Do I Need Supplements at All? Blood Tests Instead of Gut Feeling
A full shelf, a good feeling, not a single measurement. Why the watering can disappoints in large studies of healthy people, and which handful of values counts before your next purchase.
I'm not advising you to get more jars. I'm advising you to get fewer, but well-founded ones. A supplement without a measurement is a guess. A supplement with a measurement is a decision.
I bet you know this moment. You're standing in the drugstore in front of a shelf that's longer than your kitchen counter. Vitamin D, magnesium, zinc, an immune complex, something for skin, hair and nails.
Every package sounds sensible. Every one promises a little bit of security. And because you don't want to get anything wrong, three jars end up in your basket. Better safe than sorry.
At home, five others are already waiting. Some you take every day, some only when you remember. Whether you need any of them, you don't know. You hope so.
Can I ask you an uncomfortable question? How do you know what you're lacking?
This article is my case against the shelf and for a handful of targeted blood values. Not because supplements are bad. But because they are a tool. And a tool needs a reason.
How your body regulates nutrients, and why single supplements can tip balances, is explained in the article When supplements make sense, and when they interfere. This one is about the step before that. About the question of whether you need anything at all, and how to find out before you spend money.
If you're healthy and eat a varied diet, you usually don't need supplements on a hunch. Large studies show hardly any benefit in people without a deficiency, and some individual products were even associated with risks. Supplementing makes sense where a blood test shows a deficiency or a special life situation applies, such as trying to conceive, a vegan diet or too little sun.
What to expect in this article
- Why we reach for the jar before we measure
- What large studies show in healthy people
- Why the same nutrient sometimes does little and sometimes can do a lot
- Where well-meant self-dosing can tip over
- Eight blood values that count before you buy
- Three levers for your next decision
Twelve jars on the table
I know this pattern very well. Someone comes to my practice for a consultation and puts a cloth bag on the table. Inside are twelve supplements, collected over months. Jars, little bottles, blister packs.
The sentence I often hear in moments like this:
We laid the back labels side by side and took blood. Not to take anything away from anyone. But to ask every jar: What are you here for?
After the blood tests, three supplements remained for which there was a measurable reason. For the other nine, there was no finding that would have supported them.
The lesson: it doesn't matter how many jars you take. What matters is how many of them have a reason.
Why do we reach for the jar before we measure?
Supplements feel like insurance. Small premium, big protection. Who would say no to that?
This feeling is not a personal failing. It's widespread, and it's growing. Figures from Germany and the US show just how much.
In 2017, the Robert Koch Institute in Berlin analyzed the data of 7,091 adults from the nationwide health study DEGS1.
18.1 percent took vitamins, minerals or supplements as self-medication. That was around six percentage points more than in the late 1990s. Women, older people, people with higher social status, non-smokers and former smokers, and physically active people reached for them more often.
What this means for you: the people reaching for the jar are mainly those who already pay attention to their health. Whether they are lacking anything is a different story.
Knopf H. Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz. 2017;60(3):268-276. DOI: 10.1007/s00103-016-2500-yWhy do so many people reach for them? A team in the US asked exactly that.
A team led by Bailey at the Office of Dietary Supplements of the US health agency NIH asked 11,956 adults from the representative NHANES survey about their motives.
The most common reasons were to improve overall health (45 percent) or to maintain it (33 percent). Only 23 percent of the supplements were taken on the recommendation of a doctor or other health professional. People who took supplements were also more likely to rate their own health as very good.
What this means for you: most jars end up in the cupboard because they feel good. Not because a finding calls for them.
Bailey RL, Gahche JJ, Miller PE et al. JAMA Intern Med. 2013;173(5):355-361. DOI: 10.1001/jamainternmed.2013.2299This fits with a finding that makes me smile every time. The watering can often hits the beds that are already wet. In 2020, a review from Giessen and Zurich drew, among other sources, on the German National Nutrition Survey II. According to it, most people in Germany get enough vitamins through their diet. Supplies tend to run short mainly for vitamin D and folate. And supplements are often taken by precisely those adults who already eat a balanced diet.
In two analyses of the same NHANES data, Bailey's team compared how many minerals and vitamins people with and without supplements took in through their food.
People who took supplements were already eating more nutrient-rich food. At the same time, precisely these people more often exceeded the tolerable upper intake level. This applied to calcium, zinc, iron and magnesium as well as folic acid and vitamins A, B6 and C.
What this means for you: several jars add up on top of your plate. What looks harmless on its own can overshoot the mark when stacked.
