The 1987 Team That Invented GABA Tea Tested It on Blood Pressure
The tea sold today for stress relief and better sleep was tested in 1987, by the team that made it, on hypertensive rats. The blood-pressure claim for GABA-rich foods 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.
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In 1987, four Japanese food scientists 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 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 (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, appears.
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.
What the blood-pressure trials found, precisely
A trial 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. 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.
The paper's co-author visited Taiwan
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4).
National Chung Hsing University's Professor Ou Shaomei names 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 Ou 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 (see the station's 2026 release).
The newer claim argues from a weaker place
A Taiwanese agriculture-ministry outlet, Agriharvest, ran the tea under a headline promising a drinker "need not fear sleeplessness" because the tea "helps dissolve nervous tension" (Agriharvest3). GABA is the brain's principal calming neurotransmitter, but the brain guards itself with the blood-brain barrier. A 2015 review in Frontiers in Psychology found the existing evidence often 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 an industrial company (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. One proposed mechanism skips the brain: swallowed GABA acting on the peripheral nervous system through the gut-brain axis.
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, GABA-rich food aimed at blood pressure, has two independent, placebo-controlled trials behind it. The newer case, aimed at calm, has 14 studies, and not one that has shown the swallowed molecule reaching a human brain.