Low protein intake in middle aged adults reduces cancer risk
A recent survey confirms that a low protein consumption diet reduces the risk of some diseases, including cancer, especially for middle aged adults.
Researchers believe that this is due to effects on the growth hormone and its receptor that lead to a decrease in insulin and insulin-related growth factor 1 (IGF-1) levels and activity.
The observation that humans and mice carrying mutations that reduce the activity of this pathway are less susceptible to age-related diseases supports this hypothesis.
Humans with growth factor receptor deficiency often suffer from obesity but almost never develop cancer or diabetes, and are no more likely to develop heart disease than their unaffected relatives.
In mice, a specific restriction of protein intake will reduce IGF-1 levels and can increase longevity even if the overall calorie intake is maintained at normal levels.
A group of researchers led by Valter Longo of the University of Southern California, Los Angeles, CA, USA have now explored the links between protein intake, age-related diseases and mortality using a combination of an epidemiological study with experiments in animal models. The epidemiological study involved a sample of 6.381 adults aged 50 and over taken from participants in the NHANES III US dietary survey.
These participants formed a representative sample of US older adults in terms of ethnicity, education and health; the mean age of the sample was 65 years and the mean daily calorific intake was 1,823.
They were divided into three categories according to their protein intake: high (over 20% protein by mass); medium (10-19% protein) and low (less than 10% protein).
During the follow-up period of 18 years, about 40% of the participants died, with 19% of the complete sample dying from cardiovascular disease, 10% from cancer and 1% from diabetes.
Subjects with high and moderate protein consumption were found to be at increased risk of diabetes-related mortality, but not of mortality from all causes.
The participants were stratified into two groups based on age, 50-65 (3,039 subjects) and over 65 (3,342 subjects) and the relative risks re-calculated for these subgroups.
Subjects with a high protein diet in the younger subgroup were found to be at a significantly higher risk than their peers of death from any cause.
The risk of death from cancer in these individuals was over four times that of those in the same age group with a low protein diet (hazard ratio 4.33; 95% confidence interval 1.96-9.56).
These associations were significantly reduced by controlling for the proportion of calories derived from animal protein, suggesting that a high intake of animal protein is largely responsible for the increased mortality.
Middle-aged subjects with a moderate protein intake were also at increased risk of death compared to those with a low intake, although this effect was less significant.
In contrast, however, individuals over 65 in the high protein group had a significantly lower risk of death from all causes and of cancer-related death than those in the low protein group.
Mean levels of IGF-1 were found to correlate positively with protein intake in both age groups, and high IGF-1 levels in the younger age group were associated with a raised cancer risk.
Animal models and diet
The researchers used a mouse model to explore the effects of a range of protein intakes roughly similar to those reported by their human subjects on IGF-1 levels and cancer development.
Male mice of the robust C57BL/6 laboratory strain that had been continuously fed a diet with either 18% or 4%-7% of calories derived from protein were injected subcutaneously with murine melanoma cells.
Mice on the low protein diet developed tumours later and those tumours remained smaller than those in mice on the high protein diet, indicating slower cancer progression in the low protein group.
The mice were tested for levels of IGF-1 and the IGF-1 inhibitory protein IGFBP-1 in their serum; those fed on the low protein diet had significantly lower IGF-1 levels and higher IGFBP-1 levels than those fed on the high protein diet.
Similar results were observed when female mice from the same strain and fed on similar diets were injected with murine breast tumour cells.
However, healthy old (24-month-old) mice fed a low protein diet were observed to lose 10% of their body weight within two weeks, suggesting that the observed detrimental effect of a low protein diet in the elderly human subjects was replicated in mice.
Taken together, these results suggest that a low protein intake in middle age and a higher one in old age might carry the lowest risk of developing several diseases associated with ageing, including cancer.
Source: eCancer News: http://ecancer.org/news/5364-low-protein-intake-in-middle-age-is-associated-with-lowered-cancer-risk.php