Bailey RL, Fulgoni VL, Keast DR, Dwyer JT. Am J Clin Nutr. 2011;94(5):1376-1381. DOI: 10.3945/ajcn.111.020289 · Bailey RL, Fulgoni VL, Keast DR, Dwyer JT. J Acad Nutr Diet. 2012;112(5):657-663. DOI: 10.1016/j.jand.2012.01.026One more point that many people aren't aware of. In Germany, supplements are legally classified as food. Unlike a medicine, they don't need marketing authorization; a notification to the Federal Office of Consumer Protection and Food Safety is enough. So no procedure checks whether a product suits you. You check that, ideally with a measurement.
A jar is not an insurance policy. It's a hypothesis: I'm lacking something. You test a hypothesis before you build on it.
That sounds sober. But it takes a weight off your shoulders. You no longer have to guess.
And now you know why the full shelf serves a feeling more than a finding.
What do large studies show in healthy people?
Maybe you're thinking right now: well, it can't hurt. And if it protects me even a little, it's worth it.
Some of the largest studies in nutritional medicine have asked exactly this question. With tens of thousands of people, over years, with placebo as a comparison. The results are sobering, and they are instructive.
In 2022, the Kaiser Permanente Evidence-based Practice Center in Portland reviewed 84 studies with a total of 739,803 people for the US Preventive Services Task Force (USPSTF). The studies looked at adults without cardiovascular disease, without cancer and without a known nutrient deficiency.
Vitamins and minerals showed little or no benefit against cardiovascular disease, cancer and premature death. For multivitamins, there was a small benefit for cancer incidence, with important limitations. Beta-carotene, on the other hand, was associated with more lung cancer, especially in people at high risk.
What this means for you: without a deficiency, the jar does little on average. Some products can even do harm.
O'Connor EA, Evans CV, Ivlev I et al. JAMA. 2022;327(23):2334-2347. DOI: 10.1001/jama.2021.15650A team led by Manson at Brigham and Women's Hospital and Harvard Medical School gave 25,871 men aged 50 and older and women aged 55 and older daily vitamin D or a placebo.
After a median of 5.3 years, cancer and major cardiovascular events occurred about equally often in both groups. Overall mortality was also practically the same.
What this means for you: vitamin D for everyone, without measuring first, did not make any of these diseases less common in this large study.
Manson JE, Cook NR, Lee IM et al. N Engl J Med. 2019;380(1):33-44. DOI: 10.1056/NEJMoa1809944A team led by Sesso, also at Brigham and Women's Hospital, tested a daily multivitamin in 21,442 older women and men in the COSMOS trial.
After a median of 3.6 years, overall cancer and cardiovascular disease occurred about equally often in both groups. A single favorable signal appeared for lung cancer. Isolated findings like this need confirmation before they can carry weight.
What this means for you: even the colorful all-in-one pill is no protective shield for healthy people.
Sesso HD, Rist PM, Aragaki AK et al. Am J Clin Nutr. 2022;115(6):1501-1510. DOI: 10.1093/ajcn/nqac056That leaves the question of whether nutrients make a difference at all. They can, but it seems to depend on where they come from.
A team led by Chen at Tufts University in Boston followed 30,899 adults from the NHANES survey for a median of 6.1 years.
Taking supplements as such was not associated with lower mortality. An adequate intake of vitamin A, vitamin K, magnesium, zinc and copper was associated with fewer deaths, but only when it came from food. A high intake of calcium from supplements, on the other hand, was linked to more cancer deaths.
What this means for you: the plate seems to achieve more than the jar. As an observational study, however, this work cannot reliably separate cause and effect.
Chen F, Du M, Blumberg JB et al. Ann Intern Med. 2019;170(9):604-613. DOI: 10.7326/M18-2478No effect on average doesn't mean no effect for you. It means: if you don't measure, you end up in the average.
The large studies mainly examined people without a known deficiency. For them, the watering can is hardly a gain. Things get interesting where something is actually missing. And you can only tell that if you take a look.
And now you know why I tend to advise healthy people without a finding to hold back rather than add another jar.
Watering can or targeted shot: when supplementing can make a difference
Imagine two gardens. In one, the soil is moist. In the other, it's cracked from drought. You water both with the same can. In the first garden, nothing happens. In the second, the plant straightens up.
This picture keeps coming up in research. Whether a nutrient can make a difference apparently depends heavily on how full the store was beforehand.
