The short answer
Your body has three ways to part with a nutrient it does not need: the kidney, the gut, or no exit at all. Water-soluble vitamins that pass a kidney threshold leave in urine, minerals such as zinc and copper leave through bile and intestinal secretions, and iron has no regulated exit, so it is kept and stored.
Where does a nutrient actually go after it is absorbed?
It is used, stored, or routed out through one of two exits: the kidney or the gut. The absorbed fraction does not simply sit in the blood. In one study, the most riboflavin a single oral dose could deliver was 27 mg [1], and the kidney accounted for only half of its removal from plasma [1].
Three fates are possible after absorption: a nutrient is used, stored, or excreted, and excretion happens by one of two physical routes, urine or faeces. Which route applies depends on the nutrient, not on the brand on the bottle.
Why does riboflavin turn your urine bright yellow?
Riboflavin is a strongly coloured molecule, intensely yellow-green, and it is the B vitamin that colours urine. When you take a B-complex, part of the riboflavin passes through the body unchanged and leaves in urine, carrying its colour with it. In a pharmacokinetic study, urinary excretion accounted for about half of riboflavin's removal from plasma [1].
A randomised cross-over study in nine adults tested oral riboflavin at 20, 40 and 60 mg plus an intravenous dose of 11.6 mg [1]. The maximal amount absorbable from a single dose was 27 mg, with a half-life of absorption of 1.1 hours [1]; the kidney contributed only half of the removal from plasma, with metabolism and tissue uptake accounting for the rest [1].
Does bright urine mean the vitamin was wasted?
Not by itself. Excretion is not proof of waste: the colour shows that some riboflavin passed through, not that none of the dose was used. The same study put the ceiling for a single oral dose at 27 mg [1], and the kidney handled only half of the removal from plasma; tissues and liver metabolism handled the other half [1].
What is a renal threshold, and how is it different from an absorption ceiling?
A renal threshold is the point below which the kidney reclaims a nutrient from the blood. It is not the same as an absorption ceiling. In the vitamin C trial, no vitamin C appeared in the urine of six of seven volunteers until the 100-mg daily dose [2]; below that point, the kidney took nearly all of it back.
In that study, seven volunteers were hospitalised for 4 to 6 months on less than 5 mg of vitamin C daily, then given doses from 30 mg to 2500 mg [2]. Plasma saturated completely at 1000 mg daily; at single doses of 500 mg and higher, bioavailability declined and the absorbed amount was excreted [2]. Immune cells saturated at 100 mg daily and held concentrations at least 14 times higher than plasma [2].
Thiamine shows the ceiling side of the contrast: in a four-way cross-over study of single doses of 100, 500 and 1500 mg, blood levels rose nonlinearly, steepest at the lowest doses, because thiamine is absorbed by both an active and a nonsaturable passive process [11]. An absorption ceiling is set at the gut wall and caps how much gets in from one dose; a renal threshold is set in the kidney and caps what the blood keeps.
Which nutrients does your body actually store, and where?
Your body stores vitamin D in adipose tissue, B12 in the liver, and iron in dedicated storage proteins. The size of a store does not show in a blood value. One study measured serum 25-hydroxyvitamin D of 27 ± 2 versus 26 ± 2 ng/mL, while total body vitamin D stores were 2.3 ± 0.6 mg versus 0.4 ± 0.8 mg [3].
The researchers measured vitamin D directly in fat tissue in 36 women, obese and normal weight [3]. Serum values were similar (p = 0.71), yet stores were significantly greater in the obese group (p less than 0.01); the authors call the enlarged adipose mass a reservoir [3]. Vitamin D contributes to the maintenance of normal bones, and surplus is banked rather than spilling, which is why our Vitamin D3 + K2 drops are not a daily product.
Why does vitamin D behave like body fat?
Because it dissolves into body fat and stays there instead of leaving. Vitamin D is fat-soluble, so absorbed amounts partition into adipose tissue, which the study's authors describe as a reservoir [3]. Unlike riboflavin, it has no fast renal exit, so a surplus adds to the store rather than colouring the toilet. Blood is the corridor; fat is the warehouse.
Why is vitamin B12 not flushed out like other water-soluble vitamins?
Because it is recycled, not discarded. B12 is taken up in the distal ileum by the receptor formed by cubilin and amnionless, after intrinsic factor hands it over from haptocorrin in the duodenum [5]. Part of liver B12 is excreted in bile and undergoes an enterohepatic circulation: much of what leaves is picked up again [5].
Water-soluble describes the solvent a vitamin dissolves in, not the body's policy toward it. B12 is water-soluble and still the opposite of disposable [5].
Which nutrients leave through the gut instead of the kidney?
