The 1987 Paper That Invented GABA Oolong Was About Blood Pressure
The tea sold today for stress relief and better sleep began as a 1987 Japanese food-science paper aimed at hypertension. That claim has real, if modest, human trial evidence behind it. The calm claim now on the label rests on thinner proof, and on a molecule no human study has shown reaches the brain at all.
In 1987, four food scientists at Japan's National Research Institute of Tea published a short paper on a new way to process leaf: starve it of oxygen for six to ten hours, and its gamma-aminobutyric acid content, GABA for short, climbs past 150 milligrams per 100 grams (Tsushida, Murai, Omori and Okamoto, 19871). The paper is chemistry, not marketing, and the same team went straight at blood pressure: a companion 1987 study in the same journal fed the tea to hypertensive rats and tracked their pressure, years before "calm" ever appeared on a GABA tea label (Omori, Yano, Okamoto, Tsushida, Murai and Higuchi, 19872). The blood-pressure claim has held up reasonably well since. The calm claim on today's package rests on a molecule that has never been shown, in a human study, to reach the brain at all.
What "GABA" means in a bag of oolong
GABA is an amino acid every tea leaf already makes in small amounts. Raise it deliberately and you get a different product. The method is not roasting or oxidation, the two axes that define ordinary oolong (see Oxidation and Roast), but a separate step layered on top: fresh leaf goes into a sealed chamber flushed with nitrogen, cut off from oxygen entirely. Under that stress, an enzyme in the leaf, glutamate decarboxylase, converts glutamic acid into GABA and releases carbon dioxide as a byproduct. Caffeine and catechin levels barely move. GABA does, by a wide margin, and the 1987 paper is where the 150 milligram per 100 gram line, still the threshold a tea has to clear to carry the name, first appears in print.
Oxidation level is a separate choice from the GABA step. The nitrogen-chamber treatment works on green, oolong, and black tea alike, which is why GABA versions of all three exist. Oolong turned out to be the variety it suited best, for reasons a later round of testing in Taiwan would spell out.
A tea built to move a number
From the outset, the tea from the Tsushida team's process was aimed squarely at hypertension, not at anyone looking to unwind: the group's own rat blood-pressure study ran the same year as the chemistry (Omori et al., 19872). The pitch was medical: something a person with high blood pressure could add to an existing regimen, sold on a specific physiological claim rather than a feeling. That framing is easy to lose sight of now that calm is the word on the package, but it explains why the earliest serious human trials of oral GABA were run on hypertensive patients, not anxious ones.
What the blood-pressure trials found, precisely
The best-known of those trials, published in 2003 in the European Journal of Clinical Nutrition, gave 39 people with mild hypertension a fermented milk enriched with GABA, daily, for 12 weeks. Systolic blood pressure fell by a mean of 17.4 mmHg, a drop that held up against both baseline and a placebo group; diastolic fell by 7.2 mmHg against baseline, though the paper does not report that second figure clearing the placebo comparison too (Inoue et al., 20035). A separate 2009 trial, using a different GABA source, GABA-rich chlorella, in 80 people with borderline or high-normal blood pressure, found the same direction of effect over the same 12-week window: systolic pressure came down significantly against placebo, with the biggest drops in the people who started closest to the hypertension threshold (Clinical and Experimental Hypertension, 20096).
Neither trial tested tea specifically, and 39 and 80 people are not large samples by the standard of a drug trial. Read carefully rather than generously, they still describe something the calm claim cannot: two independent, placebo-controlled, blinded human studies, using two different GABA-rich foods, both finding a real, significant drop in systolic blood pressure over 12 weeks, in the population the tea was designed for in the first place.
The paper's own co-author helped bring it to Taiwan
The technology did not stay in Japan. Dr. Masashi Omori, the same Omori who co-authored the 1987 paper with Tsushida, visited Taiwan in late 2004 to see how the island's tea makers were adapting the process for themselves (The Epoch Times, 台灣茶後起之秀 GABA烏龍茶 風味全球迷). Taiwan's own Tea and Beverage Research Station tested the nitrogen treatment across the island's major styles, green tea, Tieguanyin, Wenshan Baozhong, and high-mountain oolong among them, and found oolong held up best: the oxygen-starvation step tends to leave a sour, fermented edge on raw leaf, and oolong's own oxidation and roasting steps were the most effective at folding that edge back into something drinkable while the GABA content stayed high.
National Chung Hsing University's Professor Qu Shaomei is credited with fixing the 150 milligram per 100 gram figure, the same one Tsushida's group had already reported in 1987, as the line Taiwanese producers hold their own tea to. GABA oolong, jiayelong cha (佳葉龍茶), became known there as the country's own rising specialty, what The Epoch Times' Chinese edition called tea beverages' up-and-coming star (後起之秀). One recent example is the Tea and Beverage Research Station's own high-GABA oolong made from Tai Cha No. 24, Taiwan's only native wild tea species, which measured 249 milligrams per 100 grams, well past the standard, and for which the station itself declined to make a health claim beyond the raw number (see the station's 2026 release).
The newer claim argues from a weaker place
Ask what GABA does once swallowed for calm, and the honest answer is that nobody has directly tested it. GABA is the brain's principal calming neurotransmitter, but the brain guards itself with the blood-brain barrier, and GABA's own chemistry, small, polar, charged at the body's normal pH, is exactly the profile that struggles to cross that barrier by simple diffusion. A 2015 review in Frontiers in Psychology found the existing evidence flatly contradictory: some studies report GABA failing to cross, others claim a real transport route exists, and, as the reviewers put it, no human study has ever directly tested whether an oral dose of GABA reaches the brain at all (Boonstra et al., 20157).
A 2020 systematic review in Frontiers in Neuroscience went looking for the human evidence anyway, screening 5,912 publications down to 14 that actually tested oral GABA against stress or sleep. Its verdict: "limited evidence for stress and very limited evidence for sleep benefits." Of those 14 studies, 11 had at least one author employed by the company selling the product under test (Frontiers in Neuroscience, 20208). The review does note a handful of small studies reporting real effects, more alpha brain waves, lower stress-hormone readings, alongside the conflict of interest. Where a mechanism is proposed at all, it usually skips the brain entirely: GABA made by gut bacteria acting on the vagus nerve, rather than the swallowed molecule crossing into the skull.
The two numbers, side by side
The GABA content is real and measurable, the 150 milligram line is one a lab can check, and the original case, a food aimed at blood pressure, has two independent, placebo-controlled trials behind it. The newer case, aimed at calm, has 14 studies, most of them small, most compromised by the funding behind them, and not one that has shown the swallowed molecule reaching a human brain. Both numbers are on record. Only one of them was ever actually measured against the claim on today's label.