In
a new study, researchers from the Harvard School of Public Health (HSPH) have
found that eating processed meat, such as bacon, sausage, or processed deli
meats, led to a 42 percent higher risk of heart disease and a 19 percent higher
risk of type 2 diabetes. In contrast, the researchers did not find any higher
risk of heart disease or diabetes among individuals eating unprocessed red
meat, such as from beef, pork, or lamb.

This work, which appears in today’s online edition of the journal Circulation, is the first systematic
review and meta-analysis of the worldwide evidence for how eating unprocessed
red meat and processed meat relates to risk of cardiovascular diseases and
diabetes.

“Although most dietary guidelines recommend reducing meat consumption, prior
individual studies have shown mixed results for relationships between meat
consumption and cardiovascular diseases and diabetes,” said Renata Micha, a
research fellow in the Department of Epidemiology at HSPH and lead author of
the study. “Most prior studies also did not separately consider the health
effects of eating unprocessed red versus processed meats.”

The researchers, led by Micha and HSPH colleagues Dariush Mozaffarian,
assistant professor in the Department of Epidemiology, and Sarah Wallace,
junior research fellow in the Department of Epidemiology, systematically
reviewed nearly 1,600 studies. Twenty relevant studies were identified, which
included more than 1.2 million individuals from 10 countries on four continents
(United States, Europe, Australia, and Asia).

The researchers defined unprocessed red meat as any unprocessed meat from beef,
lamb, or pork, excluding poultry. Processed meat was defined as any meat
preserved by smoking, curing, or salting, or with the addition of chemical
preservatives. Examples include bacon, salami, sausages, hot dogs, or processed
deli or luncheon meats. Vegetable or seafood protein sources were not evaluated
in these studies.

The results showed that, on average, each 50 gram (1.8 ounce) daily serving of
processed meat (about one to two slices of deli meats or one hot dog) was
associated with a 42 percent higher risk of developing heart disease and a 19
percent higher risk of developing diabetes. In contrast, eating unprocessed red
meat was not associated with risk of developing heart disease or diabetes. Too
few studies evaluated the relationship between eating meat and risk of stroke
to enable the researchers to draw any conclusions.

“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,” said Mozaffarian. “Also, the lifestyle factors associated with eating
unprocessed red meats and processed meats were similar, but only processed meats
were linked to higher risk.”

“When we looked at average nutrients in unprocessed red and processed meats
eaten in the United States, we found that they contained similar average
amounts of saturated fat and cholesterol. In contrast, processed meats
contained, on average, four times more sodium and 50 percent more nitrate
preservatives,” said Micha. “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.”

Dietary sodium (salt) is known to increase blood pressure, a strong risk factor
for heart disease. In animal experiments, nitrate preservatives can promote
atherosclerosis and reduce glucose tolerance, effects which could increase risk
of heart disease and diabetes.

Given the differences in health risks seen with eating processed meats versus
unprocessed red meats, these findings suggest that these types of meats should
be studied separately in future research for health effects, including cancer,
the authors said. For example, higher intake of total meat and processed meat
has been associated with higher risk of colorectal cancer, but unprocessed red
meat has not been separately evaluated. The authors also suggest that more
research is needed into which factors (especially salt and other preservatives)
in meats are most important for health effects.

Current efforts to update the United States government’s Dietary Guidelines
for Americans
, which are often a reference for other countries around the
world, make these findings particularly timely, the researchers say. They
recommend that dietary and policy efforts should especially focus on reducing
intake of processed meat.

“To lower risk of heart attacks and diabetes, people should consider which
types of meats they are eating. Processed meats such as bacon, salami,
sausages, hot dogs, and processed deli meats may be the most important to
avoid,” said Micha. “Based on our findings, eating one serving per week or less
would be associated with relatively small risk.”