I feel like I went from barely functioning to unstoppable overnight.

The first trimester of pregnancy—and about half of the second—wrecked me. Exhaustion, nausea, full-on survival mode. Then, suddenly, something shifted.

I’m sleeping terribly. My brain won’t shut off. And somehow, I have more energy than ever.

By the second trimester, the placenta is fully developed, easing the body’s workload. Blood volume has surged, delivering more oxygen and nutrients. Hormones have leveled out, lifting the fog of crushing fatigue.

In short, I felt amazing. I had my energy back. I was getting things done. Everything was clicking into place.

So you can imagine my shock when my blood test results came back: I’m anemic.

Wait, what?

My first reaction: frustration, discouragement, and near disbelief—how could this be true? How could I feel this good when my bloodwork is telling me the opposite? It felt like my own biology was gaslighting me.

Honestly, I still don’t have a perfect explanation. My best guess? It was so bad before that the contrast is making me feel like a million bucks.

That led me to the real question:

👉 If I feel fine—great, even—does that mean I actually am fine?

Many women walk around with subclinical iron deficiency (SID)—low iron levels that don’t quite tip into anemia, but still affect brain function, immune response, and cardiovascular health in ways they don’t notice.

Today, I’m breaking down a critical distinction: Asymptomatic vs. symptomatic iron deficiency, and what it means for your energy, health, and long-term well-being.

Why Even “Hidden” Iron Deficiency Matters

Iron deficiency exists on a spectrum.¹

Many women with low iron stores may not meet the clinical threshold for anemia (i.e., very bad iron deficiency), but their bodies are already compensating for the lack of iron.

This means that even though you don’t feel sick, your body is working harder behind the scenes, and subtle effects may be taking a toll.

🧠 Cognitive Function: The Invisible Brain Fog

Women with low iron but no anemia²  performed worse on memory tests and had slower cognitive processing speeds than those with normal iron, even though they felt normal.

Research suggests iron levels directly affect brain performance. The more severe the deficiency, the longer it takes to process information and complete tasks.

The effects of low iron on cognition exist on a spectrum:

🔍 Asymptomatic Impact: Subtle but Present

• Small Drops in Focus: Low iron (even without anemia) can make it harder to stay focused and accurate on tasks that require concentration and memory.

• Slower Thinking, But Still Functional: You take longer to plan or solve problems, but it’s subtle. You can still get through your day.

• Better with Iron: Taking iron can sharpen focus and memory, even if you didn’t feel off before.

⚠️ Symptomatic Impact: Noticeable Struggles

• Hard to Focus: Paying attention feels like a constant battle, and distractions are harder to ignore.

• Thinking Feels Slow: Problem-solving and decision-making take much longer and feel frustrating.

• Your Body Feels It Too: You may get brain fog, headaches, extreme tiredness, and weak muscles, making everyday tasks harder.

Women who treated even mild iron deficiency with supplements saw real improvements in cognitive function.

If your iron is low, your brain may not be performing at its best, even if you don’t notice it yet.

🦠 Immune Function: The Hidden Weakness in Your Defenses

Even mild iron deficiency can impact your immune system. Research shows that iron-deficient animals suffer more severe flu infections and struggle to recover. In humans, the effects are less obvious at first, but they still matter.

Both asymptomatic and symptomatic iron deficiency weaken the immune system, just in different ways:

🔍 Asymptomatic Impact: Silent but Risky

• More Infections, More Often: Low iron makes you more prone to urinary, reproductive tract, and gut infections, even if you don’t feel sick otherwise.

• Weakened T-Cell Defense: T-cells are your immune system’s frontline soldiers. Without enough iron, they don’t activate properly, making it harder for your body to fight infections.

• Thyroid Health at Risk: Iron helps regulate the immune system and thyroid function. Deficiency is linked to higher rates of thyroid autoimmunity, which may lead to hypothyroidism and metabolic issues.

⚠️ Symptomatic Impact: Taking the Hit

• Stronger, Harder-to-Fight Infections: A weakened immune response makes serious infections, especially from viruses and intracellular bacteria, more likely.

• Vaccines May Not Work as Well: Studies show lower iron levels can make vaccines less effective, leaving you vulnerable even after immunization.

• Higher Pregnancy Risks: Anemia weakens both maternal and fetal immunity, increasing the risk of infections, preterm birth, and low birth weight.

• Slower Recovery from Illnesses: A depleted immune system prolongs illness and slows recovery, making common colds and infections drag on longer than they should.

