In rodents, calorie restriction alone reduces IGF-1 levels,902 but in humans, unless protein consumption is also reduced, even severe caloric restriction doesn’t work. Researchers were only able to get subjects’ IGF-1 levels to budge after the protein intake of calorie-restriction practitioners was cut from typical American quantities down closer to the recommended daily allowance.903 At intakes far exceeding recommended consumption, protein from plants and animals equally raises IGF-1 levels,904 but at more reasonable levels, animal protein appears to be the main culprit.
