Low iron, B12, vitamin D, magnesium or folate can sap your energy over months and months without you realizing anything is amiss. Most people think they’re getting tired or having poor sleep when they feel the symptoms. A blood test can help determine if it is due to a deficiency, and correcting a deficiency is often quite simple.
A simple blood count test can be normal despite a year of having been run down. Many are taken aback by this.
It’s due to the fact that certain imbalances begin in your body, way before becoming apparent in the numbers that most common tests measure. You’ve been drifting for a while before you feel it.
If you’ve suffered from a kind of fatigue you could not cure with sleep, then this post is for you. We’ll cover the nutrients most commonly implicated, the medications that may be lowering your nutrients without you even realizing it, the tests that will actually detect the issue, and what to do at your appointment to get a true answer.
Why Is a Vitamin Deficiency So Easy to Miss?
- It’s slow to develop, so you don’t see the change on any one day.
- Some of the nutrients are stored in the body, and it can take a while for symptoms to present themselves.
- The symptoms are non-specific. Fatigue, foggy brain, flat affect, muscle aches and pains have about fifty other reasons.
- Coffee is just a bit better so it covers up the issue for a couple of hours.
That final one is one that you’ll want to sit with. Caffeine will mask fatigue, it won’t cure it.
Which Nutrients Are Most Likely Behind Low Energy?
There are five numbers that occur more than the others. Each has different functions, so we will examine them in turn.
Iron: the one that is concealed by a normal blood count!
Red blood cells carry oxygen through your body due to iron. Less of this results in a lack of oxygen to your muscles and brain that leaves you feeling down, tired, and heavy.
The problem is, iron comes in layers. A routine blood count may not detect it, as your stored iron may be running low but your haemoglobin appears normal. It is typically better detected by a ferritin test.
Signs to watch for:
- Tiredness that doesn’t improve with rest
- Trouble concentrating
- Feeling cold when others are comfortable
- Headaches or pale skin
Some people have a higher chance of running low:
- Women with heavy periods. The National Institutes of Health (NIH) says at least 10% of menstruating women are believed to have heavy periods.
- Pregnant women. About 18% had iron deficiency in one large U.S. survey.
- Regular blood donors. Roughly 25% to 35% develop it.
- People with bowel or stomach conditions, or past stomach surgery.
- Vegetarians, since iron from plants is harder to absorb.
Vitamin B12: the slow burn
B12 helps make red blood cells and keeps your nerves in good shape! Your body stores a fair amount – so it can take years for a shortfall to show up.
When it does, the signs often go beyond tiredness. You might notice tingling in your hands or feet, trouble finding words, or a sore, smooth tongue. Some people feel their heart racing for no clear reason.
Studies put deficiency in older adults anywhere from 3% to 43%, depending on how it’s measured. Across U.S. adults, about 3.6% are deficient and about 12.5% are borderline low.
You’re more likely to run low if you:
- Are over 50, when stomach acid and absorption tend to drop
- Follow a vegan diet, since B12 comes mostly from animal foods
- Have pernicious anemia, celiac disease, or gastric bypass
- Take acid reducers like omeprazole, or metformin for diabetes
Vitamin D: the one everyone assumes they have
Vitamin D supports your bones and muscles. Low levels are tied to muscle weakness and aches, and plenty of people also say they feel tired.
We’ll be straight about one thing. Vitamin D isn’t always the cause, and a supplement won’t cure every case of fatigue. If you’ve been taking it and still feel off, that’s a sign to look further, not to take more.
You’re more likely to run low if you:
- Work indoors most of the day
- Have darker skin, since your skin makes less vitamin D from sunlight
- Are older
- Have obesity or have had gastric bypass
A national U.S. survey found about 5% of people at risk of deficiency and 18% at risk of inadequacy. Risk of deficiency was 17.5% in Black Americans and 2.1% in White Americans.
And no, living in Southern California doesn’t protect you. Sunscreen, desk jobs, and long commutes eat up a lot of daylight.
Magnesium: low intake, not always low levels
Magnesium helps turn food into energy and keeps muscles and nerves calm. Low levels can cause fatigue, weakness, and cramps.
True deficiency is uncommon in healthy people, because your kidneys are good at holding onto it. Low intake is a bigger issue. NIH data suggests about 48% of Americans eat less magnesium than they need.
It’s more likely to be a problem if you:
- Have Crohn’s disease, celiac disease, or type 2 diabetes
- Drink heavily
- Are an older adult
- Take water pills, or have used acid reducers for over a year
Folate: easy to overlook
Folate helps your body build red blood cells. When it’s low, you may feel tired, short of breath, and short-tempered.