The watering can
- For everyone, based on feeling and advertising
- In studies, hardly any benefit on average in people without a deficiency
- Several jars add up, sometimes beyond upper limits
- No yardstick for whether anything has changed
The targeted shot
- For a specific, measured reason
- Greater chance of benefit when something is actually missing
- Amount and duration are guided by the value
- A follow-up value shows whether anything is moving
How much the starting value counts is shown by a second look at the VITAL trial.
In an ancillary analysis of VITAL, a team led by LeBoff at Brigham and Women's Hospital examined whether vitamin D can prevent fractures.
Over a median of 5.3 years, similar numbers of people broke a bone in both groups. The participants had not been selected for vitamin D deficiency, low bone density or osteoporosis. Even when broken down by starting level, there was no difference.
What this means for you: for healthy, unselected people, the extra dose did not protect their bones. The authors explicitly limit their conclusion to this group.
LeBoff MS, Chou SH, Ratliff KA et al. N Engl J Med. 2022;387(4):299-309. DOI: 10.1056/NEJMoa2202106In 2017, a team led by Martineau at Queen Mary University of London pooled the individual data of 10,933 people from 25 randomized trials on vitamin D and acute respiratory infections.
Overall, infections occurred somewhat less often with vitamin D. With daily or weekly dosing, the protection in people with a very low starting level below 25 nmol/l was considerably stronger than in those who were better supplied. A larger follow-up analysis by the same London team with 46 trials found only a small overall effect in 2021 and no clear difference by starting level.
What this means for you: those who had very little seemed to benefit the most. This isn't entirely settled, and that's exactly how it should be told.
Martineau AR, Jolliffe DA, Hooper RL et al. BMJ. 2017;356:i6583. DOI: 10.1136/bmj.i6583 · Jolliffe DA, Camargo CA, Sluyter JD et al. Lancet Diabetes Endocrinol. 2021;9(5):276-292. DOI: 10.1016/S2213-8587(21)00051-6With iron, the picture is clearer. Here there's a randomized trial that studied exactly the people who come to the practice with an unremarkable blood count and great fatigue.
A team led by Vaucher at the University of Geneva studied 198 women aged 18 to 53 from 44 general practices in France. All of them were fatigued and had no anemia, but had a ferritin below 50 µg/l.
With iron, fatigue decreased by 47.7 percent in twelve weeks, with placebo by 28.8 percent. The effect appeared regardless of whether ferritin was below or above 15 µg/l.
What this means for you: even a value that some labs still report as normal can play a role when you have symptoms. The basis was a measurement, not a gut feeling.
Vaucher P, Druais PL, Waldvogel S, Favrat B. CMAJ. 2012;184(11):1247-1254. DOI: 10.1503/cmaj.110950The question is not whether a nutrient can do something. The question is whether you're lacking it.
A supplement is like a key. It only fits one particular lock. The blood value shows you whether you're standing in front of that lock.
And now you know why I'd rather measure once than swallow things on a hunch for years.
Where well-meant self-dosing can tip over
Vitamins sound like fruit, like sunshine, like life. How could that do any harm?
Five hundred years ago, Paracelsus put it roughly like this: the dose alone decides whether something is a poison. With nutrients, that's not an empty phrase. Many have a window. Below it, something is missing. Above it, it becomes a burden. And for some, this window is narrower than the packaging would have you believe.
Beta-carotene
In the Finnish ATBC trial with 29,133 male smokers, there were 18 percent more lung cancer cases with beta-carotene.
Source 14 · RCTVitamin E
In the SELECT trial with 35,533 men, the risk of prostate cancer was 17 percent higher with vitamin E.
Source 15 · RCTSelenium
Long-term selenium supplementation in 1,202 people was associated with 55 percent more type 2 diabetes, most clearly in those with a high starting level.
Source 16 · RCTHigh-dose vitamin D
In 311 healthy people, bone density at the forearm fell more over three years with high daily doses than with a low dose.
Source 17 · RCTVitamin A as retinol
In the Nurses' Health Study with 72,337 women, a high retinol intake was linked to more frequent hip fractures.
Source 18 · CohortVitamin B6
As early as 1983, seven adults were described with severe nerve disorders after high doses of B6. After they stopped, the condition improved in all of them.
Source 19 · Case seriesIron without measuring
In a North American screening study, 0.44 percent of white participants carried two copies of the most common gene variant for an iron storage disease.
Source 20 · CohortCalcium from supplements
A high intake of calcium from supplements was associated with more cancer deaths; calcium from food was not.
Source 9 · CohortNone of these findings means that a particular supplement is harming you. What they mean is: more is not automatically safer. Most of these risks involve high doses, long-term use or people who weren't lacking anything at all. Two examples show particularly clearly why a value before you start matters.
