You may have an iron deficiency and not even know it.

A large proportion of adults in the United States – nearly 1 in 3 – may have one of two forms of iron deficiency, even if they don’t have a health condition for which they would be screened for it, such as anemia, heart failure or chronic kidney disease, a new study suggests.

The study, published Tuesday in the medical journal JAMA Network Open, warns that iron deficiency may be a “widespread” and “underrecognized” public health problem in the United States.

An estimated 14% of adults in the study met the criteria for absolute iron deficiency, which results from a severe reduction or absence of iron stored in the body, and an estimated 15% of the adults met the criteria for functional iron deficiency, which happens when there are adequate iron stores but they aren’t mobilized enough to sufficiently support the body.

“I think it’s important to consider the distinction between absolute and functional iron deficiency. Functional iron deficiency was fairly common across all ages and sexes. Conditions like obesity, diabetes, and kidney disease, which have become very common in the US, can cause functional iron deficiency,” Dr. Leo Buckley, lead author of the study and a clinical pharmacy specialist in the Department of Pharmacy at Brigham and Women’s Hospital, said in an email.

Absolute iron deficiency can be treated with iron supplements, and functional iron deficiency is managed by treating the underlying conditions leading to it, Buckley said, adding that the long-term consequences of functional iron deficiency remain under investigation.

“The prevalence of absolute iron deficiency was highest in pre-menopausal women, which is consistent with prior studies. Absolute iron deficiency was also common among older women and men, a group that may be more susceptible to the effects of iron deficiency,” he said. “In older adults, unknown blood loss through the gastrointestinal tract and inadequate intake may be the causes. Older adults also have many chronic conditions that can interfere with iron absorption.”

Iron is a mineral essential for maintaining many of the body’s functions and development, and the body uses iron to make a protein in red blood cells called hemoglobin. Iron deficiency has been tied to restless leg syndrome, fatigue, hair loss, heart failure and other adverse outcomes, including anemia, in which the blood doesn’t have enough healthy red blood cells and hemoglobin to carry oxygen to the body’s tissues.

Certain groups of people are more prone to iron deficiency: women who menstruate, pregnant people, children, vegetarians and frequent blood donors. A 2022 study estimated that more than 70% of iron deficiency may go undiagnosed among children and pregnant women.

Iron deficiency can be prevented or treated by using iron pills, intravenous iron infusions or nonprescription iron supplements or by including iron-rich foods in the diet, such as lean meats, seafood, nuts, beans, lentils and spinach – but the new study suggests that many people may not even know that they are deficient in iron.

The researchers – from Brigham and Women’s Hospital in Boston and other institutions – analyzed data on a nationally representative sample of 8,021 adults in the United States. The data came from the National Health and Nutritional Examination Survey, and it included blood test assessments on each person’s iron levels, from 2017 to 2020.

Even among adults without conditions that may be associated with iron deficiency – anemia, heart failure, chronic kidney disease or pregnancy – the estimated prevalence of absolute iron deficiency was 11% and that of functional iron deficiency was 15%.

Only about 33% of the adults with absolute iron deficiency and 14% of adults with functional iron deficiency had a potential medical reason to screen for iron deficiency, such as anemia, heart failure, chronic kidney disease or pregnancy. That suggests that most adults with either type of condition probably would not ever be screened for it unless they specifically asked their doctor.

Additionally, the researchers found that the use of iron supplements was infrequent among the adults with iron deficiency: Only up to 35% of women and up to 18% of men with the condition reported taking them.

“Many of us in medicine suspect that iron deficiency is more common than is currently appreciated, and this study may add evidence to support that,” Dr. Jacob Cogan, assistant professor of medicine at the University of Minnesota, said in an email.

“However, I do wonder whether this study suffers from selection bias, in that only patients in whom there was concern for iron deficiency may have been tested,” said Cogan, who was not involved with the new study. “So these estimates are only reflective of the population in which there was initial concern, rather than reflecting the population as a whole.”

Cogan’s own research, published last month, found that 58% of people diagnosed with iron deficiency had no resolution in their condition three years later, meaning they still did not have normal iron levels.

It’s unclear exactly how prevalent iron deficiency truly is, he says, but he thinks it’s more common than people realize.

“People who menstruate lose iron to some extent with every menstrual cycle, and we do not absorb iron very well in our diet, so it is relatively easy to become iron deficient. Since we do not currently screen for iron deficiency - and I am not necessarily saying that we should, as we do not have enough data yet to support anything like that - many people are likely walking around deficient without realizing it, since the symptoms can be subtle,” Cogan said.

“Iron deficiency is typically associated only with anemia. However, we know that iron deficiency can cause symptoms before a patient becomes anemic, as anemia is a late-stage manifestation of iron deficiency,” he said. “But if a patient gets basic blood testing and is found to not be anemic, providers may not look further than that,” such as by ordering iron tests.

The new findings reflect what has been seen on the ground in the medical community: Iron deficiency can be something that’s underlying and may be going under the radar, said Dr. Theodore Strange, the chairman of medicine at Northwell’s Staten Island University Hospital, who was not involved in the new study.

But for most people, routine checkups or physicals at a doctor’s office will include testing for a complete blood count, and that measurement can indicate whether someone may be iron deficient and could benefit from further testing of their iron levels, he said.

Someone also can always ask their doctor to test their iron levels if they have any concerns, and “it’s important to eat a healthy diet, taking in your fruits and vegetables. Vegetables, especially, have a lot of iron stores, as does red meat,” Strange said. “And if there’s any thought that you have excess bleeding … we should be looking for those sources of bleeding and determine then if there may potentially be the risk of iron deficiency.”