When supplements make sense, and when they get in the way

Take a look at your bathroom shelf. Or at that drawer in the kitchen. How many tins are standing there? And for how many of them could you say in one sentence why exactly you take them and when you will stop?

I ask this question almost every day in my consultations. It is not meant as a reproach. I ask it of myself in the same way.

The most common answer is: because it is supposed to be good. Because someone recommended it. Because I feel like I need something. Those are entirely understandable reasons. They are simply not a medical indication, and that is exactly where the topic starts to get interesting.

This text is the foundation article for everything that follows in this guide. It does not answer the question of which product is best. It answers the question before that: when can a capsule move something, when does it move nothing, and when does it move something in the wrong direction?

Your body has no warehouse, it has a feedback loop

Picture an old radiator with a thermostat. You turn the dial, it gets warmer. You keep turning, it does not get twice as warm. At some point the valve throttles back, and the rest of the energy goes into the wall.

Your body behaves in a similar way with almost every micronutrient. It has sensors, transporters and brakes. It decides at the gut wall how much gets in at all. It decides in the kidney how much goes back out. And it decides in the cell how much is needed right now.

That is homeostasis. Not a store, but a feedback loop.

A particularly clear example is iron. Your body has a hormone called hepcidin, which you can picture as a doorman. Hepcidin binds to ferroportin, the only known exit through which iron passes from the gut cells into the blood. Once hepcidin binds, that exit is broken down. Supply can dry up even though there is still iron sitting in the gut.

This principle does not apply to iron alone. In varying forms it applies to zinc, to calcium, to magnesium, to vitamin D. For each of these substances the body has feedback mechanisms that it does not like to have overridden from the outside.

And now you know why I ask about the feedback loop first with every product, and only then about the dose.

The large prevention trials: sobering, and easily oversimplified

You may have seen the headline: supplements do nothing. Sometimes it says: they even do harm. Both rest on real, large and well-conducted trials. And both fall short.

I will show you the numbers, because I believe you can place them in context.

It becomes even clearer in two individual trials that still echo through the field today.

With the classic multivitamin the picture looks somewhat different, though not euphoric either.

An evaluation of three large US cohorts with a combined 390,124 adults who had no cancer or chronic disease at baseline found no mortality advantage over up to 27 years for people who took a daily multivitamin. The US Preventive Services Task Force came to a similar conclusion in 2022 and clearly advises against beta carotene and vitamin E for cancer and cardiovascular prevention. For multivitamins it rates the evidence as insufficient to weigh benefit and harm against each other.

And now comes the part the headlines like to leave out. There are signals in the other direction too.

And now you know why, with headlines on this topic, my first question is always: who was actually studied here?

When one dial turns another

Now we come to the point I meet most often in daily practice and that gets discussed least.

You take something for A. And you move B along with it, without noticing.

Picture a mobile above a cot. You pull on one figure and all the others move. This is not a metaphor for some marginal phenomenon. It is the basic structure of mineral balance.

Zinc and copper: the best known pair

Zinc is one of the most popular supplements of all. It can move a lot when a deficiency has been documented. And it has a well-documented shadow.

An important qualification: these are case reports from the upper end of the dose scale, not a description of what happens with a moderate, time-limited course of zinc. But the mechanism is measurable at far lower amounts too. In a small study in adult women over ten weeks with 50 milligrams of zinc daily, both a copper-dependent enzyme in the red blood cells and ferritin fell in a before and after comparison. There was no placebo group, so the strength of the evidence is limited.

Iron and zinc: competing at the same door

Both metals partly use the same transport routes in the gut. A review from Chile gathered the human data on this. At a low iron dose the threshold for inhibition was at a zinc to iron weight ratio of about 5.9 to 1, and at a higher iron dose already at 1 to 1. Interesting detail: in real meals this effect was often not detectable. The competition shows up mainly when both arrive together, dissolved and on an empty stomach.