A team led by Stranges at the University at Buffalo analyzed a trial in which 1,202 people received selenium or a placebo over several years.
Type 2 diabetes occurred more often with selenium; the risk was 55 percent higher. The effect was clearest in those who already had the highest selenium levels in their blood at the start. In that group, the risk with selenium was about 2.7 times that of the placebo group.
What this means for you: of all people, those who were well supplied carried the greatest risk. A measurement before you start can make exactly this group visible.
Stranges S, Marshall JR, Natarajan R et al. Ann Intern Med. 2007;147(4):217-223. DOI: 10.7326/0003-4819-147-4-200708210-00175In the HEIRS study, a team led by Adams at the London Health Sciences Centre in Canada screened 99,711 people from primary care practices and blood collection centers for iron overload.
Among white participants, 0.44 percent carried two copies of the most common genetic variant for an iron storage disease, roughly one in 230 people. Most of them had elevated ferritin levels and elevated transferrin saturation, many without a previous diagnosis.
What this means for you: iron on a hunch can be exactly the wrong thing for some people. Measuring ferritin and transferrin saturation beforehand isn't pedantry. It's diligence.
Adams PC, Reboussin DM, Barton JC et al. N Engl J Med. 2005;352(17):1769-1778. DOI: 10.1056/NEJMoa041534How to put iron overload into context, and why it can be kept well under control, is explained in the article Iron overload and too much iron.
Through the KPNI lens, none of this is very surprising. Nutrients act on metabolism, the immune system, hormones and nerves all at once. Selenium is a building block of enzymes in the thyroid and the immune defense; in excess, it seems to strain sugar metabolism. Vitamin B6 is involved in nerve metabolism; in very high doses, it can irritate exactly those nerves. You can read more about this in the article Too much vitamin B6. And iron is the taxi for the oxygen in your blood. In excess, it can be deposited in the liver, heart and joints.
Fewer jars are not a sacrifice. They are freedom: from the worry of missing out, and from a shelf that tells you what you're supposedly lacking. Clarity feels lighter than any capsule.
Shukri Jarmoukli, ViveCura BerlinMore is not safer. With nutrients, the dose decides whether something is nourishment or a burden.
High doses can make sense under medical supervision, with a measurement beforehand and a check afterwards. The difference between covering your needs and therapy is described in the article Orthomolecular medicine or supplements?
And now you know why, for me, a measurement comes before any higher dose.
Which blood values count before you buy anything?
Maybe you're now wondering: do I have to get a huge lab panel straight away? No. It's not about as many values as possible. It's about the right ones for your question.
In my experience, eight values cover the most common questions before someone spends money on supplements. This selection is a clinical weighting, not a list from a guideline. Not everyone needs all eight. Which ones make sense depends on your symptoms, your diet, your medications and your stage of life.
Ferritin with CRP and transferrin saturation
Shows how full your iron store is. It's the basis for every decision about iron, in both directions.
Ferritin is also an acute-phase protein of the immune system and rises with inflammation. Without CRP, a normal value can hide a deficiency. High ferritin with high transferrin saturation, on the other hand, can point to overload.
25-OH vitamin D
The storage form of vitamin D. It reflects your supply over weeks.
In the nationwide DEGS1 study, 61.6 percent of adults were below 50 nmol/l, and values were lower in winter. Watch the unit: ng/ml times about 2.5 gives nmol/l.
Holo-TC, the active B12
Shows the share of B12 that your cells can take up via their receptor.
The total B12 value often raises the alarm late. If the result is unclear, methylmalonic acid can add information. More in the article Vitamin B12 deficiency despite a normal value.
Folate, with homocysteine if needed
Shows whether enough folate is available for cell division and methylation.
According to the German National Nutrition Survey II, folate is, alongside vitamin D, one of the vitamins that run short more often in Germany. Homocysteine reveals whether the metabolism behind it is running smoothly. More in the article Elevated homocysteine.
Omega-3 Index
Measures the share of EPA and DHA in the membranes of your red blood cells, in other words your supply over months.
In the original 2004 paper, an index of 8 percent or more was associated with the strongest protection against cardiac death, and an index of up to 4 percent with the weakest. More in the article Measuring the Omega-3 Index.
Magnesium, preferably in whole blood
Gives an indication of your magnesium status. Most of your magnesium sits in bones and cells, not in serum.
The usual serum reference range of 0.75 to 0.95 mmol/l is based on a population distribution from 1974, not on clinical outcomes. A subclinical deficiency can hide behind it. More in the article Micronutrient analysis.
