Researchers studying a group of vegetarians who'd maintained a diet relatively low in protein and calories found that they had lower blood levels of several hormones and other substances that have been tied to certain cancers.
A comparison group of distance runners also had lower levels of most of these substances compared with sedentary adults who followed a typical American diet -- that is, relatively high in protein from meat and dairy.
However, the low-protein group also had a potential advantage over the runners: lower levels of insulin-like growth factor 1 (IGF-1), a body protein that helps cells grow and multiply. High IGF-1 levels in the blood have been linked to breast, prostate and colon cancers.
The findings are based on a small sample of middle-aged adults, including 21 who'd been vegetarians for at least two years; they were recruited through a local vegetarian society and a magazine on "raw" foods.
They were compared with 21 long-time endurance runners the same age, and 21 sedentary adults who ate a typical American diet.
On average, the vegetarians ate just below the recommended daily amount of protein -- 0.8 grams per kilogram of body weight. Both the runners and the sedentary group ate significantly more than the recommended amount.
Fontana's team found that, compared with their sedentary counterparts, the runners and vegetarians had lower levels of several hormones and inflammatory proteins linked to cancer risk.
When it came to IGF-1, specifically, the low-protein group had lower levels than runners did, even though they were equally lean -- suggesting an effect of diet and not just body weight, according to the researchers.
In addition, IGF-1 levels in the sedentary group generally rose in tandem with their protein intake.
"Many people are eating too many animal products," Fontana said, as well as too many processed foods and sugars.
He advised that people try to eat more fruits and vegetables, fiber-rich whole grains, beans and fish, and less red meat. Doing so could bring the amount of calories and protein the average American eats closer to recommended levels -- and possibly lower IGF-I levels, according to Fontana.
"We hope to further clarify what happens to cancer risk when we are chronically eating more protein than we need," he said.
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