Calcium, magnesium and vitamin D: a triangle

This is where it gets interesting, because the three meet not only in the gut but throughout metabolism. Magnesium is a cofactor in the activation of vitamin D, in its transport and in its breakdown. A review in Advances in Nutrition describes how calcium intake in the United States rose considerably faster than magnesium intake between 1977 and 2012. The calcium to magnesium ratio there is now above 3 to 1, while the authors consider a ratio of around 2 to 1 more favourable.

They phrase the consequence carefully, and I join them in that caution: whoever supplements calcium and vitamin D while magnesium intake is low is moving a triangle and not just one corner. How relevant this is clinically has so far been studied directly far too rarely.

Folic acid and vitamin B12: the quietest antagonism

This point is one of the most important and at the same time professionally contested. So I will explain both sides.

The classic concern goes like this: during a simultaneous B12 deficiency, folic acid can make the blood count look unremarkable again while neurological damage continues in the background. The deficiency is then discovered later. This idea shaped the regulatory rules for folic acid for decades.

A detailed review from 2020 worked through the literature from 1945 to 2017 and disagrees in part. The authors argue that the separation between haematological and neurological signs of a B12 deficiency is not well explained by the masking model. They consider the upper limit of one milligram of folic acid per day to be safe.

Selenium: the window is narrower than many people think

Selenium is a prime example of the fact that with trace elements a deficiency is not the only problem. Too little can do harm, too much as well. Specialists speak of a U-shaped curve.

And now you know why it makes me uneasy when someone takes five single-nutrient products in parallel without a single measurement behind them.

Covering requirements and high-dose therapy are two different worlds

Here lies a mix-up that creates a lot of confusion. And it runs in both directions.

On one side there is supplementation in the narrow sense. It covers requirements. It moves in the range of the official reference values. It fills gaps. No therapeutic effect is to be expected from it, and that is not a put-down, it is a description.

On the other side there is therapeutic high-dose use, often called orthomolecular medicine. Here a nutrient is used like a medication. In a clearly higher dose, targeted, for a limited time, under medical supervision and with laboratory monitoring.

This second world is no esoteric fringe. Conventional medicine uses it every day. Thiamine in a high dose in Wernicke encephalopathy. Zinc in a high dose in Wilson disease. Folinic acid as a rescue after methotrexate. Niacin in gram doses in lipidology. Vitamin B6 in certain inherited enzyme defects. In all of these cases the same applies: targeted, supervised, time-limited.

The sharpest example of this difference is vitamin D. I describe it in detail here on purpose, because the whole principle becomes visible in it.

To cover requirements, the German Nutrition Society names a reference value of 800 International Units per day for the case where the body forms no vitamin D of its own. The European Food Safety Authority names 4,000 International Units per day as the tolerable upper intake level for adults. Over-the-counter products move in that order of magnitude. Which amount fits you belongs in the conversation with your doctor.

In the so-called Coimbra protocol for multiple sclerosis and other autoimmune diseases, by contrast, doses in the range of tens of thousands up to over 100,000 International Units daily are used. That comes with a strict calcium restriction, a high fluid intake and close monitoring of calcium in serum and urine, parathyroid hormone and kidney values.

How quickly the other direction tips over is shown by a review on vitamin D intoxication. Confusion, apathy, repeated vomiting, abdominal pain, intense thirst and increased urination are among the described signs. As a marker of overdosing the authors name serum levels above 150 nanograms per millilitre. For context: the usual supply range sits far below that, mostly between 30 and 60 nanograms per millilitre. A value clearly above that is not a safety buffer, it is a reason to have the dose reviewed medically.

And an Australian trial shows particularly vividly that more is not better even when the total amount is right.

A similar picture emerges with vitamin B6. The exact threshold is disputed, and honesty requires saying so. A review in an American journal describes a narrow therapeutic range, yet explicitly considers the link between raised pyridoxine levels and nerve damage to be poorly established. It recommends the activated form pyridoxal-5-phosphate and less frequent rather than daily dosing. Alongside this there are case reports in which gait disturbances improved after stopping an over-the-counter product containing B6.

