Tired in the afternoon, a few more hairs in the brush, the odd cramp at night. Put those into any online checklist and you'll come out missing iron, magnesium, B12, vitamin D and half the alphabet, usually with a supplement bundle waiting at the bottom of the page. By those lists, everyone is deficient in everything.
Some people are running low on something, and it matters when they are. What gives them away is rarely a symptom. It's usually something about their life: what's on their plate, what their body is going through, or a tablet they've taken every morning for years.
Why symptom lists mislead
Tiredness has dozens of causes, from short sleep to a slow thyroid to plain stress, and a deficiency is only one of them. Hair loss, cramps, low mood and brittle nails are the same: each one points in several directions at once. By the time a deficiency produces clear signs of its own, it has usually been building for months.
A better starting point is your life. Deficiencies follow what you eat, what your body is going through, how well your gut absorbs, and what medicines you take. Look at those four and you can usually tell which one or two nutrients are worth checking, and which test will answer the question.
What's on your plate
Vegans and vegetarians have the clearest risks. Vitamin B12 comes almost entirely from animal foods, so anyone who eats none of them needs fortified foods or a supplement, and a breastfeeding mother on a vegan diet passes the shortfall to her baby. Iron from plants is absorbed less well than iron from meat. Popeye had it half right: spinach carries plenty of iron, but the oxalates in it hold on to most of it, so it's a poor way to fix low iron.
Iodine is the one people forget. The best sources are fish and seafood, eggs and dairy, and seaweed carries the most of all, sometimes more than is good for you in a single serving. Sea salt and pink salt, however fashionable, carry little of it, and iodised salt is a backup rather than a main source, since most of the salt in processed food is the plain kind. Zinc can run low on a plant diet as well, because compounds in whole grains, beans and seeds hold on to it in the gut.
Carnivore and keto eaters have a different list. In the first weeks of cutting carbohydrates, the body sheds water, and salt, potassium and magnesium go with it. That's behind much of the headache, weakness and cramping people call keto flu, and it usually settles once those minerals are topped up. On a meat-only diet, iodine is easy to get from eggs, fish and dairy, and easy to miss on beef alone.
Vitamin C is the question people ask most. British sailors were nicknamed "limeys" after the lime juice that kept scurvy away on long voyages, but they lived for months on salted meat and ship's biscuit. Fresh meat, and organ meats above all, carry vitamin C of their own, and scurvy on a well-built carnivore diet is rare. Bleeding gums would be the first sign to watch for.
Long stretches of low-calorie dieting shrink everything at once. Eat less food for months and you eat fewer vitamins and minerals along with the calories, which is why a strict diet with a narrow menu is a common road to a shortfall.
What your body is going through
Heavy periods are the most common cause of low iron in women, and they're easy to overlook because they feel normal to the person having them. Pregnancy and breastfeeding raise the need for iron, folate and iodine at the same time, and folate matters before a pregnancy begins, because the baby's brain and spine start forming in the first weeks, often before anyone knows. Endurance athletes lose iron through sweat and through the pounding their feet take, and regular blood donors give some away with every donation. Teenagers grow fast and eat unpredictably, which makes iron a common gap once periods start, and vegan teenagers need B12 as much as vegan adults do.
Age changes absorption. After about fifty, many people make less stomach acid, and B12 in food needs that acid to be released, so older adults can eat plenty of B12 and still run low. Vitamin D depends on sunlight on bare skin. That puts people who spend most of the day indoors, people who cover up, and people with darker skin living far from the equator at the front of the queue, along with older adults, whose skin makes less of it. Season matters too: far from the equator the sun sits too low for part of the year, and from roughly October to March skin makes little or none, however long you spend outside. Those are the months when a test, or a vitamin D3 supplement through the darker part of the year, earns its place.
How well your gut absorbs
A healthy diet only counts once it's absorbed. Coeliac disease damages the part of the small intestine that takes up iron, folate and calcium, so unexplained low iron is one of its most common first signs. Crohn's disease, long-running gut inflammation and weight-loss surgery all cut absorption too, and people who have had part of the stomach or bowel removed usually need their levels checked for life. Heavy drinking adds its own losses, most of all thiamine, folate, magnesium and zinc.
