New CR Study and IGF-1
A new report on caloric restriction (CR) came out today. You will remember that CR is a method that has been known to extend lifespan in animals for more than 70 years now, while our intermittent fasting (IF) technique has only been studied for the past 20 years or less. Most studies have shown that IF has all the benefits of CR, without the need to restrict calories, so people often assume CR studies are always relevant to IF — but there are recorded differences between the two methods.
First, let’s look at the the new report, called Differences Between People And Animals On Calorie Restriction. This study seems to rely primarily on the measurement of Insulin-Like Growth Factor 1 (IGF-1), which is itself rather surprising since IGF-1 seems to be one of those hormones that are beneficial at certain levels, while more or less are detrimental — yet the exact optimal levels don’t seem to be known yet. Animal studies of IGF-1 in CR animals generally shows a reduction, but when researchers conducting this study looked at real people who voluntarily have undertaken CR diets, their IGF-1 levels were indistinguishable from people on normal American diets. When some of these CR dieters were asked to reduce their protein intake, their IGF-1 levels fell substantially, mimicking the results found in CR animal studies. The above report states that:
In the majority of the animal models of longevity, extended lifespan involves pathways related to a growth factor called IGF-1 (insulin-like growth factor-1), which is produced primarily in the liver. Production is stimulated by growth hormone and can be reduced by fasting or by insensitivity to growth hormone.
The problem is, there is no hard evidence that IGF-1 levels are a major factor in the longevity effect of CR. Certainly, the evidence suggests that the insulin pathways are relevant, but this particular link in the chain has not been shown to be of major significance. In fact, an earlier study: Intermittent fasting dissociates beneficial effects of dietary restriction on glucose metabolism and neuronal resistance to injury from calorie intake — showed that IGF-1 was increased in IF animal experiments, while it was reduced in CR animals. Yet both groups of animals showed the increased longevity that sparked interest in the protocols to begin with.
Another recent study: Hormone May Hold Key To Helping Elderly Men Live Longer showed that, at least for one group (elderly men) increased IGF-1 levels were associated with greater longevity, largely expressed through reduced cardiovascular risk. In that study, where researchers evaluated 376 healthy elderly men between the ages of 73 and 94 years:
Subjects with the lowest IGF-1 function had a significantly higher mortality rate than subjects with the highest IGF-1 bioactivity. These results were especially significant in individuals who have a high risk to die from cardiovascular complications.
The picture is not a simple one, however, because high IGF-1 levels have also been associated with higher risk of cancer. It seems, in short, that IGF-1 can help cells live longer — even if those cells are cancerous. Researchers thought that the lower IGF-1 levels in CR animals was indicative of a lowered cancer risk, but the IF studies suggest something else is at work here.
When animals are given CR diets, they receive their normal diet at reduced levels. With humans trying to emulate these CR benefits, their concern for nutrition leads them to increase the percentage of protein in their diets, and the results in regard to IGF-1 are different than those in the animal studies — but IF dietary results suggest that may not be such a bad thing. More IGF-1, in the absence of cancer, is probably a good thing — within limits.
