Low-Protein, Low-Calorie Intake May Lower Cancer Risk

This study:
Long-Term Low-Protein, Low-Calorie Diet And Endurance Exercise Modulate Metabolic Factors Associated With Cancer Risk, American Journal of Clinical Nutrition, 2006

Looked at 3 groups of middle-aged (53 +/- 11 years old), nonsmoking, healthy men and women:

• 21 sedentary subjects, low-protein (about 9% of calories), low-calorie (about 2000) diet*
• 21 endurance runners
• 21 sedentary subjects (non-obese), Western diet

And found:

• Levels of insulin, free sex hormones, leptin (released from body fat), and C-reactive protein (marker for inflammation) were lower and sex hormone–binding globulin (protective against cancer) was higher in the low-protein, low-calorie diet and runner groups than in the sedentary Western diet group.
• Levels of insulin-like growth factor I (IGF-I) was lower in the low-protein, low-calorie diet group.

So, even though the low-protein group was sedentary, they had lower levels of growth factors and anabolic hormones that are linked to an increased risk of cancer, plus they had the lowest levels of of IGF-I, also linked to cancer. (IGF-I "promotes tumor development by stimulating cell proliferation and inhibiting cell death." IGF-I is linked, in humans, to a greater risk of breast, prostate, and colon cancers.):

"Subjects eating a low-protein, low-calorie diet had much lower plasma IGF-I concentrations and IGF-I:IGFBP-3 than did BMI-matched endurance runners, which suggests that dietary factors may provide additional protective effects, independent of body fat mass."

Perhaps this is why Campbell could turn cancer on and off in rats by changing the amount of protein they ate (Liver Cancer: Now You See It, Now You Don't).
________* They ate uncooked, unprocessed, plant-based foods, "vegetables, fruit, nuts, seeds, sprouted grains and cereals, and olive oil and strictly avoided processed and refined foods (eg, partially hydrogenated oils, refined flours, sweets, free sugars, and soft-drinks) and foods of animal origin." Lots of fat, about 42% of calories, although most of it was unsaturated. They ate the least saturated fat. They ate an ungodly amount of fiber, the women about 40 grams, men about 60 grams a day.