Somewhere between 43-63% of people find their urine has a strong, unusual smell after they eat asparagus. Have you ever wondered why that is? In the late 1980s, researchers established that this pungent odor is caused by a combination of several sulfur-containing compounds — including compounds that we now know to be highly toxic, such as the neurotoxin dimethyl sulphoxide (DMSO).

DMSO is produced as a natural byproduct of human metabolism of asparagusic acid, a mildly poisonous constituent of asparagus that appears to be useful to the plant in warding off insects, parasites, and nutrient competition from other nearby plants. In one case study, a participant who ate 500g of asparagus was found to have over half a milligram of DMSO in their urine and another was found to have over 4mg of derivative sulfur-containing volatiles — suggesting that although DMSO may eventually be metabolized into other products within the body, there could still be well over 4mg of DMSO present in the body at any one time, for several hours after ingesting asparagus.

Unfortunately, the fact that asparagus naturally metabolizes into DMSO within the body and causes “stinky urine” was seen as a mere curiosity 30 years ago. Back then, DMSO was generally recognized as safe and routinely used as a synthetic medical solvent.

However, in recent years, researchers have found that as little as 0.000015mg of DMSO is enough to permanently destroy neurons in rats. And slightly more DMSO was enough to severely brain-damage young mice. To put this in perspective, a sub-group of mice in this DMSO exposure study (n=15) were given large doses of PCP in order to compare the amount of brain damage caused versus DMSO. PCP is a known neurotoxin that quickly causes devastating and permanent brain damage. The researchers found the expected amount of brain damage from PCP, but the effects of DMSO were both more severe and more widespread. This is especially remarkable considering that the researchers used 5x as much PCP in order to achieve less brain damage than with DMSO. This naively implies that eating 500g of asparagus could cause over 4x as much brain damages as one recreational dose of PCP. Further research has shown that DMSO damages astrocytes and other brain cells that normally survive more common neurotoxins like alcohol or ketamine. Scientists familiar with these results have gone so far as to suggest that DMSO should perhaps replace PCP as the standard compound for inducing brain damage in research animals.

These findings are part of a growing mosaic of evidence that has convinced some countries (like Australia) to outright ban commercial use of DMSO — although it’s still widely used in places like the United States. And although it began it’s life as a medical solvent, DMSO is now more widely used as an industrial solvent. For example, DMSO efficiently dissolves photoresist off LCD-panels during fabrication and is also a highly effective paint stripper.

As an interesting side-note: One of the many groups of ex-NFL players who brought brain damage lawsuits against the NFL were also recently revealed to have habitually abused large doses of DMSO acquired from horse veterinarians[!]

So is eating asparagus as bad as drinking paint stripper, playing in the NFL, or doing PCP? It’s hard to say. But it may still be prudent to exercise restraint around asparagus if you care about your brain, the same way one might exercise restraint around large doses of alcohol, PCP, or ketamine. Since asparagus ingestion creates neurotoxins that must be at least partially cleared by the kidneys, those with poor renal function should almost certainly avoid asparagus. And of particular concern, parenting guides often suggest introducing asparagus into a child’s diet at 8 months. Obviously, this advice is simply outdated and not informed by these new connections in the literature between asparagus metabolism and modern day knowledge of DMSO toxicity. But just to be clear, letting a child eat asparagus would be exceedingly reckless at this time, given that DMSO is a known developmental neurotoxin and eating asparagus has been documented to cause DMSO exposure 40,000x higher than the amount shown to kill neurons.

And for those skeptical that humans could be dumb enough to eat something that retards brain function for thousands of years, one reference suggested that the pungency of asparagus, as well as garlic and onions have all increased significantly over the past 50 years as sulfur-based fertilizers have gone from being nonexistent —> ubiquitous. So even though humans have eaten asparagus for millennia, the kind of asparagus that is commonly available today likely has a much greater asparagusic acid content than asparagus from even 60 years ago (and would therefore be much more neurotoxic). It would be valuable if a biologist or chemist could assay asparagus that was grown in a sulfur-depleted region vs more conventionally grown stock to confirm if this speculation is correct.

Also, for those wondering if perhaps eating asparagus is safe for those who don’t seem to produce “stinky urine” after ingesting asparagus I would offer two points of caution:

1) The ability to smell asparagus-urine seems to be an independent trait that is almost completely uncorrelated with producing it. So not smelling a foul odor in your own urine is only ~50% evidence that you are a “non-excreter”. If you actually want to know, a better source of evidence is finding someone who knows they can smell asparagus-urine and having them check your urine roughly an hour after you eat a large meal of asparagus.

2) It is not entirely clear that “non-excreters” aren’t producing the same metabolites and simply excreting them differently (or not at all). Scientists have suggested that the human metabolic system could potentially degrade DMSO into carbon dioxide and inorganic sulphate, at which point it could be removed via standard respiration. But this is only a theorized reaction, and there is currently no data on whether asparagus “non-excreters” fail to absorb the asparagusic acid, fail to metabolize it, excrete the products by some other process, or fail to excrete the DMSO and other toxic byproducts of asparagus ingestion altogether. Urgent study is required to better understand how asparagus is metabolized in humans so as to determine exactly how neurotoxic it is and what factors contribute to this toxicity.