Zinc and selenium
Two trace elements for immune defense, skin, thyroid and many enzymes.
Both have a narrow window between too little and too much. This is exactly where supplementing without measuring can tip over, as the selenium data above show. More in the article Minerals in balance.
TSH as thyroid context
Shows whether fatigue, feeling cold or hair loss might come from the thyroid rather than from a nutrient.
TSH is often just the beginning. If anything looks unusual, the free thyroid hormones belong in the picture. More in the article The TSH value alone is not enough.
Ferritin above 100: my perspective, clearly separated from the evidence
On ferritin, I have a clear position. In my practice, I aim for a value above 100 µg/l in people with symptoms. This is a functional perspective based on experience, not a target that large studies have established for all healthy people.
Still, this value isn't pulled out of thin air. In cardiology, a ferritin below 100 µg/l already counts as iron deficiency in heart failure.
In the FAIR-HF trial, a team led by Anker at the Charité in Berlin studied 459 people with heart failure and iron deficiency. Iron deficiency was defined as a ferritin below 100 µg/l, or up to 299 µg/l with a transferrin saturation below 20 percent.
After iron infusions, 50 percent felt much or moderately better, compared with 28 percent on placebo. This applied to people with and without anemia.
What this means for you: a ferritin that looks unremarkable on the lab report can already count as a deficiency in certain situations. Applied to healthy people, this is a perspective, not proof.
Anker SD, Comin Colet J, Filippatos G et al. N Engl J Med. 2009;361(25):2436-2448. DOI: 10.1056/NEJMoa0908355Sleep medicine also draws the line higher than many labs. A guideline from the International Restless Legs Syndrome Study Group considers oral iron possibly effective for restless legs up to a ferritin of 75 µg/l. You can read more about this in the article Restless legs syndrome and iron. How the debate about a target above 100 is being conducted is shown in the article Ferritin above 100 as a target.
Before you have blood drawn: supplements can distort values
This is where it gets almost curious. Some supplements change not only your body but also the measurement. Biotin, for example, is often found in products for skin, hair and nails.
A team led by Li at the University of Minnesota had six healthy adults take biotin for one week, in an amount common in over-the-counter products. The team then checked 37 lab tests for eleven analytes.
In 9 of 23 tests that work with biotin technology, there was interference. Depending on the analyzer, TSH, for example, came out falsely low and vitamin D falsely high.
What this means for you: before every blood draw, say what you're taking. Whether and for how long you pause anything beforehand is something to discuss with your doctor.
Li D, Radulescu A, Shrestha RT et al. JAMA. 2017;318(12):1150-1160. DOI: 10.1001/jama.2017.13705From the perspective of Clinical Psychoneuroimmunology, KPNI for short, these values talk to each other. Silent inflammation can raise ferritin via the immune system and thereby hide a deficiency. The thyroid needs iron, iodine and selenium to make its hormones, and a supplement can distort its lab picture. B12 and B6 affect the nervous system, both in deficiency and in excess. And with selenium and calcium, metabolism reacts not only to too little but also to too much. That's why a single value is never the whole picture. Only together do they tell a story.
How iron, selenium, zinc and vitamin D interact with the thyroid is shown in the article The most important nutrients for the thyroid.
A lab value is not a verdict. It's a rationale.
It doesn't tell you who you are. It tells you what a supplement would be for, and how you'll later recognize whether it has changed anything.
Blood tests instead of gut feeling doesn't mean: never without a lab. A review in Deutsches Ärzteblatt names well-founded situations in which supplementing can make sense even without a prior measurement. These include folic acid when trying to conceive and during pregnancy, vitamin B12 with a vegan diet or impaired absorption, and vitamin D when your body can't make enough on its own.
And please never stop supplements prescribed by your doctor on your own, even if this article invites you to clear things out.
And now you know why a handful of targeted values can bring more clarity than an entire shelf.
Three levers for your next decision
I'm deliberately not giving you any dosages. They depend on your values, and I don't know them. What I'm giving you are three directions you can act on today.
Take inventory before you buy
Put all your jars on the table and read the back labels side by side. Many products contain the same nutrients in different amounts. Add them up. Often this look alone shows where something is doubled up.
Have the values measured that fit your question
Take the list with you to your doctor's practice. Ask for the values that fit your symptoms and your stage of life, rather than an all-in-one package. Mention everything from the drugstore, especially biotin.
Give every supplement a reason and a recheck date
Next to each jar, write down which value justifies it and when it will be measured again. A supplement without a reason can go. A supplement with a reason deserves a recheck.
