“In patients with hemochromatosis, we can clearly show that excessive iron kills the beta cells that make insulin in the pancreas,” says Donald McClain of the University of Utah School of Medicine.
The same could be happening in people who have large stores of iron in their bodies but who don’t have hemochromatosis.
“Very large studies across several different populations and ethnicities have shown that people with high levels of ferritin have a two to seven times greater risk of developing type 2 diabetes than those with low ferritin levels,” says McClain. Ferritin levels reflect the amount of iron stored in the body. (Routine blood screening doesn’t include a ferritin test, which typically costs $50 to $100 if ordered separately.)
In the Nurses’ Health Study, which tracked more than 32,000 women for 10 years, those with the highest ferritin levels were nearly three times more likely to be diagnosed with diabetes than those with the lowest levels. And in England, men and women with the highest ferritin levels were seven times more likely to have diabetes.
“But we don’t know for certain which is the cart and which is the horse, since ferritin also rises with inflammation,” cautions McClain. “And diabetes usually involves inflammation of some tissues.”
Still, there are good reasons to think that excessive iron is a cause, rather than a consequence, of diabetes.
“If you take rats and mice that have been bred to be prone to diabetes, you can lower their risk dramatically by decreasing the amount of iron in their bodies,” McClain notes.
What’s more, several studies in Europe found that when people with high-normal iron stores donated blood regularly, their insulin sensitivity improved. (People who are insulin insensitive—or resistant—cannot easily transfer sugar from the bloodstream into cells. Insulin resistance often leads to diabetes.)
McClain is leading a similar study in Utah, sending volunteers with high levels of ferritin (but not hemochromatosis) to the Red Cross to donate blood three to five times over the course of several months to lower their ferritin. Results aren’t in yet.
If too much iron can cause diabetes, the easiest solution, says McClain, is to cut back on beef and lamb, which are the major sources of heme iron in the diet. Poultry, pork, and fish also contain heme iron, but much less than beef.
Heme is the form of iron in the hemoglobin of red blood cells and in the myoglobin of muscle cells. It’s much more easily absorbed than the non-heme iron that’s in plant foods, supplements, and fortified foods. And it’s heme iron in the diet that’s linked to a higher risk of diabetes.
In the Nurses’ Health Study, for example, women who consumed the most heme iron had a 28 percent greater risk of being diagnosed with diabetes than women who consumed the least. And participants in the Health Professionals Follow-Up Study, which tracked more than 38,000 men for 12 years, had a 63 percent greater risk if they consumed the most heme iron. In both studies, non-heme iron in the diet wasn’t linked to a higher risk of diabetes.
“Clearly, consuming large amounts of heme iron is a risk factor for type 2 diabetes,” concludes Harvard University’s Frank Hu. Even so, he adds, “weight control remains the most important way to lower that risk.”
As for donating blood to remove iron, “we’re not at the point yet where we can recommend that people do that to avoid developing diabetes,” says McClain.
“But I wouldn’t feel bad about suggesting that it’s a nice thing to donate a unit of blood occasionally.”
Sources: JAMA 291: 711, 2004. Diabetologia 50: 949, 2007. Am. J. Physiol. Endocrinol. Metab. 298: E1236, 2010. Diabetes 51: 1000, 2002. Diabetes Care 29: 1370, 2006. Am. J. Clin. Nutr. 79: 70, 2004.