If you’re frequently sick, take longer to recover, or struggle with inflammation, low iron may be playing a bigger role than you think.

Your immune system might be working harder than it should, leaving you vulnerable in the long run.

🫀 Cardiovascular Health: The Silent Strain on Your Heart

Your heart depends on iron-rich blood to fuel every beat. When iron levels drop, your heart works harder to compensate, even if you don’t feel it.

Over time, this silent strain can turn into noticeable symptoms, increasing the risk of heart disease.

🔍 Asymptomatic Impact: Working Overtime

• Hidden Changes in Cholesterol: Even mild iron deficiency can throw off cholesterol balance, slowly raising heart disease risk.

• Heart Beating Faster: With less oxygen available, the heart pumps harder to keep up, even if you don’t notice.

• Lower Endurance: Oxygen fuels both muscles and the heart. Without enough iron, stamina can decline gradually, making workouts feel harder over time.

⚠️ Symptomatic Impact: The Breaking Point

• Fatigue & Shortness of Breath: Daily tasks and exercise feel harder because your heart and muscles aren’t getting enough oxygen.

• Higher Risk of Heart Disease: Severe iron deficiency increases the risk of coronary disease, heart-related death, and cardiovascular issues.

• Weakened Heart Function: In extreme cases, prolonged iron deficiency can weaken heart muscle function, increasing the risk of heart failure.

• Menopause & Heart Health: As estrogen declines, some women develop iron deficiency from poor absorption, while others accumulate too much iron. Both can increase heart disease risk.

Even if you feel fine, your heart may be working overtime behind the scenes. If you’re struggling with fatigue, poor endurance, or shortness of breath, it might not be just being out of shape.

⏳ The Long-Term Health Risks of Ignoring Asymptomatic Iron Deficiency

Iron deficiency isn’t just a short-term energy drain. It can quietly set the stage for serious health consequences down the road.

Many women assume that if they aren’t experiencing noticeable symptoms, they’re in the clear. But iron deficiency can silently chip away at your health for years before the damage becomes obvious.

Here’s what’s at stake if even asymptomatic iron deficiency goes unchecked:

• 🩸 Higher Risk of Full-Blown Anemia → Mild deficiency can progress to anemia, which is harder to reverse and can take months to recover from.

• 🫀 Increased Risk of Heart Disease & Heart Failure → Low iron forces the heart to work harder, increasing the risk of coronary disease, heart failure, and cardiovascular-related death.

• 🧠 Lasting Cognitive & Mental Health Effects → Chronic iron deficiency slows cognitive processing and may contribute to long-term brain changes linked to mental fog and reduced focus.

• 🦴 Weaker Bones & Mineral Imbalance → Iron deficiency disrupts phosphate regulation, which could impact bone strength and increase the risk of fractures later in life.

💡 The good news? Optimizing your iron levels now can help protect your long-term health and prevent serious complications down the road.

Take a few minutes to check in with yourself today:

1. Track your next workout. Notice if you tire faster, breathe heavier, or take longer to recover.

2. Set a reminder to check your focus at the end of your workday. Take notes. Were you mentally sharp, or did you struggle with concentration and brain fog?

3. Climb a set of stairs and check your breathing. If you feel more winded than usual or your heart races, your body may be compensating for low oxygen.

4. Look back at your recent health. If you’ve been sick more often or taking longer to recover, low iron could be weakening your immune system.

Small actions today can prevent bigger health issues later. Which one will you start with?

This whole experience made one thing impossible to ignore: feeling fine doesn’t always mean you’re functioning at your best.

Iron deficiency is sneaky. It doesn’t always hit like extreme exhaustion. Sometimes, it works in the background, subtly affecting your brain, immune system, and heart in ways you don’t notice until much later.

Luckily, you don’t have to wait until you feel terrible to do something about it.

Paying attention to the right signs now can make all the difference.

🤔 Even if you feel fine, low iron can silently impact your brain, heart, and immune system. Taking action now can help prevent long-term health consequences.

🩸 Are women designed to be iron-deficient? In next week’s issue, we’re uncovering how our food system quietly changed—making iron harder to get—and what you can do about it.

1  This is likely why we have such trouble establishing globally recognized thresholds on what constitutes iron deficiency, not to mention challenges in comprehensive screening guidelines and practices.

2  This study excluded women with anemia, defining it as a hemoglobin (Hb) level below 120 g/L.