small increased risk of death from any cause.
They suggest: "These findings have important public health implications and should warrant further investigations."
The benefits of eating a balanced diet with high intake of fruit, vegetables, wholegrains, lean protein and low in sugar, salt and saturated fat are well established. This reduces chances of overweight and obesity and helps to decrease risk of various diseases including CVD and cancer. More specifically, higher intakes of red meat, especially processed meat, has been linked with certain cancers, particularly bowel cancer.
This study gathers a large quantity of observational cohort data to try to quantify the effect these specific meats have on cardiovascular disease risk.
They found a general trend for increased consumption of processed and red meat and poultry to be linked with increased risk. However, it's important to highlight that the relative risk increase was very small. For red meat and poultry in particular, the risk associations only just reached statistical significance (95% confidence interval of 1.01). The difference that this would make to an individual's baseline risk of developing CVD was accordingly very small: at the maximum 2 servings of processed meat would give you a 1.7% increase on your risk over 30 years.
Therefore, although the links seem to be clear and further study is warranted, the specific effect of meat may be small alongside other factors that could contribute to your overall cardiovascular risk such as age, genetics, obesity, smoking or alcohol.
Related to this, although the researchers have made careful attempts to adjust for other such health and lifestyle factors to isolate the effects of meat, we cannot know if their influence has been fully removed. So we still cannot be certain of direct cause and effect for these small risk increases.
It's worth noting that serving or portion sizes used in the study are more than those recommended in the UK. For example, a portion of red meat was 4oz (113g) whereas current UK recommendation is a portion size of 70g (2.5oz) per day.
Other limitations include that food intake was assessed only once at study recruitment. This may not give a reliable representation of lifetime patterns. Food questionnaires may also give inaccurate estimation of food intake, especially when estimating portion or serving sizes. For example, "2 small links of sausage" may mean different things to different people, while a portion of red meat could mean a lean cut or a fatty one, or poultry could mean plain meat or deep-fried.
The studies all came from the US. Similarly, they all took food assessments 20 to 30 years ago. Therefore we cannot be sure they are representative of people from the UK or other countries, or of dietary intakes today.
The study overall supports understanding around a healthy diet that limits processed meat and saturated fat, salt and sugar. But it's important to note that lean meats are good sources of protein and many nutrients, and can still form part of a healthy balanced diet.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Mail Online, 3 February 2020
The Sun, 3 February 2020
Links to the science
Jama Internal Medicine. Published online 3 February 2020