Having low iron means your body doesn’t have enough of a mineral it relies on to carry oxygen, produce energy, and support muscle function. Iron is central to making hemoglobin, the protein in red blood cells that delivers oxygen to every organ and tissue. About two-thirds of your body’s iron is tied up in hemoglobin alone. When iron runs low, this system starts to falter, and the effects can range from mild fatigue to significant impairment of daily life.
Low iron exists on a spectrum. You can have depleted iron stores without being anemic, or you can progress to full iron deficiency anemia, where your red blood cells become too few and too small to supply your body with adequate oxygen. Understanding where you fall on that spectrum, and why, matters for figuring out what to do about it.
How Iron Works in Your Body
Iron does three major jobs. First, it’s the core component of hemoglobin, which picks up oxygen in your lungs and drops it off wherever your body needs it. Second, it’s built into myoglobin, a related protein in your muscles that stores oxygen locally so your muscles can access it during activity. Third, iron is essential for producing energy at the cellular level. Inside your cells’ mitochondria, iron-containing enzymes drive the chemical reactions that create ATP, your body’s primary energy currency.
When iron is low, all three systems suffer. Your blood carries less oxygen per trip, your muscles can’t buffer their own oxygen supply as well, and your cells literally produce less energy. That’s why the fatigue from iron deficiency feels so pervasive. It’s not just tiredness from poor sleep or overwork. It’s a bottleneck in the basic chemistry that powers your body.
The Stages of Iron Deficiency
Low iron doesn’t happen all at once. It progresses through stages, and blood tests can reveal where you are.
In the earliest stage, your iron stores drop but your body still manages to make enough hemoglobin. A blood test would show low ferritin (the protein that stores iron) while your hemoglobin stays normal. You might feel fine, or you might notice subtle dips in energy and exercise tolerance. Ferritin below 30 ng/mL is a strong indicator of depleted iron stores, catching about 92% of cases while still being highly accurate. The WHO uses a similar threshold: below 30 in children and below 70 in adults who have any ongoing infection or inflammation, since inflammation artificially inflates ferritin levels.
In the next stage, your body can no longer compensate. Hemoglobin production drops, red blood cells become smaller, and you develop iron deficiency anemia. For adult women, anemia is typically defined as hemoglobin below 12.2 g/dL. For adult men, it’s below about 13.7 g/dL in younger adults and 13.2 g/dL after age 60. These thresholds vary by age, sex, and pregnancy status. In pregnancy, hemoglobin below 11 g/dL in the first or third trimester (or below 10.5 g/dL in the second trimester) signals anemia.
What Low Iron Feels Like
The most common symptom is fatigue that doesn’t improve with rest. Because your tissues receive less oxygen and your cells produce less energy, you may feel winded doing things that used to be easy, like climbing stairs or walking briskly. Dizziness, headaches, and cold hands and feet are also typical, all reflecting reduced oxygen delivery.
Some symptoms are less obvious. Restless legs syndrome, that irresistible urge to move your legs accompanied by tingling, crawling, or aching sensations, has a strong connection to low iron. Interestingly, the link isn’t just about iron levels in your blood. Restless legs may be driven by low iron levels in the brain specifically, which is why some people develop it even when their blood work looks borderline rather than severely low.
Pica, an unusual craving for non-food items like ice, dirt, or starch, is another hallmark of iron deficiency. Craving and compulsively chewing ice (called pagophagia) is especially common. Brittle or spoon-shaped nails, where the nail curves inward like a small scoop, can develop in more advanced deficiency. Pale skin, a sore or swollen tongue, and frequent infections round out the picture, since iron also plays a role in immune function.
Cognitive effects are real too. Difficulty concentrating, brain fog, and irritability often accompany low iron, particularly in children and adolescents where it can affect learning and development.
Common Causes of Low Iron
There are really only three ways to end up low on iron: you’re losing it, you’re not absorbing it, or you’re not getting enough from food.
Blood loss is the most common cause in adults. For people who menstruate, heavy periods are the leading driver. Any source of slow, chronic bleeding can also deplete iron over time, including peptic ulcers, colon polyps, colorectal cancer, hiatal hernias, and regular use of over-the-counter pain relievers like aspirin or ibuprofen that can cause subtle GI bleeding.
Poor absorption matters even if your diet is adequate. Conditions like celiac disease, inflammatory bowel disease, and prior gastric surgery can reduce how much iron your gut actually pulls from food. This is also where the type of iron you eat becomes important (more on that below).
Insufficient intake is particularly relevant for vegetarians, vegans, and people with restrictive diets. Plant-based diets contain only non-heme iron, which the body absorbs less efficiently than heme iron from animal sources. The NIH estimates that vegetarians need 1.8 times more dietary iron than meat eaters to compensate for this difference.
Increased demand is a fourth factor. Pregnancy dramatically raises iron needs because blood volume increases and the developing baby requires its own iron supply. Without supplementation, many pregnant people develop iron deficiency anemia. Rapid growth in infants and children creates similar demand.
Who Needs the Most Iron
Daily iron needs vary widely by age and sex. Adult men and women over 51 need about 8 mg per day. Women between 19 and 50 need 18 mg, more than double, primarily to offset menstrual losses. Pregnant women need 27 mg per day, the highest requirement of any group. Teenage girls need 15 mg daily, and teenage boys need 11 mg.
Athletes, especially endurance athletes, often need more iron than these baseline recommendations suggest. Intense training increases iron loss through sweat, GI micro-bleeding, and a phenomenon called foot-strike hemolysis, where red blood cells are physically damaged by repetitive impact. People with chronic inflammatory conditions also face higher effective requirements because inflammation interferes with iron metabolism, trapping iron in storage and keeping it out of circulation.
Getting More Iron From Food
Iron from food comes in two forms. Heme iron, found in meat, poultry, and seafood, is absorbed significantly better than non-heme iron, which comes from plant foods like beans, lentils, spinach, and fortified cereals. Most people get a mix of both, but the ratio matters.
You can boost absorption of non-heme iron by pairing it with vitamin C at the same meal. Adding tomatoes to a lentil soup, squeezing lemon over sautéed greens, or eating strawberries alongside fortified oatmeal all help your gut absorb more iron from those plant foods. Eating heme iron (even a small amount of meat) alongside non-heme sources also improves absorption of the plant-based iron.
Certain substances work against you. Bran fiber, phytates (found in whole grains and legumes), tannins (in tea and coffee), and large amounts of calcium, particularly from supplements, all inhibit non-heme iron absorption. This doesn’t mean you should avoid these foods entirely. Timing matters more than elimination. If you’re trying to maximize iron intake, avoid drinking tea or coffee with your iron-rich meals and take calcium supplements at a different time of day.
What Happens if Low Iron Goes Untreated
Mild iron depletion is common and often correctable with dietary changes. But when iron deficiency progresses to anemia and stays there, the consequences compound. Your heart works harder to pump oxygen-poor blood, which can lead to a rapid or irregular heartbeat and, over time, an enlarged heart. Exercise capacity drops substantially. In pregnancy, severe iron deficiency increases the risk of preterm birth and low birth weight. In children, prolonged deficiency can impair cognitive development in ways that may not fully reverse even after iron levels are restored.
Perhaps the most important reason not to ignore low iron: in adults over 50, new iron deficiency anemia can be a sign of hidden bleeding in the GI tract, which sometimes points to colon polyps or colorectal cancer. The iron deficiency itself may be the first clue to a more serious underlying condition, making it worth investigating rather than simply supplementing without understanding the cause.