What's in your medicine cabinet
Some everyday medicines lower nutrient levels slowly enough that nobody connects the two. Metformin, the most common tablet for type 2 diabetes, reduces B12 absorption over years. Tablets that reduce stomach acid, taken long term, lower both B12 and magnesium. Water tablets for blood pressure and fluid can drain potassium and magnesium. Keep taking the medicine, and check the nutrient it affects every so often.
Which test fits
For iron, the useful test is ferritin, which shows how much iron you have in storage, usually alongside a full blood count. One catch: ferritin rises with inflammation, so a normal result during an infection or a flare of something inflammatory can hide low stores.
For B12, a standard blood level is the start. When the result sits in the grey zone near the bottom of the normal range, a second test, methylmalonic acid, shows whether your cells are short of it.
For vitamin D, the test to ask for is 25-hydroxy vitamin D. Magnesium is harder: almost all of it sits inside cells and bone, so a normal blood level can hide low stores, and your diet, medicines and symptoms tell you as much as the number. Iodine and zinc have no reliable single blood test for an individual, which makes looking at your diet the practical check for both. An or a similar finger-prick kit for ferritin can be a reasonable start. Hair mineral analysis, sold online for the same job, is unreliable for finding deficiencies.
Whatever the test, ask for the number as well as the verdict. "Normal" covers a wide range, and a result sitting at the bottom of it, or one that keeps falling from one test to the next, tells you more than the label does.
Before any blood test, stop biotin for a few days if you take it. It's in many hair, skin and nail supplements, and it can throw off a range of lab results, including thyroid tests, making them look better or worse than they are.
Fix what's low, and stop there
Once you know what's low, food is the first fix. Red meat, liver and shellfish are the richest sources of iron, B12 and zinc together, and the iron in them is the kind the body absorbs best, though liver is best left out during pregnancy because of how much vitamin A it carries. Oily fish and eggs bring vitamin D, fish, dairy and eggs bring iodine, leafy greens and beans bring folate, and nuts and seeds bring magnesium. Food closes a gap with a whole package of other nutrients attached. When the gap is large, or your diet rules a food out, a supplement does the job faster.
When you do take one, how you take it matters as much as what you take. Iron is absorbed best with vitamin C and away from tea, coffee and calcium, dairy included. Vitamin D goes in better with a meal that has some fat in it. And the B12 in supplements such as and in fortified foods is already in a form that needs no stomach acid, which is why it works for people over fifty whose food B12 no longer gets through.
More of a good thing is a poor rule, though. Iron builds up in the body when you take it without needing it, vitamin D can climb too high with long-term large amounts, and a daily multivitamin taken on the off chance rarely fixes a real shortfall while adding plenty you never needed. Recheck after a few months, so you know the fix worked and when to ease off.
Start with your own list
The checklists work backwards, from symptoms everybody has to nutrients everybody seems to lack, and they end at a shopping basket. Start from your own life instead: your plate, your body, your gut and your medicine cabinet. For most people that points to one or two nutrients at most, and one or two tests that settle the question. The afternoon tiredness might still turn out to be short sleep. At least you'll know, and the supplement bundle can stay in the basket.
Go through those four today, write down the nutrients that match your life, and get the test for the one that comes up most.
Want to know which mineral signs are real? The Quiet Signals of Mineral Deficiency — seven minerals, the signs that hold up, the ones that are folklore, and which tests can be trusted.
Tired all the time, however much you sleep? Why Am I Always Tired: When Chronic Exhaustion Has a Cause That Sleep Can't Fix — how to tell ordinary tiredness from fatigue with a cause, from minerals to thyroid to stomach acid.
Know someone who's bought a supplement bundle after an online checklist? Send them this. It might point them to the one test that settles it.
Disclaimer: This article is general information. For advice about your own diet, medication or health, talk to a qualified health professional. If you're pregnant or planning a pregnancy, take regular medication, or have a gut condition, get professional advice before starting or changing supplements, and have your levels checked rather than guessing.
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