People, who eat a diet rich in animal proteins during middle age, are four times more likely to die from cancer than those with low-protein diet, according to a University of Southern California study. Researchers said that the mortality risk is equivalent to that of smoking.

Protein-lovers were 74 percent more likely to die of cancer and diabetes than their low-protein counterparts. It is already known that excessive protein consumption causes dramatic rise in cancer mortality.

"There's a misconception that because we all eat, understanding nutrition is simple. But the question is not whether a certain diet allows you to do well for three days, but can it help you survive to be 100?" corresponding author Valter Longo, the Edna M. Jones Professor of Biogerontology at the University of Southern California Davis School of Gerontology and director of the USC Longevity Institute, said in a press release.

For the study, the researchers observed biological changes in a group of people for over two decades. The study was based on data from 6,318 adults over the age 50 years.

They claim that a diet that is good at one age can prove harmful at another stage in life.

Protein controls the growth hormone IGF-I. The hormone is responsible for body growth and is linked to cancer susceptibility. The levels of IGF-I decreases drastically post the age 65, causing weakness and muscle loss. The study showed that high protein intake during middle age is very harmful but is beneficial for older adults. People, who are aged over 65 and consumed moderate- or high-protein diet, were less likely to suffer from cancer and diabetes.

"The research shows that a low-protein diet in middle age is useful for preventing cancer and overall mortality, through a process that involves regulating IGF-I and possibly insulin levels," said co-author Eileen Crimmins, the AARP Chair in Gerontology at USC. "However, we also propose that at older ages, it may be important to avoid a low-protein diet to allow the maintenance of healthy weight and protection from frailty."

Longo said that the majority of Americans are eating twice the amount of proteins recommended by health experts. They should try to reduce the daily intake of proteins, especially animal-derived proteins. The researchers advise not to drastically cut the protein intake as it can quickly lead to malnutrition.

The finding is published in Cell Metabolism.