Your inventory in five questions
- Which nutrients are in several of my supplements at once?
- Which supplement has a measured reason?
- Am I taking anything that can interfere with lab values, such as biotin?
- Which medications am I taking that could interact with supplements?
- When was it last checked whether anything has changed?
Well established: In healthy adults without a known deficiency, vitamins and minerals show little or no benefit against cardiovascular disease, cancer and premature death in large studies. Beta-carotene, vitamin E and selenium were associated with risks in randomized trials.
Plausible, with mixed data: People with a genuine deficiency are more likely to benefit. For iron, a randomized trial supports this; for vitamin D, the analyses are inconsistent.
My clinical perspective: The ferritin target above 100 µg/l for people with symptoms and the selection of the eight key values are based on experience and on thresholds from specialties such as cardiology and sleep medicine, not on studies in healthy people.
Supplements are neither a lifestyle accessory nor the enemy. They are a tool that needs a reason. You won't find the reason on the shelf, but in your blood.
Measure first, then decide. That way, twelve jars can become a handful that suits you.
If you'd like your values not just read but put into context: below this article, you'll find the option to book an appointment.
Frequently asked questions
Do supplements make sense if I eat a healthy diet?
For healthy people without a deficiency, large studies mostly show little benefit. An analysis of 84 studies with 739,803 people found little or no benefit from vitamins and minerals against cardiovascular disease, cancer and premature death. According to the German National Nutrition Survey II, most people in Germany get enough vitamins from their food; supplies tend to run short mainly for vitamin D and folate. Supplementing makes sense when a deficiency has been measured or a special life situation applies.
Which blood values should I have checked before taking supplements?
That depends on your question, not on a standard package. Often informative are ferritin with CRP and transferrin saturation, 25-OH vitamin D, Holo-TC as active B12, folate, the Omega-3 Index, magnesium, zinc and selenium, and TSH as a look at the thyroid. A basic blood count and a kidney marker round out the picture. Not everyone needs all of these values. Symptoms, diet, medications and stage of life decide which ones fit.
Why do large studies like VITAL show hardly any benefit?
Because they mainly studied people who weren't lacking anything. In VITAL, 25,871 adults received vitamin D or placebo, and they were not selected for a deficiency. Cancer and cardiovascular events occurred about equally often in both groups, and fractures did not become less common either. If you're well supplied, you gain little from more of the same. Studies in genuine deficiency can turn out differently, as a randomized trial on fatigue shows for iron.
Can I overdose on vitamins and minerals?
Yes, especially when several supplements add up. In US data, people taking supplements more often exceeded the tolerable upper intake levels, for example for zinc, iron, folic acid and vitamins A and B6. Studies link very high doses of vitamin B6 with nerve disorders, a lot of retinol with more hip fractures and long-term selenium supplementation with more type 2 diabetes. Iron on a hunch can be a burden if you have an undetected iron storage disease. That's why it pays to look at the sum of all doses.
Should I stop my supplements before a blood test?
Discuss this with your doctor beforehand instead of deciding on your own. Biotin can interfere with some lab tests: in one study, one week of biotin distorted 9 of 23 susceptible test methods, including TSH and vitamin D, depending on the analyzer. A supplement you've only just started can also raise a value before the cause of a deficiency has been clarified. Please never stop medications prescribed by your doctor on your own. What matters is that the lab knows what you're taking.
What is a good ferritin level?
There's no single answer; the thresholds depend on the situation and the lab. In a randomized trial, iron was able to reduce fatigue in women with a ferritin below 50 µg/l, even when the value was above 15. In cardiology, a ferritin below 100 µg/l already counts as iron deficiency in heart failure. In my practice, I aim for values above 100 µg/l in people with symptoms. This is a functional perspective, not a target proven for all healthy people, and ferritin should always be read together with CRP and transferrin saturation.
Do I need a multivitamin as a safety net?
As all-round insurance for healthy people, the evidence for it is weak. In the COSMOS trial with 21,442 older adults, a daily multivitamin lowered neither overall cancer incidence nor cardiovascular disease. A large review found a small benefit for cancer incidence, but with important limitations. For a specific deficiency, targeted supplementation is usually a better fit than a colorful mix.
When does supplementing make sense even without a blood test?
In a few well-founded life situations. A review in Deutsches Ärzteblatt names folic acid when trying to conceive and during pregnancy, vitamin B12 with a vegan diet or impaired absorption, and vitamin D when the body can't make enough on its own. Supplements prescribed by your doctor also belong here. Outside such situations, measuring is the better first step.
How often should I have my values checked?