And now you know why I never skip the question of dose, not even with a vitamin that has a reputation for being harmless.

Real food first, and not as a platitude

If you have read this far, you can already sense where this is going.

Nutrient supply from natural, real foods of good origin and good husbandry is always the first choice. That is not a romantic sentence. It has a physiological reason.

A food never gives you a nutrient on its own. It gives it to you embedded in a matrix. In fibre, in fats that make the absorption of fat-soluble vitamins possible, in plant compounds, in other minerals in a ratio that grew that way. This matrix helps determine how much arrives and how quickly.

This is probably where part of the explanation for the disappointing antioxidant trials lies. Observational data show a link between plenty of fruit and vegetables and fewer diseases. When a single substance is then isolated from that and given in a high dose, it behaves differently. Beta carotene is the best known example.

I do not say this to lecture you. I say it because I keep meeting people who spend a lot of money on products while sleeping five hours. That order of priorities costs strength and money at the same time.

And now you know why in my consultation the question about breakfast often comes before the question about the product.

And why targeted topping up can be more often justified today

At this point you might think: so just eat well and everything is fine. Unfortunately it is not that simple, and this section matters to me.

There are four developments that have changed the equation. I present them with their limits, not as a scandal story.

First: soils, cultivars and harvest times have changed

I show you this study with its limitations, because in advertising copy it is often quoted as proof of empty soils. It does not support that. The comparability of analytical methods over 50 years is methodologically contested, and part of the effect is explained by the dilution effect in high-yield cultivars. What it does support is a hint: the assumption that an apple today is the same apple as in 1950 is not a given.

Second: the share of heavily processed foods is high

Ultra-processed products deliver energy reliably. With micronutrient density it looks different.

Third: environmental exposure and chronic stress can raise consumption

Heavy metals, mould toxins, plasticisers and pesticide residues are present in the environment. The body meets them with detoxification and protection systems. These systems work with cofactors such as zinc, selenium, magnesium, glutathione precursors and B vitamins.

The same could apply to chronic stress. A permanently active stress axis can raise the requirement for cofactors and increase excretion through the kidney. Solid human studies that put a number on this extra requirement are, however, scarce. What I describe here is a physiological consideration, not a documented effect.

Fourth: medications can deplete nutrients

This is the best documented of the four points, and of all things the one discussed least.

Further examples are discussed in the literature: diuretics and potassium or magnesium, statins and coenzyme Q10, hormonal contraception and B6, B12, folate and zinc. The strength of the data varies considerably.

And now you know why in every new conversation I ask about the medication list first.

The four lenses: why a nutrient never plays just one part

In clinical psychoneuroimmunology we look at a symptom through four lenses. With micronutrients that is not a detour, it is simply cell biology.

The reason is simple: most micronutrients are not active agents, they are cofactors. They do nothing themselves. They make it possible for enzymes to do something. And these enzymes sit in quite different systems at the same time.

These four lenses explain why the same nutrient turns up in quite different guides. With sleep, with exhaustion, with hair loss, with mood, with susceptibility to infection. Not because it is good for everything. But because a cofactor that runs along in many enzyme reactions can make itself noticed in many places when it runs short.

And they explain the flip side. When you pull on one lens, the others move with it. That is exactly the reason for the antagonisms from section three.

Three levers you can put into practice today

No protocol, no brands, no dosing recommendation. Just three things that lie in your hands and cost nothing.

Before that, the three questions I ask myself with every single product. They are the core of this whole article.

And now you know why my answer to the question of whether supplements make sense begins with a question back: sensible for what, in whom, and for how long?

Common questions about supplements and homeostasis

How this topic connects to the others

This article is the starting point. If you want to go deeper at a particular place, these paths lead onward.