Researchers recently discovered that the health impacts of eating protein change as we age. Their study looked at the impact of protein consumption on overall mortality, cancer, and diabetes risk in a nationally representative epidemiological study of 6,381 individuals age 50 and better in the United States.
Overall, the individuals in this study averaged an 1,823-calorie diet, with 51 percent of those calories coming from carbohydrates, 33 percent coming from fat, and 16 percent from protein. A little over two-thirds of that protein came from animal sources. The participants in this study were divided into three groups based on their protein consumption: high (over 20 percent of their diet from protein), moderate (11 to 19 percent of calories from protein), and low (less than 10 percent of calories from protein). The study participants’ average age at the start of this study was 65, and they were followed for 18 years. During the 18 years, 40 percent of the participants died, with 19 percent of all participants dying from cardiovascular disease, 10 percent dying from cancer, and one percent dying from diabetes.
When the researchers examined the groups of participants between 50 and 65 and those who were 66 and older, very different patterns emerged. For participants between 50 and 65, the overall risk of death and of death from cancer were both higher in the high-protein group when compared to the low-protein group. For overall risk of death, there was a 74 percent increase in risk of death and for risk of a cancer-related death, the risk was over 4 times greater for the high-protein group in comparison to the low-protein group. When the impact of animal proteins was controlled for, this association was significantly reduced or eliminated, leading the authors to conclude that “animal proteins are responsible for a significant portion of these relationships.” Individuals in the moderate-protein consumption group also showed an increased risk of cancer death, which was three times higher than in the low-protein group.
Surprisingly, for individuals over 66, the researchers found the opposite outcomes for the relationship between overall mortality and cancer mortality and protein consumption. For this age group, the high-protein consumption group showed a 23 percent reduction in overall mortality, and the moderate-consumption group showed a 21 percent reduction in overall mortality than the low-protein group. This was not affected by the percentage of calories from fat, carbohydrates, or animal proteins. The older group also showed a 60 percent reduction of cancer mortality in the high-protein group compared to the low-protein group.
To look at why different levels of protein consumption are optimal at different ages, the researchers used blood tests and animal models. For middle-aged adults, it appears that high protein consumption leads to an increase in the molecule IGF-1. Higher levels of this molecule in the blood are associated with an increased risk of cancer death. Why this pattern was not observed in older adults is not clear. Other studies have suggested that higher levels of IGF-1 may be beneficial in older adults. It also appears that the benefits of higher protein consumption in older adults may be related to low protein intake, leading to difficulty maintaining a healthy weight and possibly leading to malnourishment. When they tested this on mice, old mice on a low-protein diet for 30 days lost 10 percent of their weight by day 15 on the diet. This was not observed in younger mice on the same diet.
One association of protein consumption and mortality that did not differ with age was the association of protein consumption and diabetes. For all age groups, higher protein consumption was associated with diabetes-related causes of death. However, it should be noted that this was a cause of death for only one percent of the overall study population.
Levine ME, Suarez JA, Brandhorst S, et al. Low protein intake is associated with a major reduction in IGF-1, cancer, and overall mortality in the 65 and younger but not older population. Cell Metabolism (2014); 19(3): 407–417.