You’re dragging. By 2 p.m., the office lights feel too bright, your legs feel like lead, and that third cup of coffee isn’t doing a thing. Most people just shrug and blame "getting older" or a bad night’s sleep. But often, the culprit is much more microscopic. When we talk about what are the causes of low iron, we aren't just talking about not eating enough spinach. It’s actually way more complicated than that.
Iron is the backbone of your hemoglobin. Think of hemoglobin as the taxi driver in your blood that ferries oxygen from your lungs to your brain, muscles, and heart. When you run low, your organs are basically gasping for air.
Honestly, it’s a global problem. According to the World Health Organization (WHO), iron deficiency is the most common nutritional disorder on the planet. But the "why" behind it varies wildly depending on who you are, what you eat, and how your gut functions.
The Math Problem: Why You’re Losing More Than You’re Making
At its simplest level, low iron is a budget deficit. Your body has a specific daily requirement, and if the "withdrawals" exceed the "deposits," you end up in the red.
Blood loss is the most frequent thief. This isn't always a dramatic, ER-visit kind of bleeding. It’s often subtle. For women of childbearing age, heavy menstrual periods (menorrhagia) are the leading cause of iron deficiency. If you’re changing a pad or tampon every hour, your body simply cannot keep up with the iron replacement. It's a constant uphill battle.
But what if you aren't menstruating?
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Internal bleeding is the sneaky version. We’re talking about slow leaks in the gastrointestinal tract. A tiny, oozing peptic ulcer or a polyp in the colon can drip blood for months without you ever seeing a red drop in the toilet. This is why doctors get worried when a man or a postmenopausal woman shows up with low iron—it’s often a red flag for something happening in the gut. Even the regular use of over-the-counter pain relievers like aspirin or ibuprofen (NSAIDs) can irritate the stomach lining enough to cause chronic, microscopic bleeding.
What Are the Causes of Low Iron in Your Diet?
Diet is the obvious one, but it’s rarely just about "not eating meat."
There are two types of iron: heme and non-heme. Heme iron comes from animal sources like red meat, poultry, and fish. Your body loves this stuff; it absorbs it easily. Non-heme iron comes from plants—beans, lentils, spinach, and fortified grains. Here’s the kicker: your body is actually pretty bad at absorbing non-heme iron.
If you’re a vegan or vegetarian, you have to eat significantly more iron-rich foods than a meat-eater to get the same net gain.
The Blockers You Didn't Know About
You might be eating a massive bowl of fortified cereal or a steak, but if you’re washing it down with a huge cup of black tea or coffee, you’re basically sabotaging yourself. Tannins in tea and polyphenols in coffee bind to iron and prevent it from crossing the intestinal wall.
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Calcium is another frenemy. If you take a high-dose calcium supplement right alongside your iron-rich meal, the calcium competes for the same pathways. The iron usually loses that fight.
When Your Gut Just Says No
Sometimes the food is there, but the body can't grab it. This is the "malabsorption" category. Most iron absorption happens in the duodenum, the first part of your small intestine. If that area is damaged or bypassed, you’re in trouble.
Celiac disease is a classic example. When someone with celiac eats gluten, their immune system attacks the lining of the small intestine. The villi—those tiny, finger-like projections that soak up nutrients—flatten out and stop working. Frequently, unexplained low iron is the only symptom of silent celiac disease.
Other culprits include:
- Gastric bypass surgery: When surgeons bypass the duodenum to help with weight loss, they also bypass the primary "loading dock" for iron.
- Inflammatory Bowel Disease (IBD): Crohn’s disease or ulcerative colitis causes inflammation that prevents nutrient uptake and often adds the complication of bleeding.
- Autoimmune Gastritis: This is where the body attacks the cells in the stomach that produce acid. You need stomach acid to break down iron into a form your intestines can actually use.
The Pregnancy Drain
Pregnancy is an iron-demanding marathon. Your blood volume increases by nearly 50% when you're growing a human. You aren't just fueling your own organs anymore; you’re building a placenta and an entire fetal blood supply.
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The National Institutes of Health (NIH) notes that pregnant women need about 27 milligrams of iron per day, compared to the 18 milligrams required for non-pregnant women. If you start the pregnancy with low stores—which many women do—you’ll likely hit a wall by the second trimester. This isn't just about feeling tired; severe deficiency can lead to premature birth or low birth weight. It's a high-stakes scenario.
Genetics and the Rare Stuff
Sometimes, it’s just in the blueprints. There are rare genetic conditions like IRIDA (Iron-Refractory Iron Deficiency Anemia). People with this condition produce too much hepcidin.
What's hepcidin? It's the master hormone that regulates iron. When hepcidin is high, it "locks" the iron doors in your gut. No matter how many iron pills you swallow, the body refuses to let them in. These cases are frustrating because standard treatments fail, and patients usually require intravenous (IV) iron infusions to bypass the gut entirely.
Surprising Culprits: Intense Exercise and "Foot Strike"
If you're a long-distance runner, you might be literally crushing your red blood cells. It's called "foot strike hemolysis." Every time your foot hits the pavement with force, the impact can rupture small blood vessels in the soles of your feet, destroying red blood cells.
Also, athletes lose a small amount of iron through sweat. Over the course of a 20-mile training week, that adds up. Combine that with the inflammation caused by heavy training—which raises hepcidin levels—and you have a recipe for "sports anemia."
How to Actually Fix It
If you suspect your levels are tanking, don't just grab a bottle of "Iron 65mg" from the grocery store. Self-treating is risky. Too much iron is toxic and can damage your liver or heart (a condition called hemochromatosis).
- Get the "Full Monty" of Blood Work. Don't just check Hemoglobin. You need a Ferritin test. Ferritin is your storage tank. Your hemoglobin can look "normal" while your ferritin is sitting at a 5, meaning you’re running on fumes and about to crash. Also, ask for Total Iron Binding Capacity (TIBC) and Transferrin Saturation.
- Pair Iron with Vitamin C. If you're eating plant-based iron or taking a supplement, have it with a glass of orange juice or some bell peppers. Vitamin C changes the pH in your gut to make iron much more soluble.
- Time Your Supplements. Take iron on an empty stomach if you can tolerate it. If it makes you nauseous (which it often does), take it with a small amount of food, but avoid dairy, eggs, and caffeine for at least two hours before and after.
- Consider Heme Iron Supplements. If traditional ferrous sulfate (the green pills) wreaks havoc on your digestion, look into heme iron polypeptides. They are derived from bovine sources and are much gentler on the stomach because they use a different absorption pathway.
- Investigate the "Why." This is the most important step. If your iron is low, your doctor must find out why. Is it your diet? Is it your period? Or is there a silent ulcer? Taking a pill without finding the leak is like trying to fill a bathtub with the drain open.
Iron deficiency develops in stages. First, your stores drop (low ferritin). Then, your red blood cells start getting smaller and paler (microcytic anemia). You don't want to wait until you're breathless walking up a flight of stairs to take action. Listen to the fatigue. It's usually trying to tell you something specific about your blood chemistry.