Zinc, copper and manganese leave mainly through the gut, in bile and in secretions back into the intestine. The clearest data are for zinc: women eating 5.2 ± 0.2 mg of zinc per day lost 1.30 ± 0.07 mg of endogenous zinc into faeces daily, versus 2.34 ± 0.20 mg in women eating 8.1 ± 0.2 mg [4].
Copper has a working tap at both ends: absorption ranges between 12 and 60 percent depending on intake and copper status, and biliary excretion is regulated too [6]. Manganese is exported by a dedicated transporter, Slc30a10, which sits on the bile-facing side of liver cells; mutations in the same gene cause an inherited human manganese excess first identified in 2012 [7].
How does your body dial zinc losses up and down?
By cutting what it secretes back into the intestine, not by absorbing more. In the study above, fractional absorption was unchanged between the marginal and control groups (0.31 ± 0.03 versus 0.34 ± 0.03, p = 0.45), while endogenous faecal losses fell significantly (p less than 0.001) [4]. The dial sits on the exit, not the entrance.
The study used stable isotopes, 70Zn given intravenously and 67Zn orally, with complete faecal collection [4]. Zinc leaves through the gut, not the urine. Zinc also contributes to the normal function of the immune system, which is one reason we offer it as zinc picolinate capsules.
Why can your body not simply get rid of surplus iron?
Because it has no regulated exit. The kidney filters circulating iron and reabsorbs it almost completely in the tubules, in the words of one paper, to prevent urinary iron wasting [8]. Instead of an exit, the body controls the entrance: the hepcidin-ferroportin axis sets how much iron moves from cells into the blood [9]. Surplus iron is stored, not disposed of.
The reabsorption finding comes from 20 healthy subjects, 20 patients with systemic iron overload and 18 patients with renal tubular dysfunction [8]. Systemic iron homeostasis runs through the hepcidin-ferroportin axis [9]. This is also why chronic iron overload, whether from hereditary haemochromatosis or from regular transfusions, leads to progressive iron accumulation in tissue: with no route out, what enters is kept [10]. We do not sell iron; it appears here as a worked example.
Which exit does each nutrient use?
Vitamin C and riboflavin leave by the kidney; B12, copper and manganese leave by bile; zinc leaves by the gut; and iron has no exit at all. The table gathers each nutrient's main route, whether the body keeps a store, and what a surplus does. Nothing in it is a dosing instruction.
| Nutrient | Main exit | Is there a store? | What a surplus does |
|---|---|---|---|
| Vitamin C | Urine, above a threshold | No meaningful store | Appears in urine once the dose passes about 100 mg [2] |
| Riboflavin (B2) | Urine, about half of plasma removal [1] | No meaningful store | Colours the urine; single-dose absorption stops at about 27 mg [1] |
| Vitamin B12 | Bile, then largely recycled [5] | Held in the liver | Recycled rather than discarded [5] |
| Vitamin D | Not readily excreted | Adipose tissue [3] | Adds to the body store rather than leaving |
| Zinc | The gut, as endogenous faecal zinc [4] | No large store | Losses into the gut are turned up or down [4] |
| Copper | Bile [6] | Liver | Biliary excretion is regulated [6] |
| Manganese | Bile and the gut [7] | Liver | Exported by a dedicated transporter [7] |
| Iron | No regulated exit; filtered iron is reabsorbed [8] | Stored in the body | Accumulates rather than leaving [8][9] |
Two patterns stand out. The nutrients with renal exits are the ones people notice leaving. Iron is not disposed of the way vitamin C is: it is filtered, taken back and stored [8][9].
So is "expensive urine" true?
Partly, and only for nutrients with a renal exit. Urine does carry away riboflavin and vitamin C above the 100-mg daily threshold [2]. But zinc never appears in it [4], iron is pulled back out of it [8], and vitamin D is banked in fat [3]. Excretion is not proof of waste; it is one exit of three.
How much of a dose reaches the blood is covered separately in our article on how much of a supplement actually reaches your bloodstream.
Does any of this change how often you should take a supplement?
Yes. A nutrient that leaves through the kidney and has no store, such as riboflavin, fits a daily rhythm: 20 mg a day sits under the 27 mg single-dose ceiling [1]. A nutrient held in a large store, such as vitamin D [3], tolerates longer intervals. The exit and the store set the schedule, not the percentage on the label.
How long a supplement takes to do anything is the subject of our article on how long before a supplement actually does anything.
What does this mean for the labels on our own products?
Our Vitamin B-Complex carries 20 mg of riboflavin as riboflavin-5-phosphate, which is 1429 percent NRV, and yes, it will colour your urine. That is expected, not a fault. The dose sits under the measured single-dose absorption ceiling of 27 mg [1], and riboflavin contributes to normal energy-yielding metabolism, which is the authorised EU claim for it.
