After reviewing worldwide evidence, US researchers concluded that eating processed red meats like bacon, sausage, hot dogs and deli meats, but not unprocessed red meat, for example from beef, lamb and pork, is linked to a significantly higher risk of developing heart disease and type 2 diabetes.

The Harvard School of Public Health (HSPH) researchers found that every 50 gram (1.8 oz) increase in daily intake of processed meat (about 1or 2 slices of deli meat or 1 hot dog), was linked to a 42 per cent higher risk of developing heart disease and a 19 per cent higher risk of developing type 2 diabetes.

However, they cautioned that the evidence does not prove the link is causal, although it appears strong, and that further research should be done to understand the biology of how processed meat might cause these diseases.

You can read a 17 May before print issue of their findings online at the website of Circulation, a journal of the American Heart Association.

Lead author Renata Micha, who is a a research fellow in the department of epidemiology at HSPH, said in a statement that although people are often advised to cut down their meat consumption, the evidence is not very clear:

"Prior individual studies have shown mixed results for relationships between meat consumption and cardiovascular diseases and diabetes," said Micha, adding that:

"Most prior studies also did not separately consider the health effects of eating unprocessed red versus processed meats."

Micha said the results of this study suggest that perhaps people should consider which types of meat they are eating in order to lower their risk of heart attack and diabetes.

"Processed meats such as bacon, salami, sausages, hot dogs and processed deli meats may be the most important to avoid," said Micha, who advised that:

"Based on our findings, eating one serving per week or less would be associated with relatively small risk."

Micha and two colleagues also from the department of epidemiology at HSPH, assistant professor Dariush Mozaffarian and junior research fellow Sarah Wallace, carried out a systematic review and meta-analysis of the available evidence for links between red (unprocessed), processed and total meat consumption with incident coronary heart disease (CHD), stroke and type 2 diabetes (diabetes mellitus).

A systematic review and meta-analysis is a way of pooling results from many studies that examine the same variables and then re-analysing them as if they came from one large study. You have to be strict about which studies to include and exclude, for instance consistency of design, definitions of variables and data sampling are important factors to consider (known as "heterogeneity", the greater this is, the less sure you can be that the studies are saying the same thing).

In this case Micha and colleagues looked for three types of studies that assessed consumption of processed and unprocessed red meat and the incidence of CHD, stroke and type 2 diabetes in generally healthy adults.

The three types of study they searched for were: prospective cohort study (where researchers follow a group of people over a period of time), case-control study (where a group is compared with matched controls that did not have the same "exposure"), or randomized trial (where participants are randomnly assigned to the "treatment" and the control groups).

A media statement from HSPH said the researchers defined unprocessed red meat as "any unprocessed meat from beef, lamb or pork, excluding poultry", and processed meat as "any meat preserved by smoking, curing or salting, or with the addition of chemical preservatives", for instance "bacon, salami, sausages, hot dogs or processed deli or luncheon meats".

They did not look at the effect of vegetable or seafood protein sources.

By examining the abstracts of nearly 1,600 studies, Micha and colleagues found 20 that met their inclusion criteria: 17 prospective cohort studies and 3 case-control studies, covering over 1.2 million participants, nearly 24,000 incidents of CHD, about 2,300 of stroke and nearly 11,000 of diabetes. The participants came from 10 countries on four continents (US, Europe, Australia and Asia).

They extracted and pooled the data and then using statistical tools they looked for "dose-response" patterns, ie where increased consumption linked to increased or decreased risks of the diseases.

The results showed that: On average, each 50 gram (1.8 oz) daily serving of processed meat (about 1or 2 slices of deli meat or 1 hot dog), was associated with a 42 per cent higher risk of developing CHD (data came from 5 studies, relative risk RR 1.42, confidence interval CI ranged from 1.07 to 1.89, P for heterogeneity=0.04). This was also linked to a 19 per cent higher risk of developing type 2 diabetes: (7 studies, RR 1.19, 95% CI 1.11-1.27, P less than 0.001).
Consumption of red meat was not linked to CHD (4 studies, RR per 100g serving per day 1.00; 95% CI 0.81-1.23, P=0.36) or type 2 diabetes (5 studies, RR 1.16, 95% CI 0.92-1.46, P=0.25).
Links between total meat intake and risk of developing CHD and type 2 diabetes were intermediate between the processed and unprocessed meat intake patterns.
Consumption of processed and unprocessed meat did not show any links with raised risk of stroke, but only 3 studies had data on this, which is not really enough for a plausible analysis. The researchers concluded that consumption of processed but not unprocessed red meat is linked to a higher incidence of CHD and type 2 diabetes, and this highlights the need to better understand the potential mechanisms involved and perhaps think more carefully about policy and dietary advice on processed meat consumption.

Mozaffarian told the media that:

"Although cause-and-effect cannot be proven by these types of long-term observational studies, all of these studies adjusted for other risk factors, which may have been different between people who were eating more versus less meats."

"Also, the lifestyle factors associated with eating unprocessed red meats and processed meats were similar, but only processed meats were linked to higher risk," he added.

Micha said when they compared the nutritients in uprocessed and processed red meats eaten by Americans, they found they had on average about the same amount of saturated fat and cholesterol, but processed meat had on average about 4 times more sodium and 50 per cent more nitrate preservatives.

"This suggests that differences in salt and preservatives, rather than fats, might explain the higher risk of heart disease and diabetes seen with processed meats, but not with unprocessed red meats," said Micha.

Dietary sodium, which gets into processed food in the form of added salt, is known to increase blood pressure which puts people at higher risk of heart disease. Also animal experiments suggest that nitrate preservatives can reduce glucose tolerance (which increases risk of diabetes) and promote atherosclerosis (which increases risk of heart disease).

"Red and Processed Meat Consumption and Risk of Incident Coronary Heart Disease, Stroke, and Diabetes Mellitus. A Systematic Review and Meta-Analysis."
Renata Micha, Sarah K. Wallace, and Dariush Mozaffarian.
Circulation, May 2010.
DOI:10.1161/CIRCULATIONAHA.109.924977

Source: HSPH.

: Catharine Paddock, PhD