'Pine Nut Syndrome' Leaves a Bitter Taste in Your Mouth—Literally

Bitter black coffee is one thing, but a bacon-and-egg sandwich shouldn’t taste like a mouth full of batteries. Recently, for several days, everything I ate was accompanied by a harsh metallic taste that negated all the usual pleasure of eating. And the culprit, it seems, was pine nuts.

At first, I had no idea why my tastebuds were suddenly so messed up. I Googled “persistent bitter taste” and weighed up the possibilities. Menopause and pregnancy were easy to rule out (I’m a dude), and I wasn’t undergoing chemotherapy, or taking lithium, antibiotics, or cardiac drugs. I had neither burning mouth, which is accompanied by a scalding sensation, nor dry mouth, which messes with salivary production. The bitter taste, it seemed, was caused by a handful of pine nuts I’d added to a pumpkin and feta pasta the night before.

Pine Nut Syndrome (PNS), also known as Pine Mouth, was first identified by the Belgian Poisons Center in 2001. In most instances, symptoms of PNS kick in after one to three days and last up to two weeks. The delayed onset makes it hard to diagnose, and easy to unwittingly imbibe more of the little bastards, perpetuating the problem.

The medical term for a distorted sense of taste is dysgeusia. It’s not quite as nightmarish as ageusia, which is the complete lack of taste, but it’s not much fun. Testament to the misery of PNS are the 572 members of the Facebook group Damn You Pine Nuts. They commiserate over symptoms that some victims claim can last up to ten months.

How exactly pine nuts cause PNS is not yet settled, but Dr Gregory Möller, Professor of Environmental Chemistry and Toxicology at the University of Idaho, has one of the most compelling theories. He posits that pinolenic acid stimulates the production of cholecystokinin, a hormone that slows the emptying of the stomach and causes more bile to be produced and released. Möller argues that the production of excess bile can cause a cross-wiring of the brain’s neurological bitter sensation of the tongue and the bitter taste receptors you never knew you had in your gastrointestinal tract. That’s right: Pine nuts might be making you ‘taste’’ what’s inside your digestive system, instead of what’s in your mouth. Vom emoji.

Möller first learned about PNS in 2010 when a journalist asked him about the phenomenon. “The reporter’s question clawed its way into my mind, and would not let go,” he says. “It was a ‘curious case’ just like in the Sherlock Holmes detective stories. I asked myself, ‘What would Sherlock do?’ in trying to solve the case of PNS. That reporter's question led me down a path of Holmesian deductive analysis.”

So which pine nuts cause PNS? The suspects initially included rancid nuts, raw nuts, nuts processed certain ways, and nuts from certain countries or trees. Rancid nuts weren’t to blame; pine nuts still well within their use-by dates caused PNS. It wasn’t processing, as even nuts eaten straight from pine cones could cause PNS. Cooked pine nuts, and even pine nuts used in pesto have also caused PNS. A large number of the guilty pine nuts came from China, but not all of them. The plot thickened.

Another researcher, food scientist Grace Hui Shan Tan, explored PNS in her MSc thesis at Wageningen University. She asked people to send her samples of pine nuts that had caused the bitter taste, and found volunteers crazy enough to eat them. Tan arrived at the conclusion that it was nuts from the Chinese white pine ( Pinus armandii) that caused PNS. There are over 100 sub-species of pine nuts, of which the World Health Organisation recognises 29 as edible. Pinus armandii—a smaller, pale beige pine nut with a dark tip —isn’t one of them.

Tan’s thesis aligns with what was happening in the pine nut market in 2008 and 2009, when a poor global crop of pine nuts saw prices skyrocket and new sources of nuts come onto the market, including pine nuts from China. The incidence of PNS spiked, leading to protests at supermarkets like Whole Foods in the US and Sainsbury’s in the UK, with many retailers deciding to stop sourcing Pinus armandii and Pinus massoniana (Chinese red pine). My own miserable experience was caused by pine nuts bought from the otherwise excellent Sanyuanli market in Beijing, China, where I live.

Dr Möller says that while Pinus armandii is the most likely offending subspecies of pine nuts, the data is not conclusive. The American Academy of Clinical Toxicology also hedges its bets, stating that “affected nuts would appear to include all, or some portion, of nuts harvested from species Pinus armandii, but could include nuts from other species.”

The China Chamber of Commerce of Imports and Exports of Foodstuffs, Native Produce, and Animal ByProducts (CCCFNA) says it has implemented measures to stop nuts from Pinus armandii being exported, but they could easily find their way into your shopping bag even if you’re living outside China. Peter Cassell, Press Officer at the US Food and Drug Administration (FDA) told me, “There aren't any current import alerts on pine nuts.” Labelling rules are lax, with most packages simply listing ‘pine nuts’, without mentioning the sub-species (or mix of sub-species), but Dr Möller is optimistic things will improve.

“Food fraud, food safety and food spoilage all along the local to global farm-to-table distribution network will be managed by blockchain data pipelines in the very near future”, he says. “Growers, food companies, market and governmental food safety authorities will have a new and very rapid tool to identify food quality problems as they happen. Until the arrival of blockchain agriculture and food data management, consumers will be relying on producers, importers, point-of-sale vendors, and governmental food safety authorities to manage and minimize the potential for inedible pine nuts to appear in the food system.”

That offers little solace for members of Damn You Pine Nuts, many of whom have sworn off ever eating pine nuts again. I’m not doing anything quite so bigoted as that, but I have become a pine nut profiler, steering clear of those smaller, pale beige nuts with a dark tip. They just aren’t worth it.