That depends on the nutrient and the starting value. Storage markers such as ferritin or 25-OH vitamin D change over weeks to months, so a recheck usually only makes sense after you've been taking something for a while. If you have no deficiency and take nothing, you usually don't need frequent checks. It helps to set the recheck date right when you start. Then you know how you'll recognize success.
How is this article different from the other articles on supplements?
The article “When supplements make sense” explains how your body regulates nutrients and why single supplements can tip balances. The article on micronutrient analysis goes deeper into what whole blood and serum each measure. This article starts one step earlier: with the question of whether you need anything at all, why we often buy based on feeling and which handful of values counts before your first purchase. The texts complement each other.
Further reading in the Supplement Guide
Related articles from this guide
- When supplements make sense, and when they interfere
- Micronutrient analysis: what whole blood shows and the serum value misses
- Orthomolecular medicine or supplements? Why the dose changes everything
- Vitamin B12 deficiency: why the standard value lulls you into a false sense of security
- Measuring the Omega-3 Index: the one blood value hardly any doctor checks
- Vitamin D overdose: where covering your needs ends and high-dose therapy begins
- Too much vitamin B6: why of all things a vitamin can irritate the nerves
- Minerals in balance: zinc, copper, selenium and why single supplements can tip the balance
- Folate instead of folic acid: what lies behind MTHFR and methylation
- All articles in the Supplement Guide
Related topics
Supplements
Nutrients, lab tests and well-founded supplementation
This articleIron
Ferritin, storage values and fatigue
Thyroid
TSH, hormones and nutrients
Nutrition
The plate as the first source
Sources
All studies were researched via PubMed and checked against the PubMed metadata (title, authors, year, journal, DOI). Key papers were additionally cross-checked via Consensus, and individual details via the full texts in PubMed Central. The study type is given in square brackets.
- Knopf H. Self-medication with vitamins, minerals and food supplements in Germany: Results of nationwide health surveys. Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz. 2017;60(3):268-276. DOI: 10.1007/s00103-016-2500-y [Real-world, cross-sectional, DEGS1, n=7,091, article in German]
- Bailey RL, Gahche JJ, Miller PE, Thomas PR, Dwyer JT. Why US adults use dietary supplements. JAMA Intern Med. 2013;173(5):355-361. DOI: 10.1001/jamainternmed.2013.2299 [Real-world, cross-sectional, NHANES, n=11,956]
- Jungert A, Quack Lötscher K, Rohrmann S. Vitamin Substitution Beyond Childhood: Requirements and Risks. Dtsch Arztebl Int. 2020;117(1-2):14-22. DOI: 10.3238/arztebl.2020.0014 [Review, with data from the German National Nutrition Survey II]
- Bailey RL, Fulgoni VL, Keast DR, Dwyer JT. Dietary supplement use is associated with higher intakes of minerals from food sources. Am J Clin Nutr. 2011;94(5):1376-1381. DOI: 10.3945/ajcn.111.020289 [Real-world, cross-sectional, NHANES, n=8,860]
- Bailey RL, Fulgoni VL, Keast DR, Dwyer JT. Examination of vitamin intakes among US adults by dietary supplement use. J Acad Nutr Diet. 2012;112(5):657-663. DOI: 10.1016/j.jand.2012.01.026 [Real-world, cross-sectional, NHANES, n=8,860]
- O'Connor EA, Evans CV, Ivlev I, Rushkin MC, Thomas RG, Martin A, Lin JS. Vitamin and Mineral Supplements for the Primary Prevention of Cardiovascular Disease and Cancer: Updated Evidence Report and Systematic Review for the US Preventive Services Task Force. JAMA. 2022;327(23):2334-2347. DOI: 10.1001/jama.2021.15650 [Systematic review, 84 studies, n=739,803]
- Manson JE, Cook NR, Lee IM et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. N Engl J Med. 2019;380(1):33-44. DOI: 10.1056/NEJMoa1809944 [RCT, VITAL, n=25,871]
- Sesso HD, Rist PM, Aragaki AK et al. Multivitamins in the prevention of cancer and cardiovascular disease: the COcoa Supplement and Multivitamin Outcomes Study (COSMOS) randomized clinical trial. Am J Clin Nutr. 2022;115(6):1501-1510. DOI: 10.1093/ajcn/nqac056 [RCT, COSMOS, n=21,442]
