IGF-1 or Insulin Growth Factor-1 is a growth hormone. Its levels in the bloodstream increase naturally when we are kids, because we need to grow. Its levels naturally decrease when we are full grown adults, as we don’t have the need to grow any longer.
Regardless of our age or growth phase, when we consume more protein than we need, our body needs to do something with all that excess. So the liver starts producing IGF-1 which tells the cells in our body to start growing and multiplying (1). The problem reside in some of the new cell additions may be tumours. Therefore, when we are adults, cell growth is something we want to slow down, and not accelerate. So it would be reasonable to maintain an adequate, but not excessive, overall protein intake (2).
If our levels of IGF-1 stay high as adults, there is a constant message to our cells to grow and divide, and keep growing. Therefore, it’s not a surprise that the more IGF-1 we have in our bloodstream, the higher our risk for cancer. More IGF-1, more breast cancer, more IGF-1, more prostate cancer and so on (3). IGF-1 helps cancer cells break off from the main tumour, migrate into surrounding tissues, and invade the bloodstream (metastases). It helps transform normal cells into cancer cells in the first place, then helps them survive, proliferate, self-renew, grow, migrate, invade, stabilise into new tumours, and even helps hook the blood supply to the new tumours. So, too much IGF-1 when we are all grown up, could mean cancer (4).
In 2002, a research study comparing the IGF-1 levels of meat eaters, vegetarians and vegans showed that those eating plant proteins had significantly lower levels of IGF-1 compared to meat eaters, who had the highest levels of all (5). One of the largest prospective studies on diet and cancer found that the incidence in all cancer growth combined was lower among those eating more plant-based. The reason to this may be that eating less animal protein; less meat, eggs, dairy… end up with less IGF-1, which means less cancer growth (6).
Here’s the experiment that really nailed IGF-1 as the villain. On a plant-based diet, cancer cell growth drops and cancer cell death shoots up. But then, when adding back to the cancer the IGF-1 that banished from the body because of the healthier eating, cancer cells start to grow back again. It erases the diet effect. It’s like one never started to eat healthy at all (7).
In 2014, a study of middle aged men and women (50 to 65) showed that high protein animal intakes, but no plant protein, had a 75% increase in overall mortality, and four times increase in the risk of dying specifically from cancer. Which makes sense given the higher IGF-1 levels (8). The University of Southern California sent out a press release after the study with the following opening line; “That chicken wing you are eating could be as deadly as a cigarette”, explaining that a diet rich in animal proteins during middle age (50 to 65) makes you four times more likely to die from cancer. A mortality rate comparable to smoking cigarettes (9).