Wednesday March 5, 2014 (foodconsumer.org) -- A new study published March 4 in Cell Metabolism suggests that eating a high protein diet such as Atkins’s diet and Paleo diet during middle age can increase the risk of dying of cancer.

High protein diet is popular in the U.S. and many men and women use it to lose weight.  Americans have average intake of 15% or more of total calories from proteins while most people only need no more than 11%.

The study led by researchers from University of Southern California shows that men and women following a high-protein diet during middle age were nearly twice as likely to die and about four times more likely to diet of cancer.

High intake of protein has been known to be a risk factor for cancer.   Dr. T Colin Campbell, a well-known nutrition professor at Cornell University conducted a study and reported in his book titled China Study that people in China who had low intake of protein were found much less likely to develop cancer and other diseases.

People who ate high amounts of proteins from animal sources such as milk, cheese and meat were not only at high risk for cancer death, but also more likely to die prematurely, compared with those who ate less.

The current study finds men and women who ate lots of protein were 74% more likely to die of all-causes during the study period and many times more likely to die of diabetes, compared with those who had low intake of protein.

Proteins at least those from animal sources control IGF-I  (Insulin-like growth factor - I), which has been linked to  increased risk of cancer and diabetes in many early studies.   However, the current study suggests people older than 65 may benefit from having moderate intake of protein, which made them less susceptible to diseases.

Researchers also discovered that plant-based proteins do not seem to have disease promoting effects as animal proteins do, which has been observed by Dr. Campbell and colleagues decades ago.

Additionally, the study revealed both carbohydrates and fats do not affect the rates of cancer and death suggesting that animal protein is the sole culprit for the increased risk for cancer and other diseases.

Some health agencies recommend consumption of 0.8 grams of protein per kilogram of body weight per day in middle age.  That is, for a person with 130 pounds of weight, he should eat only 45 to 50 grams of protein per day, which is about 200 calories.  And the researchers suggest in a press release that preference should be given to plant proteins.

In the study, the researchers defined high protein diet as having at least 20% of total calories from proteins including plant-derived and animal derived proteins.  And a moderate protein diet should have 10 to 19% calories from proteins and a low protein diet has fewer than 10% calories from proteins.

A low protein diet is not recommended as it can cause malnutrition.  And a moderate protein diet can increase risk of cancer and other diseases compared with a low protein diet.  The difference in the risk in this case can be as much as 21%.

A high protein diet was linked to a high serum level of IGF-I and increased risk of cancer death, compared with a low protein diet.

For those who have already developed cancer, a low protein diet can help control tumor growth, according to an animal study. (David Liu)