Caloric restriction (CR) has long been known to increase longevity (often dramatically) in a number of eukaryotic models, including yeast, fruit flies, nematodes, mice, and rhesus monkeys. It's also well established that protein restriction can reduce cancer incidence and/or increase longevity in many organisms, independently of calorie intake.
Recent research by a multinational team (including scientists from Ecuador, Italy, and the United States) now indicates that restricting protein intake may be sufficient to reduce cancer in humans substantially. A new study in Cell Metabolism (4 March 2014), drawing on epidemiological data involving 6,381 U.S. men and women aged 50 and above (from the NHANES III database), has found that persons aged 50–65 reporting high protein intake had a 75% increase in overall mortality and a four-fold increase in cancer death risk during an 18-year period. (However, high protein intake was associated with reduced cancer and overall mortality in respondents older than 65.) A five-fold increase in diabetes mortality, across all age groups, was also noted for those reporting high protein intake.
According to the authors of the study:
None of these associations was significantly affected by controlling for percent calories from total fat or for percent calories from total carbohydrates. However, when the percent calories from animal protein was controlled for, the association between total protein and all-cause or cancer mortality was eliminated or significantly reduced, respectively, suggesting animal proteins are responsible for a significant portion of these relationships. When we controlled for the effect of plant-based protein, there was no change in the association between protein intake and mortality, indicating that high levels of animal proteins promote mortality and not that plant-based proteins have a protective effect.
The authors conclude (among other things) that "a diet in which plant-based nutrients represent the majority of the food intake is likely to maximize health benefits in all age groups." They suggest that a daily adult protein intake of 0.7 to 0.8 g per kilogram of body weight, currently considered a minimum acceptable amount, should in fact be considered the proper target amount—all of it as plant protein, not animal protein.
To see the complete discussion of the study's findings (non-paywalled), go here.