It’s more common in people who drink often, people who are pregnant, and people who rarely eat leafy greens or beans.
One caution: folic acid on its own can hide a B12 deficiency while it keeps getting worse. That’s a good reason to test before you start supplements.
How Do the Five Compare?
Use this as a rough guide. It can’t replace a blood test.
Nutrient | Common Signs | Who’s More Likely to Run Low | Key Test |
Iron | Fatigue, pale skin, feeling cold | Heavy periods, pregnancy, blood donors, vegetarians | Serum ferritin (below 30 mcg/L suggests deficiency) |
B12 | Fatigue, tingling, brain fog | Adults over 50, vegans, metformin or acid-reducer users | Serum B12, sometimes MMA |
Vitamin D | Muscle weakness, aches, tiredness | Indoor workers, darker skin, older adults | 25(OH)D blood level |
Magnesium | Fatigue, cramps, weakness | Diabetes, bowel disease, diuretic users | Blood test, read alongside diet and medications |
Folate | Fatigue, shortness of breath, irritability | Alcohol use, pregnancy, few greens and beans | Serum folate |
Could Your Medication Be the Problem?
It might be and almost nobody mentions this when they hand you a prescription. Some common medicines change how your body absorbs or holds onto nutrients.
- Acid reducers (PPIs and H2 blockers): They can lower B12 absorption. PPIs can also lower iron and magnesium.
- Metformin: It may lower B12 over time.
- Water pills (loop and thiazide diuretics): They can make you lose more magnesium.
- Levothyroxine: Take it at least 4 hours apart from iron.
- Calcium supplements: They can block some iron absorption, so space them out.
If you take any of these, tell your provider. And please don’t stop a prescribed medicine on your own.
Is It a Deficiency or Something Else?
Sometimes it’s neither. Fatigue has a long list of causes, and nutrients are only one.
A thorough visit will often look at:
- Thyroid problems
- Sleep apnea and other sleep disorders
- Anemia from other causes
- Diabetes or blood sugar swings
- Depression and anxiety
- Medication side effects
This is the real argument for testing instead of guessing. If you start supplements on your own, you can spend months treating the wrong thing.
How Do You Get Tested for a Deficiency?
It starts with a conversation and a blood draw. These are the tests most often used:
- Ferritin – The best early marker for iron. A level below 30 mcg/L suggests deficiency.
- Serum B12 –Many labs call anything under 200 pg/mL low, though cutoffs vary.
- MMA (methylmalonic acid) – A follow-up test when B12 lands in the borderline range.
- 25(OH)D – Below 12 ng/mL is linked to deficiency. From 12 to 20 ng/mL is generally inadequate, and 20 ng/mL or higher is generally adequate.
- Complete blood count (CBC) – Helps spot anemia, though it can miss early iron loss.
- Serum folate – Checks your folate level directly.
Your provider may add others based on your history. If you’ve taken supplements recently, say so, since that can change your results.
How Much Do You Actually Need?
These are the NIH’s recommended daily amounts for adults. Your own needs may be different, so check with your provider before changing anything.
Nutrient | Recommended Daily Amount (Adults) | Upper Limit |
Iron | 18 mg for women 19 to 50; 8 mg for men and for women over 50 | 45 mg per day |
B12 | 2.4 mcg | None set (low toxicity) |
Vitamin D | 600 IU (15 mcg) for ages 19 to 70; 800 IU (20 mcg) over 70 | 4,000 IU (100 mcg) per day |
Magnesium | 400 to 420 mg for men; 310 to 320 mg for women | Applies to supplements only |
A few safety points:
- Iron: Too much can cause nausea and constipation. Iron overdose is also a leading cause of fatal poisoning in young children, so keep it well out of reach.
- Hemochromatosis: People with this condition store too much iron and shouldn’t take iron supplements unless a doctor says so.
- Vitamin D: Very high levels over time can cause harm. More isn’t better.
What Should You Eat to Support Your Energy?
Food comes first, and supplements fill whatever gaps are left. Here’s where each nutrient shows up:
- Iron – Lean meat, poultry, seafood, beans, and fortified cereals. Pair plant sources with vitamin C, like orange slices or bell peppers, and you’ll absorb more.
- B12 – Fish, meat, eggs, dairy, and fortified foods.
- Vitamin D – Salmon, trout, egg yolks, and fortified milk.
