The real danger from eating processed meat, such as deli slices and hot dogs, may not the meat itself but their salt and chemical preservatives, a new study suggests.

The study from Harvard researchers found that eating a diet high in processed meats significantly raised the risks of heart attacks and diabetes, but that eating regular, unprocessed meat did not.

The researchers say when they looked at the nutrients in red meat and in processed meats, they contained similar amounts of saturated fat and cholesterol. But the processed meats contained, on average, four 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 Renata Micha, the lead author of the study and a research fellow in the department of epidemiology at the Harvard School of Public Health.

Dietary sodium is known to raise blood pressure, which is a strong risk factor for heart disease, the researchers note. And nitrate preservatives have been shown to promote artery clogging in animals, as well as reduce glucose tolerance, both of which could increase risk of heart disease and diabetes.

For the study, appearing in the journal Circulation, Micha's team systematically reviewed nearly 1,600 studies on diet and health. They narrowed it down to 20 observational studies that included a total of 1.2 million individuals from 10 countries.

They found that, on average, each daily serving of processed meat (about one to two slices of deli meats or one hot dog) was associated with a 42 per cent higher risk of developing heart disease and a 19 per cent higher risk of developing diabetes.

In contrast, eating unprocessed red meat did not appear to raise the risk of developing heart disease or diabetes.

The researchers say this is the first study to analyze the health risk differences between red meat and processed meat.

The team wanted to look at the relationship between eating meat and risk of stroke but there were too few studies to draw any conclusions.

Dariush Mozaffarian, a co-author of the study and an assistant professor in the department of epidemiology said the studies they used were long-term observational studies and so cause and affect is difficult to establish.

But he noted that all the lifestyle factors between the groups that ate unprocessed red meat and processed meats were similar, yet only processed meats were linked to higher risk.

The team says that given their findings, people should decrease their intake of processed meats like bacon, sausages, hot dogs and processed deli to just once a week or so.

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

They also called for further study into the differences between red meat and processed meat. For example, they note that while a high intake of meat and processed meat has been linked to colorectal cancer, unprocessed red meat has not been separately evaluated.

The American Meat Institute Foundation responded to the study by saying that epidemiological studies such as this one are poor at defining cause and effect, noting that nay number of other factors might be responsible for the findings they made.

"Too often, epidemiological findings are reported as ‘cased closed' findings, as if a researcher has discovered the definitive cause of a disease or illness," AMIF President James H. Hodges said in a statement.

"At best, this hypothesis merits further study. It is certainly no reason for dietary changes."