“Vitamin X” is not an officially recognized vitamin. Unlike vitamins A through K, which have established roles in human nutrition, “vitamin X” is an informal label that has been applied to several different compounds over the years, most commonly pyrroloquinoline quinone (PQQ) and ergothioneine. These are naturally occurring substances found in food that some researchers believe may deserve vitamin status but haven’t yet earned that classification.
Where the Term Comes From
The idea of undiscovered or unclassified vitamins has circulated in nutrition science for decades. In 2018, biochemist Bruce Ames published an influential paper in the Proceedings of the National Academy of Sciences proposing a new category called “longevity vitamins,” compounds that aren’t essential for short-term survival but may play a role in long-term health and aging. His list included ergothioneine (a fungal antioxidant), PQQ (a compound made by bacteria), queuine (another bacterial metabolite), and seven plant-based carotenoids like lutein, lycopene, and astaxanthin. Other candidates he mentioned include lipoic acid, ubiquinone, and carnitine.
None of these have been formally classified as vitamins by any regulatory body. The term “vitamin X” floats around in supplement marketing and health blogs as a placeholder, most often referring to PQQ. It’s a catchy label, but it can mean different things depending on who’s using it.
PQQ: The Most Common “Vitamin X”
PQQ is a small molecule that bacteria produce naturally. It acts as a cofactor, helping enzymes carry out chemical reactions, similar to how B vitamins function. Your body doesn’t make PQQ on its own, and it’s found in trace amounts in many foods. Proponents argue this makes it vitamin-like: essential, externally sourced, and involved in fundamental cellular processes.
The compound has drawn interest because of its potential role in mitochondrial function, the energy-producing machinery inside your cells. Animal studies suggest PQQ may stimulate the creation of new mitochondria, which could theoretically support energy metabolism and protect against age-related decline. That said, the leap from animal research to proven human benefits is significant, and the evidence in people remains thin.
Foods That Contain PQQ
PQQ exists in small quantities across a wide range of foods. The amounts are measured in micrograms per 100 grams, which gives you a sense of just how little is present. Fermented soybean products (natto) top the list at about 6.1 micrograms per 100 grams. Parsley comes in at 3.4, green tea at around 2.9, and kiwi and papaya each at 2.7. Other sources include potatoes (1.7), broad beans (1.8), bananas (1.3), spinach (0.7 to 2.2), and miso (1.7). Even wine and beer contain trace amounts.
Unpasteurized bovine milk contains a surprisingly high concentration, roughly 13 micrograms per 100 milliliters, though this drops dramatically in processed dairy products like skim milk powder (about 0.25 micrograms). The overall dietary intake from a varied diet is extremely small, typically in the low microgram range per day.
What the Human Research Actually Shows
Claims about PQQ supplements tend to focus on brain health, heart health, and anti-aging. The reality is more modest. The few human trials conducted so far have been small and short-term. There is no strong clinical evidence that PQQ supplementation improves cognitive function, reduces cardiovascular risk, or extends lifespan in humans. Most of the promising findings come from cell cultures and animal models, which don’t reliably predict what happens in people.
This stands in contrast to well-established vitamins like the B vitamin family, where the roles are clearly defined. Thiamin (B1) is essential for energy metabolism in the brain and muscles. Riboflavin (B2) powers the cellular machinery that converts food into usable energy. Niacin (B3) produces a molecule critical for the same process. These are vitamins because without them, specific deficiency diseases develop. No such deficiency syndrome has been identified for PQQ.
Safety and Supplement Dosing
PQQ disodium salt has received “no objection” letters from the FDA through the Generally Recognized as Safe (GRAS) process, meaning it can legally be sold as a dietary ingredient in the United States. Multiple manufacturers have submitted GRAS notices, and independent expert panels have agreed the compound is safe under intended conditions of use.
The maximum suggested dosage in supplements is 20 milligrams per day for adults. In the limited human studies that exist, no side effects were reported during short-term use, and there are no documented cases of allergic reactions or overdose. However, there is essentially no data on long-term safety. Interactions with medications or other supplements haven’t been studied, and PQQ is not recommended for people who are pregnant or breastfeeding due to the lack of safety data in those groups.
Why It Hasn’t Become a Real Vitamin
For a compound to be classified as a vitamin, scientists need to show that humans require it for normal biological function and that a deficiency causes a recognizable disease. Vitamin C deficiency causes scurvy. Vitamin D deficiency causes rickets. No equivalent condition has been linked to PQQ deprivation in humans. The same applies to ergothioneine and the other “longevity vitamin” candidates.
That doesn’t mean these compounds are useless. It means the bar for vitamin status is high, and none of these substances have cleared it. They may turn out to have meaningful health benefits at supplemental doses, or they may not. The research simply isn’t mature enough to say. For now, “vitamin X” remains a marketing term rather than a scientific one, and the best way to get PQQ and similar compounds is through a varied diet rich in vegetables, fruits, fermented foods, and tea.