Consuming a high protein diet when you are middle aged may be as dangerous to your health as smoking, a new study suggests.

US scientists monitored the health of more than 6,300 adults over the course of almost two decades. They found that those who consumed a diet high in animal proteins, such as meat and cheese, were four times more likely to die of cancer compared to those who consumed a low-protein diet.

This mortality risk is comparable to smoking, they noted.

Aside from the risk of cancer, the study also found that middle-aged people who ate lots of animal proteins had an overall increased risk of dying from any cause.

In fact, they were 74% more likely to die of any cause and were also significantly more likely to die as a result of diabetes.

However, the study also found that high protein intake can have a protective effect in people over the age of 65. This is because protein controls IGF-I, a hormone that helps our bodies grow, but has been linked to cancer. Levels of this hormone significantly fall after the age of 65, which can lead to muscle loss and frailty.

"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. 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," the scientists from the University of Southern California said.

They pointed out that the same results were not found in relation to plant-based proteins, such as vegetables and nuts.

For the purpose of this study, a high-protein diet was considered to be one in which at least 20% of calories were derived from protein. A moderate protein diet involved 10-19% of calories from protein, while a low-protein diet contained less than 10% protein.

The scientists noted that during middle age, even a moderate-protein diet increased the risk of death. Those who ate a moderate amount were still three times more likely to die of cancer than those who ate a low-protein diet.

If a middle-aged person managed to reduce their protein consumption from moderate to low levels, they reduced their risk of early death by 21%.

The scientists added that while many people eat too much protein, people should be careful if cutting down.

"It seems that the best change would be to lower the daily intake of all proteins, but especially animal-derived proteins. But don't get extreme in cutting out protein. You can go from protected to malnourished very quickly."

Details of these findings are published in the journal, Cell Metabolism.