Eating unprocessed beef, pork or lamb appeared not to raise risks of heart attacks and diabetes, they said, suggesting that salt and chemical preservatives may be the real cause of these two health problems associated with eating meat.
The study, an analysis of other research called a meta-analysis, did not look at high blood pressure or cancer, which are also linked with high meat consumption.
"To lower risk of heart attacks and diabetes, people should consider which types of meats they are eating," said Renata Micha of the Harvard School of Public Health, whose study appears in the journal Circulation.
"Processed meats such as bacon, salami, sausages, hot dogs and processed deli meats may be the most important to avoid," Micha said in a statement.
Based on her findings, she said people who eat one serving per week or less of processed meats have less of a risk.
The American Meat Institute objected to the findings, saying it was only one study and that it stands in contrast to other studies and the U.S. Dietary Guidelines for Americans.
"At best, this hypothesis merits further study. It is certainly no reason for dietary changes," James Hodges, president of the American Meat Institute, said in a statement.
Most dietary guidelines recommend eating less meat. Individual studies looking at relationships between eating meat and cardiovascular diseases and diabetes have had mixed results.
But studies rarely look for differences in risk between processed and unprocessed red meats, Micha said.
She and colleagues did a systematic review of nearly 1,600 studies from around the world looking for evidence of a link between eating processed and unprocessed red meat and the risk of heart disease and diabetes.
They defined processed meat as any meat preserved by smoking, curing or salting, or with the addition of chemical preservatives. Meats in this category included bacon, salami, sausages, hot dogs or processed deli or luncheon meats.
Unprocessed red meat included beef, lamb or pork but not poultry.
They found that on average, each 1.8 oz (50 grams) daily serving of processed meat a day -- one to two slices of deli meats or one hot dog -- was associated with a 42 percent higher risk of heart disease and a 19 percent higher risk of developing diabetes.
They found no higher heart or diabetes risk in people who ate only unprocessed red meats.
The team adjusted for a number of factors, including how much meat people ate. They said lifestyle factors were similar between those who ate processed and unprocessed meats.
"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," Micha said.
"In contrast, processed meats contained, on average, four times more sodium and 50 percent more nitrate preservatives," Micha added.
Last month, the Institute of Medicine urged the U.S. Food and Drug Administration to regulate the amount of salt added to foods to help Americans cut their high sodium intake.
The FDA has not yet said whether it will regulate salt in foods, but it is looking at the issue.