Always Tired? Doctors Say This Common Nutrient Deficiency Could Be To Blame

For some people, consistently feeling exhausted has become so commonplace that they just accept it as normal. PSA: It shouldn’t be. Sure, there are periods of life when fatigue is going to happen—such as after having a baby or during finals week in school—but fighting exhaustion on a daily basis should never be someone’s experience long-term.

Not only can feeling fatigued all the time get in the way of enjoying daily activities, but it’s also often a sign of an underlying health condition that shouldn’t be ignored. Dr. Michael Billington, MD, the chief science officer at PrimaryMD, says that when a patient comes to him about fatigue, he starts by ruling out the worst-case scenarios first. 

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“Anyone who has unintentional weight loss, fever or night sweats, shortness of breath, chest pain, abnormal bleeding, including heavy menstrual periods or blood in the stool, or new lymph node swelling needs to be worked up very expeditiously,” he says.

Fortunately, the majority of the time, the reason for fatigue is not as serious. In fact, Parade asked three primary care doctors what the most common reason for fatigue is, and they all said that it often comes from a specific nutrient deficiency. 

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The Common Nutrient Deficiency That Can Cause Fatigue

“Fatigue can indicate just about anything, and this is the reason that I do not immediately draw a conclusion,” says Dr. Alexander Acosta, MD,a primary care doctor with SonderCare, saying that one of the first questions he asks patients experiencing fatigue is how they sleep. After all, not getting enough sleep is one of the most straightforward reasons for feeling tired. Besides asking about sleep, Dr. Acosta says that he also asks patients about their diet and stress, two other factors that are greatly connected to energy levels.    

All three of the primary care doctors we talked to say that getting a basic metabolic panel and testing nutrient levels are key steps toward figuring out the root cause of fatigue. They say that, often, fatigue is caused by being deficient in either iron or vitamin B12. 

“In the United States, iron, vitamin D, B12, folate and magnesium are common deficiencies,” says Dr. Alexa Mieses Malchuk, MD, MPH, FAAFP, a family practice physician at Geisinger. She explains that iron and vitamin B12 deficiencies are most common among people who eat primarily plant-based and adds that menstruating women are also at higher risk for iron deficiency.  

Dr. Acosta adds that many people are deficient in iron, vitamin B12 and vitamin D at the same time, which can certainly cause fatigue.

“B12 is needed for red blood cell formation, and red blood cells deliver oxygen to all tissues in the body. If B12 is too low, oxygen is reduced, which can lead to fatigue, brain fog and muscle weakness. B12 is also used to keep the myelin sheath around your nerves healthy, and, if your B12 levels remain low over an extended period of time, symptoms become more than just fatigue and begin to impact the nervous system,” he explains.

To this point, according to a 2022 scientific article, even vitamin B12 levels that are on the low side of normal can lead to fatigue and exhaustion. Scientific research has also shown that iron deficiency can lead to fatigue and trouble concentrating. 

Related: It’s Not Just Hormones—Here Are 6 Sneaky Reasons You’re Exhausted All the Time

How To Get More Iron and Vitamin B12

If your doctor tells you that you have low iron or vitamin B12, you’ve identified the potential cause for your fatigue. The next question is how you can get more of these nutrients. “Vitamin B12 is made only by bacteria. No plant, fungus or animal makes it, which is why it reaches us almost entirely through animal foods,” Dr. Billington says.

To get more iron and vitamin B12, he recommends eating more eggs, shellfish (such as clams, mussels and oysters) and oily fish (such as salmon, mackerel and sardines). Beef is another food source of these nutrients, he adds.

“For people eating vegetarian or vegan, fortified foods are non-negotiable,” Dr. Billington says, explaining that this includes fortified breakfast cereals, fortified soy or almond milk, fortified meat substitutes and fortified nutritional yeast. 

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Unfortunately, Dr. Billington says that fermented foods like tempeh, miso, kimchi and sauerkraut don’t produce enough vitamin B12 to matter. And unfortunately, spirulina, which is often marketed as being rich in vitamin B12, doesn’t have enough of the nutrient to really move the needle.

To get more iron while sticking to a plant-based diet, up your intake of dark leafy greens, beans, lentils and tomatoes. 

When To Consider a Supplement

“Sometimes, for people whose levels are already low, food alone isn’t enough to get them back up and running and that’s where supplements can be a more convenient way of getting it,” Dr. Acosta says. 

Dr. Billington adds to this, saying that if someone is deficient in vitamin B12 because they cannot absorb it, rather than because they are not eating it, no amount of dietary change will fix it. This, he explains, can include people who have had gastric bypass surgery or who have an autoimmune condition.

All three doctors say that many people who are vegan or vegetarian should consider taking iron and vitamin B12 supplements if their bloodwork indicates a deficiency because it can be hard to get enough of these nutrients without eating meat. 

“Anyone on long-term metformin, or on a proton pump inhibitor or H2 blocker for years, should have their status checked,” Dr. Billington says. He explains that there are several other drugs associated with lower levels of vitamin B12 or iron, including colchicine, cholestyramine and oral contraceptives. “Patients on GLP-1 drugs are also at risk for deficiency due to appetite suppression,” he adds.

Dr. Acosta tells Parade that older adults may also want to consider supplementation. “Beyond age 50, individuals have reduced amounts of stomach acid and intrinsic factors, which are essential for the absorption of B12 from food, and even individuals with regular consumption of animal products can become deficient. In such instances, sublingual tablets or B12 injections are more effective as the vitamin is not affected by gut absorption,” he says.

As with all supplements, Dr. Malchuk issues a reminder that iron and vitamin B12 supplements are not regulated by the Food and Drug Administration, so it’s important to only buy ones that are third-party tested. That way, you can make sure you’re getting the appropriate dose and there aren’t any toxic ingredients.

If you’re not deficient in vitamin B12, Dr. Billington says that taking a supplement won’t make you feel more energized. He emphasizes that it’s only people who are truly deficient who should consider a supplement.

What’s most important to remember is that it’s not normal to feel exhausted every day. “Patients normalize the fatigue for months to years before getting tested. They believe it is stress or aging and do not even consider their nutrient levels. It is important to have a blood test first, as there may be other problems that need to be addressed and supplementing may hide them,” Dr. Acosta says.

The secret to feeling more energized may simply require a change in what you eat. With this in mind, if you regularly feel exhausted, it’s worth it to get your nutrient levels checked.

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Related: 5 Things To Do Before 10 a.m. To Have More Energy, According to Doctors

Sources 

• Dr. Michael Billington, MD, chief science officer at PrimaryMD.
• Dr. Alexander Acosta, MD, primary care doctor with SonderCare.
• Dr. Alexa Mieses Malchuk, MD, MPH, FAAFP, family practice physician at Geisinger.
• Herrero Pfnür, I. A. (2022). Vitamin B12 Deficiency as the Cause.Deutsches Ӓrzteblatt International. 119(12):216.
• Auerbach, M., DeLoughery, T. G. and Tirnauer, J. S. (2025). Iron Deficiency in Adults: A Review. JAMA. 333(20):1813-1823.
• Food Sources of Iron. Dietary Guidelines for Americans. 
• Vitamin B12. National Institutes of Health.