Researchers estimate that up to 10 percent of Americans
have iron stores high enough that, when coupled with excessive
iron intake, could increase their likelihood of developing
cancer.
The study, involving a nationally representative sample of
U.S. adults, looked at a measure called transferrin saturation.
Transferrin is a protein that transports iron through the body,
and transferrin saturation levels in the blood are an indicator
of the body's iron stores.
Research has linked excessively high transferrin saturation
to an elevated risk of death from any cause, and possibly to an
increased cancer risk. In general, the evidence has suggested
that transferrin saturation levels of 60 percent or higher --
found in about 1 percent of the U.S. population -- may present
a cancer risk.
However, the new findings implicate lower levels of
transferrin saturation -- 41 percent or higher -- that are seen
in about 10 percent of Americans.
The problem appears to arise when these individuals consume
higher-than-recommended levels of iron, which is found in foods
like red meat, liver and fortified cereals.
"It's the combination that's the key," said lead study
author Dr. Arch G. Mainous of the Medical University of South
Carolina in Charleston.
In contrast, he told Reuters Health, men and women with
similarly elevated transferrin saturation levels but normal
iron intakes did not show an increased cancer risk.
The findings are published in the current issue of the
Annals of Family Medicine.
High transferrin saturation is seen in hemochromatosis, an
inherited disorder in which the body absorbs and stores more
dietary iron than it needs. Without treatment, this iron
buildup can damage organs over time and eventually cause them
to fail.
Hemochromatosis affects mainly people of Northern European
descent, and in the U.S., about one in 200 Caucasians are at
high risk of developing the disorder because they carry two
copies of the flawed gene underlying the disorder.
"We're talking about a lot more people," Mainous said,
referring to the individuals whose transferrin saturation
levels may raise their cancer risk if they consume too much
iron.
Mainous and his colleagues based their findings on data
from a national study that followed 6,300 adults ages 25 to 74
over a two-decade span. At the outset, about 10 percent had
transferrin saturation levels of 41 percent or higher, and when
these individuals consumed more than 18 milligrams of iron a
day, their risk of developing cancer over the study period was
twice that of men and women with normal iron stores and lower
iron intake.
Eighteen milligrams (mg) is the recommended daily intake of
iron for premenopausal women; men and older women are advised
to get 8 mg per day.
Most people with elevated transferrin saturation won't know
it unless they get the blood test for it -- similar, Mainous
said, to the case with high cholesterol. But that doesn't mean
transferrin saturation should be tested routinely, as
cholesterol is, according to the researcher.
Instead, he said, this study suggests that people should be
cautious about substantially boosting their iron intake-whether
by taking supplements or going on an "extreme diet" that's
heavy in red meat, for example. In these cases, Mainous said, a
person may do well to get a transferrin saturation test before
deviating from a normal iron intake.
Exactly how iron overload may lead to cancer is unclear,
but Mainous said his theory is that the excess mineral
contributes to oxidative stress -- damage to cells that is
caused over time by natural byproducts of metabolism known as
reactive oxygen species. Oxidative damage is thought to play a
role in a number of diseases, including cancer.
