Eating lean proteins, such as nuts, chicken and fish, reduces a person's risk of dying compared to a diet high in red meats, eggs and dairy, according to a large study out Monday.

The findings in the Journal of the American Medical Association (JAMA) Internal Medicine confirm what many health experts have said for decades, but also contained some surprises.

For instance, diets high in red meat and fatty proteins like eggs and cheese were not linked to a higher risk of dying among people who were otherwise healthy.

However, people with one other risk factor -- such as drinking lots of alcohol, being overweight, inactive or smoking -- were more likely to see their death risk rise if they ate more red meat.

The study led by researchers at Harvard University spanned three decades and more than 130,000 people, but was observational in nature so it did not probe the biological reasons behind the changes in death risk according to diet, nor did it demonstrate cause and effect.

"Our findings suggest that people should consider eating more plant proteins than animal proteins, and when they do choose among sources of animal protein, fish and chicken are probably better choices," said co-author Mingyang Song, a research fellow at Massachusetts General Hospital.

"While previous studies have primarily focused on the overall amount of protein intake -- which is important -- from a broad dietary perspective, the particular foods that people consume to get protein are equally important."

When researchers examined what kinds of protein were linked to higher mortality risk, they found them to be primarily processed and unprocessed red meats, including beef and pork.

The lowest mortality rate was seen among people who got their protein mainly from breads, cereals, pasta, beans, nuts and legumes.

Song said researchers were surprised to see there was no apparently higher risk of dying among otherwise healthy red meat eaters.

"While we expected we might find the associations to be weaker in the healthy lifestyle group, we did not expect them to completely disappear," he said.

"But when we looked deeper into the data, we found that -- at similar levels of animal protein intake -- those in the unhealthy lifestyle group consumed more red meats, eggs and high-fat dairy, while the healthy lifestyle group consumed more fish and poultry."

According to Ian Johnson, a doctor and nutrition researcher at the Institute of Food Research who was not involved in the study, the research is "robust" and "seems to support the growing consensus that diets based largely on plant foods are better for long term health than diets containing large quantities of meat and dairy products."

However, he pointed out the study, being based on questionnaire data, "tells us little about mechanism. It is far from clear whether plant proteins are protective or animal proteins are detrimental to health, or whether these protein levels are simply markers for something else."