- Chen F, Du M, Blumberg JB et al. Association Among Dietary Supplement Use, Nutrient Intake, and Mortality Among U.S. Adults: A Cohort Study. Ann Intern Med. 2019;170(9):604-613. DOI: 10.7326/M18-2478 [Cohort, NHANES, n=30,899]
- LeBoff MS, Chou SH, Ratliff KA et al. Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults. N Engl J Med. 2022;387(4):299-309. DOI: 10.1056/NEJMoa2202106 [RCT, VITAL ancillary study, n=25,871]
- Martineau AR, Jolliffe DA, Hooper RL et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. DOI: 10.1136/bmj.i6583 [Meta-analysis, individual participant data from 25 RCTs, n=10,933]
- Jolliffe DA, Camargo CA, Sluyter JD et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol. 2021;9(5):276-292. DOI: 10.1016/S2213-8587(21)00051-6 [Meta-analysis, 46 RCTs, n=48,488 analyzed]
- Vaucher P, Druais PL, Waldvogel S, Favrat B. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ. 2012;184(11):1247-1254. DOI: 10.1503/cmaj.110950 [RCT, n=198]
- The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group. The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. N Engl J Med. 1994;330(15):1029-1035. DOI: 10.1056/NEJM199404143301501 [RCT, ATBC, n=29,133]
- Klein EA, Thompson IM, Tangen CM et al. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011;306(14):1549-1556. DOI: 10.1001/jama.2011.1437 [RCT, SELECT, n=35,533]
- Stranges S, Marshall JR, Natarajan R et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007;147(4):217-223. DOI: 10.7326/0003-4819-147-4-200708210-00175 [RCT, secondary analysis, n=1,202]
- Burt LA, Billington EO, Rose MS, Raymond DA, Hanley DA, Boyd SK. Effect of High-Dose Vitamin D Supplementation on Volumetric Bone Density and Bone Strength: A Randomized Clinical Trial. JAMA. 2019;322(8):736-745. DOI: 10.1001/jama.2019.11889 [RCT, n=311]
- Feskanich D, Singh V, Willett WC, Colditz GA. Vitamin A intake and hip fractures among postmenopausal women. JAMA. 2002;287(1):47-54. DOI: 10.1001/jama.287.1.47 [Cohort, Nurses' Health Study, n=72,337]
- Schaumburg H, Kaplan J, Windebank A et al. Sensory neuropathy from pyridoxine abuse. A new megavitamin syndrome. N Engl J Med. 1983;309(8):445-448. DOI: 10.1056/NEJM198308253090801 [Case series, n=7]
- Adams PC, Reboussin DM, Barton JC et al. Hemochromatosis and iron-overload screening in a racially diverse population. N Engl J Med. 2005;352(17):1769-1778. DOI: 10.1056/NEJMoa041534 [Cohort, HEIRS screening study, n=99,711]
- Rabenberg M, Scheidt-Nave C, Busch MA, Rieckmann N, Hintzpeter B, Mensink GB. Vitamin D status among adults in Germany: results from the German Health Interview and Examination Survey for Adults (DEGS1). BMC Public Health. 2015;15:641. DOI: 10.1186/s12889-015-2016-7 [Real-world, cross-sectional, DEGS1, n=6,995]
- Harris WS, von Schacky C. The Omega-3 Index: a new risk factor for death from coronary heart disease? Prev Med. 2004;39(1):212-220. DOI: 10.1016/j.ypmed.2004.02.030 [Review, validation paper with laboratory and study data]
- Costello RB, Elin RJ, Rosanoff A et al. Perspective: The Case for an Evidence-Based Reference Interval for Serum Magnesium: The Time Has Come. Adv Nutr. 2016;7(6):977-993. DOI: 10.3945/an.116.012765 [Review, expert perspective]
- Anker SD, Comin Colet J, Filippatos G et al. Ferric carboxymaltose in patients with heart failure and iron deficiency. N Engl J Med. 2009;361(25):2436-2448. DOI: 10.1056/NEJMoa0908355 [RCT, FAIR-HF, n=459]
- Allen RP, Picchietti DL, Auerbach M et al. Evidence-based and consensus clinical practice guidelines for the iron treatment of restless legs syndrome/Willis-Ekbom disease in adults and children: an IRLSSG task force report. Sleep Med. 2018;41:27-44. DOI: 10.1016/j.sleep.2017.11.1126 [Consensus guideline, IRLSSG]
- Li D, Radulescu A, Shrestha RT et al. Association of Biotin Ingestion With Performance of Hormone and Nonhormone Assays in Healthy Adults. JAMA. 2017;318(12):1150-1160. DOI: 10.1001/jama.2017.13705 [In vitro, lab assay testing after biotin intake, n=6, 37 test methods]