The same capsule carries 500 micrograms of vitamin B12 as methylcobalamin (20000 percent NRV), all eight B vitamins, and 50 mg each of choline and inositol, which have no EU NRV. For water-soluble nutrients, a high percentage is a statement about dose size, because the exit exists: surplus riboflavin leaves in urine, and B12 is recycled through bile [5].
For nutrients without a quick exit, a high percentage is a different kind of decision. Our Vitamin D3 + K2 drops provide 5000 IU (125 micrograms) of D3 plus 120 micrograms of K2 as MK-7, dosed at 0.25 ml every 3 days rather than daily, because vitamin D is stored instead of leaving. Zinc Picolinate Capsules provide 30 mg of elemental zinc, one capsule daily with a meal. Magnesium 7 in 1 provides 251 mg of elemental magnesium from seven forms in two capsules a day, and magnesium contributes to normal muscle function. EU reference intakes are set by EFSA for the population, not for an individual [12].
Frequently Asked Questions
Why is my urine bright yellow after a B vitamin?
Riboflavin (vitamin B2) is a strongly coloured molecule, intensely yellow-green, and part of an absorbed dose leaves in urine unchanged, carrying the colour with it [1]. Urinary excretion accounted for about half of riboflavin's removal from plasma, with metabolism and tissue uptake handling the rest [1]. The colour means riboflavin passed through you, not that the whole capsule did nothing.
Does excreting a vitamin mean I wasted my money?
Not necessarily. Excretion is not proof of waste: urine carries only what the blood could not hold or the kidney did not reclaim. Below the vitamin C threshold, six of seven volunteers excreted none until the 100-mg daily dose [2]. What leaves is the surplus above what the body took, and whether it was worth buying depends on whether your diet needed the top-up.
Which vitamins can build up in the body?
The fat-soluble ones and the minerals with storage forms. Vitamin D partitions into adipose tissue, which one study describes as a reservoir [3]. Vitamin B12 is held in the liver and recycled through bile [5]. Iron is stored and has no regulated exit, since filtered iron is reabsorbed almost completely [8][9]. Water-soluble vitamins with renal thresholds do not build up.
Is it better to take a small dose every day or a large dose once a week?
It depends on the exit and the store. Riboflavin behaves like a daily nutrient: the ceiling for one dose is 27 mg, and the surplus leaves in urine [1]. Vitamin D is banked in fat [3], which is why our D3+K2 drops are taken every 3 days rather than daily. Match the rhythm to the nutrient, not to habit.
Can drinking more water flush out a supplement faster?
The evidence in this article does not answer that. What the studies show is that the renal threshold is set by the kidney's transport capacity and by the dose: below 100 mg per day, vitamin C was fully reclaimed in six of seven volunteers [2]. The deciding factor is the dose relative to the threshold, not the volume of water you drink.
The Bottom Line
A nutrient your body does not use has three possible destinations: urine, gut, or a store. Riboflavin and vitamin C leave by the kidney, above limits of 27 mg per single dose and 100 mg per day respectively [1][2]. Zinc, copper and manganese leave through the gut. Vitamin D and B12 are banked. Iron has no exit at all [8].
The practical reading of any label follows from this. For nutrients with an exit, the surplus is shed; for nutrients with a store, or with no exit, it stays with you. Excretion is not waste, and storage is not automatically a virtue; both are chemistry you can know in advance.
This article describes physiology and is not medical advice, nor a guide to dosing. If you have a diagnosed condition or take medication, speak to your doctor or pharmacist before changing what you take.
Sources
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- Carrelli A, Bucovsky M, Horst R, et al. Vitamin D Storage in Adipose Tissue of Obese and Normal Weight Women. Journal of Bone and Mineral Research, 2017. PubMed 27542960
- Sian L, Mingyan X, Miller LV, Tong L, Krebs NF, Hambidge KM. Zinc absorption and intestinal losses of endogenous zinc in young Chinese women with marginal zinc intakes. American Journal of Clinical Nutrition, 1996. PubMed 8602591
- Gueant JL, Gueant-Rodriguez RM, Alpers DH. Vitamin B12 absorption and malabsorption. Vitamins and Hormones, 2022. PubMed 35337622
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- Mercadante CJ, Prajapati M, Conboy HL, et al. Manganese transporter Slc30a10 controls physiological manganese excretion and toxicity. Journal of Clinical Investigation, 2019. PubMed 31527311
- van Raaij SEG, Rennings AJ, Biemond BJ, et al. Iron handling by the human kidney: glomerular filtration and tubular reabsorption both contribute to urinary iron excretion. American Journal of Physiology, Renal Physiology, 2019. PubMed 30623722
- Galy B, Conrad M, Muckenthaler M. Mechanisms controlling cellular and systemic iron homeostasis. Nature Reviews Molecular Cell Biology, 2024. PubMed 37783783
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