Signs you might be low on vitamin D, iron, B12, or magnesium

Tired, foggy, run-down, achy. That short list is the symptom profile for a low thyroid, poor sleep, depression, anemia, low vitamin D, low B12, low magnesium, doing too much, and being a normal person in a stressful month. Nutrient deficiencies are real and common, but their symptoms are so nonspecific that you genuinely cannot diagnose them from how you feel. You can only develop a reasonable suspicion — and then confirm it with a blood test.

So treat everything below as a "this is worth asking your clinician about" guide, not a "this explains everything" answer. The point of knowing the symptoms and the risk factors isn't to self-diagnose; it's to know when a $30 lab panel is worth ordering and which foods to lean on while you wait for the result. The four deficiencies below — vitamin D, iron, B12, and magnesium — are among the most common worldwide, which is exactly why they get over-blamed for symptoms they didn't cause.

Vitamin D

Common symptoms. Frank vitamin D deficiency causes bone pain, muscle weakness, and in children, rickets. The hard truth is that mild-to-moderate low vitamin D often causes no clear symptoms at all, which is why it's so often missed. People report fatigue and low mood, but the evidence that supplementing fixes those in people who aren't deficient is weak. The real risk of long-term deficiency is to bone: osteomalacia in adults, and worse outcomes with falls and fractures in older adults.

Who's at risk. People with limited sun exposure, darker skin (more melanin means less cutaneous synthesis), older adults (skin synthesizes less with age), people with obesity (vitamin D gets sequestered in fat tissue), anyone with fat-malabsorption conditions (Crohn's, celiac, bariatric surgery), and people at higher latitudes through winter. This is the one deficiency where "are you outdoors much?" is a genuinely useful screening question.

Food sources (rough amounts). Few foods contain meaningful vitamin D naturally. Fatty fish leads: a 3 oz serving of cooked salmon has roughly 570 IU, and canned trout is similar. A teaspoon of cod liver oil is around 450 IU. Beyond that you're mostly relying on fortified foods — a cup of fortified milk has about 120 IU, fortified cereals and orange juice somewhat less. The NIH Office of Dietary Supplements vitamin D fact sheet lists the RDA at 600 IU/day for adults up to 70, and 800 IU after. For most people, diet alone struggles to hit that without sun or a supplement.

When to test. A 25-hydroxyvitamin D blood test is the standard. It's worth ordering if you have bone pain, malabsorption, osteoporosis, or several risk factors stacked together. Routine population-wide screening of healthy people is not recommended by most guideline bodies — test when there's a reason, not reflexively.

Iron

Common symptoms. Iron deficiency is the most common nutrient deficiency in the world, and it's the one most likely to actually explain "tired all the time." Symptoms include fatigue, weakness, pale skin, shortness of breath on exertion, cold hands and feet, brittle nails, and — a weirdly specific tell — pica, the craving to chew ice or non-food items. Restless legs at night is also associated with low iron. These appear well before full-blown anemia, in the "low stores but normal hemoglobin" stage.

Who's at risk. Menstruating women (blood loss is the dominant cause), pregnant women, frequent blood donors, endurance athletes, people with GI bleeding (which can be silent — ulcers, polyps, colorectal cancer), people with celiac or after bariatric surgery, and vegetarians and vegans, because plant (non-heme) iron is absorbed far less efficiently than the heme iron in meat.

Food sources (rough amounts). Heme iron from animal foods is the most bioavailable. A 3 oz serving of beef has roughly 2 mg; a similar serving of canned light tuna about 1 mg; oysters are exceptional at around 8 mg per 3 oz. On the plant side, the numbers look high but absorb poorly: a cup of cooked lentils has about 6.6 mg, firm tofu around 3 mg, a cup of cooked spinach about 6 mg, and fortified breakfast cereals are often dosed near 18 mg per serving. Per the NIH ODS iron fact sheet, the RDA is 8 mg/day for adult men and 18 mg/day for women aged 19–50. Pair plant iron with vitamin C (peppers, citrus, tomatoes) to boost absorption, and keep coffee and tea away from iron-heavy meals, since they inhibit it.

When to test. This is the one where testing matters most, because iron is genuinely harmful in excess and you should never supplement long-term on a hunch. A ferritin test (which reflects iron stores) plus a CBC is the standard workup. If iron deficiency is confirmed in a man or a postmenopausal woman, the next question your clinician will ask is why — unexplained iron deficiency can be the first sign of GI bleeding and warrants investigation, not just a supplement.

Vitamin B12

Common symptoms. B12 deficiency is sneaky because it has two faces. The hematologic face looks like iron deficiency — fatigue, weakness, pallor — but from a different mechanism (megaloblastic anemia). The neurologic face is the dangerous one: numbness and tingling in the hands and feet, balance problems, memory issues, and a sore, smooth, red tongue. Crucially, the nerve damage can become permanent if deficiency runs long enough, and neurologic symptoms can show up before the anemia does. This is not a deficiency to sit on.

