Tuesday May 18 2010
It is not known whether the risk is caused by other lifestyle factors
“'Sausage not steak’ increases heart disease risk”, reported the BBC. It said that eating processed meat such as sausages increases the likelihood of heart disease, while red meat does not seem to be as harmful. The risk of diabetes is also reportedly raised, with 50g of processed meat a day increasing that risk.
This news story is based on a review and analysis of 20 studies on red or processed meat and the risk of coronary heart disease, stroke and diabetes. As reported by the BBC, red meat did not appear to be associated with increased risk, but processed meat was linked.
As it stands, this well-conducted study shows an association between eating processed meat and an increased risk of heart disease and diabetes. However, it is still not certain whether this increased risk is caused by particular components of processed meat, or if it is due to other dietary or lifestyle factors that are also associated with higher consumption of processed meats. Further research will need to address this question.
Where did the story come from?
The study was carried out by researchers from Harvard Medical School. It was funded by The Bill and Melinda Gates Foundation/World Health Organization, the National Heart, Lung and Blood Foundation, and the Searle Scholars Program. It was published in the peer-reviewed medical journal Circulation.
The newspapers tended to focus on components of processed meat, such as salt and preservatives, which may cause the effect. However, this analysis and its component studies can only show associations but not determine cause. Further intervention studies in which processed meat is removed from the diet are needed to determine whether the preservatives or salt in processed meats underlie these effects.
What kind of research was this?
This was a systematic review and meta-analysis of evidence for the relationships between eating meat and the risk of developing coronary heart disease, stroke and diabetes.
The researchers said that previous studies on the risk of developing illnesses associated with meat consumption have produced ‘considerably’ conflicting results. The purpose of this meta-analysis was to pool all of the data and examine whether the amount of meat eaten or the type of meat (processed or non-processed) affects the link between meat and these illnesses.
What did the research involve?
The researchers searched medical and scientific databases for mention of meat or processed and non-processed meat products and cardiovascular disease or diabetes. They looked for articles that had been published up to March 2009.
The researchers defined processed meat as meat preserved by smoking, curing, salting or the addition of chemical preservatives. For example, bacon, salami, sausages, hot dogs or processed deli or luncheon meats were defined as processed meat. Unprocessed meat was defined as red meat from beef, hamburgers, lamb, pork and game. The researchers did not include poultry, fish or eggs in their analysis. Also excluded were studies that had compared vegetarians to non vegetarians, as these comparisons could be biased by other differences in diet or lifestyle.
Only studies that were of a suitable design to make reliable risk estimates were included. This excluded case reports, commentaries or narrative non-systematic reviews, as these can only give crude risk estimates. Where possible, the researchers used the adjusted risk estimates from the individual studies. This was so that the figures that were used in the meta-analysis had already taken into account other factors, such as saturated fat intake or weight, which can increase the risk of diabetes or heart disease. About half of the included studies had been adjusted for these potential confounding factors.
In total, 20 studies were selected. Two researchers independently assessed the quality of these articles and extracted the data. As the serving size of meat differed between the studies, they took an average in order to do their statistical analysis. This was 3.5 oz (100g) for red and total meat (red and processed meat) and 1.8 oz (50g) for processed meat.
What were the basic results?
Of the 20 studies, 11 were conducted in the US and the others were carried out in Europe, Asia or Australia. In total, there were 1,218,380 people in the studies. Most of the studies were prospective cohort studies (17). No randomised controlled trials of red, processed or total meat consumption and incidence of CHD, stroke or diabetes were found. Among this population, 23,889 people had coronary heart disease, 2,280 had stroke and 10,797 had diabetes.
The researchers found that across the studies the average weekly red meat consumption was between 1.1 and 8.3 servings. The participants ate between 0.4 and 5.7 servings of processed meat per week.
Consumption of red meat was not associated with coronary heart disease (CHD). However, each daily serving of processed meat was associated with a 42% higher risk of CHD (Relative Risk [RR] =1.42; 95% Confidence Interval [CI], 1.07 to 1.89).
Consumption of red meat was also not associated with the risk of diabetes. However, analysis of the seven studies on processed meat and diabetes risk indicated that there was a small increase in relative risk (RR = 1.19; 95% CI, 1.11 to 1.27). The relative risk increased to 1.53 when only the American studies were included.
Five studies looked at the effect of specific types of processed meat and the risk of new onset diabetes (incidence). Each serving (two slices) of bacon per day was associated with around double the risk of diabetes (RR=2.07; 95% CI, 1.40 to 3.04), as did hot dogs (one per day) (RR=1.92; 95% CI, 1.33 to 2.78). Other processed meats (one piece per day) were linked with a 66% higher incidence (RR=1.66; 95% CI, 1.13 to 2.42).
Only three of the studies looked at the effect of meat consumption on the risk of having a stroke. The pooled analysis of these studies showed no association between either processed or non-processed meat with stroke. However, analysis of total meat consumption (a mixture of processed and non-processed meat) indicated a 24% higher risk of stroke per daily serving (RR=1.24; 95% CI, 1.08 to 1.43).
The researchers looked at the available nutritional information of processed meat compared to the red meat. They found that processed meats had slightly higher fat-derived calories and slightly lower protein-derived calories. Processed meats had slightly less iron. The largest differences were the levels of salts – processed meats contained four times the amount of salt as red meat. Processed meats also contained around 50% non-salt preservatives, such as nitrate, nitrites and nitrosamines.
How did the researchers interpret the results?
The researchers said that although meat consumption is commonly considered to be a risk for cardiovascular and metabolic diseases, their study showed that the magnitudes of risk may depend on the type of meat and the type of disease. They say that “on the basis of our evaluation of average nutrient and preservative contents of red and processed meats, constituents in meats other than fats may be especially relevant to health effects”.
This was a large systematic review and meta-analysis, which found an association between processed meats and an increase in the risk of coronary heart disease and diabetes mellitus. Red meat itself did not appear to increase the risk of these illnesses.
The meta-analysis mostly included prospective cohort studies, which are appropriate for looking at associations between dietary intake and developing a disease over the long term. The systematic review was well-conducted and had the strength of including data from a large number of individuals from different countries. The study does have some potential limitations that should be considered, including:
- Most of the studies did not extensively detail the content of specific types of deli meats, making it difficult to determine whether particular additives may have a strong contributory effect to the risk.
- The studies did not include information on how the meat was cooked (fried, baked) which may have affected the outcome.
- Some of the studies included did not adjust for other dietary and socioeconomic factors. Therefore, links between CHD or diabetes and processed meats could relate to a generally less healthy diet or lifestyle rather than being a causal effect of processed meats.
As it stands, this well-conducted study shows an association between eating processed meat and an increased risk of heart disease and diabetes. However, it is still not certain whether this increased risk is actually caused by particular components of processed meat, or if it is due to other dietary or lifestyle factors that are associated with higher consumption of processed meats. Further research will need to address this question.