- Magnesium – Pumpkin seeds, almonds, spinach, beans, and whole grains.
- Folate – Leafy greens, beans, and fortified grains.
Two small habits help more than people expect. Eat at regular times, because skipped meals are skipped nutrients. And go easy on alcohol, since it drains several of the nutrients above.
What Should You Bring to Your Appointment?
You’ll get more out of the visit if you show up prepared. A little homework makes the conversation much faster.
- A one-week log. Note your energy at morning, midday, and evening, plus sleep hours and any odd symptoms like tingling or cramps.
- Every medication and supplement you take, including over-the-counter ones and how long you’ve been on them.
- Your diet in plain terms. Vegan, vegetarian, mostly takeout, whatever’s true.
- Your period pattern, if relevant. Heavy or long cycles matter.
- Family history. Anemia, thyroid disease, and digestive conditions are all worth mentioning.
- Your questions. For example, “Can you check my ferritin and not just my blood count?”
When Should You See a Doctor About Fatigue?
Book a visit if you:
- Have felt unusually tired for more than two weeks
- Haven’t improved with more rest or better sleep
- Notice tingling, numbness, or memory trouble
- Have heavy periods or ongoing stomach problems
- Take a medication that can lower nutrient levels
- Eat a vegan or vegetarian diet and have never been tested
Get urgent care right away if you have:
- Chest pain
- Shortness of breath while resting
- Fainting
- Black or bloody stools
- Numbness that’s spreading, or trouble walking
How Can Anaheim Medical Clinic Help?
At Anaheim Medical Clinic, we provide primary care and internal medicine in Anaheim. When someone comes in worn out, we take the time to go through their history, diet, and medications, and then figure out the next step together.
Energy and eating habits are closely linked, so we also offer weight management support.
- Address: 710 S Brookhurst St, Suite J, Anaheim, CA 92804
- Hours: Monday to Friday, 8:00 AM to 5:00 PM
- Phone: (714) 576-7575
Ready to find out why you’re so tired? Book an appointment or call (714) 576-7575. You can also read more on our health blog.
FAQ
What is the vitamin deficient that gives the maximum fatigue?
Two vitamins are most closely associated with fatigue: iron and B12. Iron transports oxygen to your blood; B12 helps develop red blood cells and maintain nerves. Vitamin D, magnesium and folate can also add in.
Does low vitamin D make you tired?
It can be linked to it. Roughly, low vitamin D has been more recognized for muscle weakness and aches. The only way to determine if it is a factor is to have a blood test.
How many days after correction of a deficiency before improvement?
This varies according to the nutrient and your nutritional status. Some people feel better in a few weeks, others in a few months – particularly if their store was very low. Your provider will recheck your levels along the way.
Should I simply just go for a multi-vitamin?
You may not be getting enough of the nutrients you need from a multivitamin. Certain nutrients, such as iron, can also be detrimental at higher levels – so testing first and supplementing with only the amount that’s recommended is advisable.
Is it possible to be a deficiency and have normal blood markers?
Yes. Your iron levels may be low without a normal blood count. Providers will test ferritin separately, which is why.
May I not test if I have symptoms?
Not reliably. There are many causes of fatigue, and the symptoms can’t distinguish them.
Is there enough sun exposure for sufficient vitamin D?
Some are able to, but not many. Your skin’s production varies depending on factors such as sun exposure, skin tone, season and indoor employment. The blood test determines your position.
When is tiredness a sign of something serious?
Get urgent care if it’s accompanied with chest pain, difficulty breathing while resting, fainting, black stools or numbness spreading to other parts of the body. Seek advice from the provider if it persists for longer than two weeks and for no apparent reason.
The Bottom Line
Tiredness has a cause, even when it doesn’t feel like it. Sometimes it’s your schedule, and sometimes your body is low on something it needs to run properly.
You don’t have to keep guessing or pushing through it. One conversation and a blood test can tell you a lot.
Take the next step: Schedule a visit with Anaheim Medical Clinic.
This article is for education only. It isn’t medical advice, diagnosis, or treatment. Talk with your healthcare provider before starting any supplement.
Sources
- National Institutes of Health, Office of Dietary Supplements. Vitamin B12: Health Professional Fact Sheet
- National Institutes of Health, Office of Dietary Supplements. Iron: Health Professional Fact Sheet
- National Institutes of Health, Office of Dietary Supplements. Vitamin D: Health Professional Fact Sheet
- National Institutes of Health, Office of Dietary Supplements. Magnesium: Health Professional Fact Sheet