“Expensive urine” has become a convenient way of dismissing vitamin supplements taken in doses beyond what our bodies have an immediate need for. Vitamin C is water-soluble, and some of what enters the bloodstream is eliminated by the kidneys. However, that explanation tells us little about the portion that never gets absorbed, or what might happen to it as it continues through the intestinal tract.
The National Institutes of Health estimates that approximately 70 to 90 percent of vitamin C is absorbed at moderate intakes of 30 to 180 milligrams per day. At doses exceeding one gram, absorption falls below 50 percent. Some of that unabsorbed vitamin C remains in the gastrointestinal tract, where it may eventually encounter the enormous communities of microorganisms living in our intestines. This raises an interesting question about whether the biological activity of vitamin C extends beyond what our own cells absorb and use.
Vitamin C participates in several essential physiological processes, including collagen synthesis, immune function, and the absorption of iron from plant foods. Research has consequently focused on how it is transported into tissues, how much is used within those tissues, and what happens when we consume a surplus. But intestinal microbes also respond to the nutrients and chemical conditions around them, and several small human studies suggest that vitamin C can influence those communities.
Vitamins direct to colon
In a 2021 study published in Gut Microbes, researchers investigated whether delivering different vitamins directly to the colon could influence the composition and metabolic activity of the human gut microbiome. The randomized, placebo-controlled trial included 96 healthy participants across several vitamin and placebo groups. Those receiving colon-targeted vitamin C showed increased microbial alpha diversity, a measure that accounts for both the richness and evenness of microorganisms within a sample. Researchers also observed significantly higher fecal concentrations of the short-chain fatty acids propionate and butyrate compared with placebo.
Short-chain fatty acids, including acetate, propionate, and butyrate, are largely produced when intestinal bacteria ferment available substrates. These compounds have biological functions that extend beyond the intestine. Butyrate, for example, serves as an important energy source for cells lining the colon, whereas acetate has been shown to enhance the bactericidal activity of alveolar macrophages in mice, improving their ability to defend against secondary bacterial infections following influenza.
More recently, a 2026 study published in Cell Metabolism found that propionate enhanced MHC class I antigen presentation on tumor cells, promoting their recognition by CD8+ T cells and improving responses to anti-PD-1 immunotherapy in experimental models. Although these findings come from separate investigations and do not establish whether similar effects would produce clinically meaningful benefits in humans, they illustrate the potential biological significance of changes in intestinal concentrations of these microbial metabolites.
Important qualifications
There are, however, some important qualifications regarding the aforementioned Gut Microbes study. Of the 96 participants, only 12 received vitamin C alone, and the researchers used a specially formulated supplement designed to release its contents directly into the colon. We therefore cannot assume that an ordinary vitamin C tablet would produce the same effects.
Moreover, although short-chain fatty acids produced in the intestine are known to enter systemic circulation, the higher fecal concentrations observed in this trial do not necessarily indicate that more of these metabolites entered the bloodstream. The amounts recovered in stool depend on several factors, including microbial production, intestinal absorption, and transit.
The researchers also did not establish whether gut bacteria were directly metabolizing vitamin C or responding indirectly to changes in their environment, nor did they determine whether the observed microbial changes produced any measurable benefits to participants’ health.
Daily Vitamin C
Another 2021 pilot study examined what happened when 14 healthy adults took 1,000 milligrams of ordinary vitamin C daily for two weeks. Researchers observed changes in several bacterial populations, including an increase in the relative abundance of Lachnospiraceae, a family containing organisms capable of producing short-chain fatty acids. Although these findings provide some support for an interaction between oral vitamin C supplementation and the microbiome, the study lacked a placebo group, and changes in bacterial abundance alone cannot tell us whether the resulting community is healthier or more beneficial to its host.
A more recent 2025 study published in Brain, Behavior, and Immunity examined microbial changes in 40 young adults with suboptimal vitamin C status who had participated in a randomized, double-blind, placebo-controlled trial. After four weeks of supplementation, researchers reported alterations in several bacterial groups, including a reduction in Desulfovibrio.
Interestingly, the extent of that reduction was associated with better performance on a cognitive task measuring attention and cognitive control. Vitamin C supplementation was also associated with reduced circulating lipopolysaccharide, a bacterial component capable of stimulating inflammatory responses. Among participants who experienced the greatest reductions in Desulfovibrio, researchers observed significantly greater decreases in IL-10 concentrations and the proportion of classical monocytes than among other vitamin C recipients. Although these findings suggest a possible relationship between changes in the gut microbiome and immune function, they do not establish that the microbial changes were responsible for those effects.
Incomplete picture
In summary, we may be evaluating some nutrients through an incomplete picture of what happens after they are consumed. Measuring how much vitamin C enters the bloodstream tells us something important about its availability to human tissues. It does not necessarily tell us everything about the effects of the vitamin that remains in the intestine. At the same time, detecting a change in the microbiome is only a starting point. Researchers still have to establish the mechanisms responsible, determine whether these effects occur at ordinary dietary intakes, and show that they produce meaningful outcomes for human health.
Researchers at the University of Glasgow have registered a study examining how daily doses of 200 and 1,000 milligrams of vitamin C affect gut microbial composition and short-chain fatty acid production. Its results are not yet available. However, the investigation may help address some of the questions raised by the existing research, particularly whether different doses of ordinary vitamin C produce distinct effects on the intestinal microbiome and its metabolic activity.
None of this establishes that consuming larger amounts of vitamin C necessarily produces additional health benefits, and the available evidence remains too preliminary to determine whether high-dose supplementation offers meaningful advantages for microbiome health. Nevertheless, the possibility that unabsorbed vitamin C can influence intestinal microorganisms raises questions that conventional measures of vitamin absorption and urinary excretion cannot fully address. The familiar argument about expensive urine may describe what happens to some excess vitamin C, but it does not necessarily account for everything that happens to the portion our bodies never absorb.
Yasin Ali Muhammad is a cell biologist, graduate researcher, and science writer. He earned his Bachelor of Science in Biology with a concentration in Molecular Genetics and Cell Biology and is currently pursuing an M.S. in Biology with a focus on Immunometabolism.
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