Who's at risk. Vegans and strict vegetarians (B12 is essentially absent from plant foods), older adults (stomach acid needed for absorption declines with age), people on long-term metformin or acid-suppressing drugs (PPIs, H2 blockers), people with pernicious anemia (an autoimmune loss of intrinsic factor), and anyone after gastric or ileal surgery. The absorption machinery is elaborate, so plenty of people eat enough B12 but can't absorb it.

Food sources (rough amounts). B12 comes almost exclusively from animal foods. Clams and beef liver are off the charts. More everyday: 3 oz of salmon has about 2.6 mcg, 3 oz of beef around 1.5 mcg, a cup of milk roughly 1.2 mcg, an egg about 0.5 mcg. The NIH ODS B12 fact sheet sets the RDA at 2.4 mcg/day for adults. For vegans, fortified foods (nutritional yeast, fortified plant milks and cereals) or a supplement are essentially mandatory — there's no reliable whole-plant source.

When to test. A serum B12 level is the first step, though it's an imperfect marker; methylmalonic acid (MMA) is a more sensitive confirmatory test when the B12 result is borderline. Test if you have neurologic symptoms, are vegan without supplementing, are over 60, or are on metformin or long-term acid suppression. Because the neurologic damage can be irreversible, err toward testing here rather than waiting.

Magnesium

Common symptoms. Magnesium deficiency is hard to pin down because the body keeps blood levels stable by pulling from bone, so a normal blood test doesn't rule out a whole-body shortfall. Symptoms of genuine depletion include muscle cramps and twitches, fatigue, weakness, poor appetite, nausea, and in severe cases abnormal heart rhythms and numbness. Low magnesium also tends to drag potassium and calcium down with it, so deficiencies cluster.

Who's at risk. People with GI disorders (Crohn's, celiac, chronic diarrhea), type 2 diabetes (more is lost in urine), heavy alcohol use, older adults, and people on long-term PPIs or certain diuretics. Surveys suggest many people simply don't hit the recommended intake from diet, though true clinical deficiency in otherwise healthy people is less common than the supplement aisle implies.

Food sources (rough amounts). This is the deficiency most fixable with food, because magnesium is in a lot of whole, unprocessed plants. An ounce of pumpkin seeds has about 150 mg, an ounce of almonds or cashews around 75–80 mg, half a cup of cooked spinach about 78 mg, a cup of cooked black beans roughly 120 mg, and two squares of dark chocolate around 65 mg. The NIH ODS magnesium fact sheet puts the RDA around 400–420 mg/day for men and 310–320 mg/day for women. Refining grains strips most of the magnesium out, which is part of why intakes run low on heavily processed diets.

When to test. A standard serum magnesium test is cheap but, as noted, can miss chronic depletion. It's worth checking if you have unexplained muscle cramps, are on a PPI or diuretic long-term, have a malabsorptive condition, or have low potassium or calcium that won't correct — because magnesium often has to be replaced first before the others will normalize.

A quick word on potassium

Potassium rounds out the "electrolyte fatigue and cramp" picture. Most people get too little — the adequate intake is around 2,600–3,400 mg/day, and typical diets fall short — though most low potassium that shows up on a blood test is driven by medications (diuretics) or fluid losses rather than diet alone. Symptoms of genuinely low potassium include muscle weakness, cramps, constipation, and heart palpitations. Food sources are easy and pleasant: a baked potato with skin has about 925 mg, a cup of cooked spinach around 840 mg, a banana about 420 mg, a cup of orange juice roughly 500 mg. One caution that's the opposite of the others here: if you have kidney disease or take certain blood-pressure medications, high potassium is the real danger — so this is another "talk to your clinician before supplementing" nutrient, not a free-for-all.

How to actually use this

The honest summary: symptoms point, blood tests confirm. The four deficiencies above share a fatigue-and-weakness core that makes them impossible to tell apart by feel, and two of them — iron and B12 — can be actively harmful to "fix" blindly, either by masking a bleed or by over-supplementing. So the move is to notice the pattern, check your risk factors, and bring a specific question to a clinician: "Given X, should we check my ferritin / B12 / vitamin D?"

While you're at it, the cheapest intervention is usually food, and the only way to know if your diet is actually short is to look at the numbers. CalBurndown reads these micronutrients straight off nutrition labels as you log meals, so you can see your weekly vitamin D, iron, B12, magnesium, and potassium intake against the RDA instead of guessing. If you log a week and your iron is sitting at half the target every single day, that's a much more useful signal than "I feel tired" — it's a concrete gap you can close with food or take to a doctor with evidence. And if the intake looks fine but the symptoms persist, that's exactly the case where a blood test, not another supplement, is the right next step.

This is not a diagnosis. Nothing here can tell you what's wrong — only a clinician working with your history and the right lab tests can. Don't start iron or high-dose supplements on the strength of a symptom list. See a clinician, get tested, and treat the actual result.