The most special characteristic of GABA tea is not having the original biological functions of tea, but having the functions of blood pressure moderation . 111 Although the catechins in tea have good benefits for reducing blood pressure, Hara et al . in 1987112 demonstrated that the mechanism for reducing blood pressure by catechins was due to their inhibition of angiotensin I converting enzyme (ACE) activity.
For GABA tea, the biological function of blood pressure moderation was even more predominant due to the mutual enhancement of GABA and catechins com-bined.113,114 Until now, research about the biological functions of GABA tea has concentrated more on its blood pressure moderation However, several foods rich in GABA, such as germinated brown rice and GABA-enriched fermented milk product, have gained considerable attention due to their biological functions Besides reducing blood pressure, many other functions were reported, such as reducing anxiety and stress,115 aiding against onset of Alzheimer's,116 helping with sleep and mood problems,116 increasing growth hormone,59,117,118 helping aging persons feel younger, and relieving postmenopausal depression .88,101,119,120 In addition, new techniques using microbial fermentation to produce a high content of GABA in foods have been developed.90,115,116,121,122
Recently, the effect of the GABA-enriched fermented milk product (FMG) on the blood pressure of spontaneously hypertensive and normotensive Vistar-Kyoto rats was studied . 123 During the chronic administration of experimental diets to SHR/ Izm, a significantly slower increase in blood pressure with respect to the control group was observed at 1 or 2 weeks after the start of feeding with the GABA or FMG diet, respectively (p < 0.005) . This difference was maintained throughout the feeding period The time profile of blood pressure change due to administration of FMG was similar to that of GABA . FMG did not inhibit ACE . Furthermore, an FMG pep-tide-containing fraction from reverse-phase chromatography lacked a hypotensive effect in SHR/Izm rats The results suggest that low-dose GABA has a hypotensive effect in SHR and that the hypotensive effect of FMG is due to GABA .
The glutamic acid-germinated brown rice extract (Gex) and chitosan/glutamic acid-germinated brown rice extract (CGex) significantly retarded the proliferation rates of L1210 and Molt4 cells, and the highest retardation rate was with CGex . In addition, the Gex and CGex enhanced significantly the apoptosis of cultured L1210 cells, but no significant apoptosis was seen with the other extracts, which have lower concentrations of GABA than Gex and CGex . The results show that brown rice extracts with enhanced levels of GABA have an inhibitory action on leukemia cell proliferation and have a stimulatory action on the cancer cell apoptosis 124
Omori and colleagues7 were the first group working on the effect of GABA tea on reducing the blood pressure of animals and humans . In 1987, they used 24 spontaneously hypertensive rats (SHR) divided into three groups to feed GABA tea, green tea, and deionized water, respectively. After feeding 2 weeks, the blood pressure of the GABA group SHR was significantly reduced and their urine contained a lot of sodium Omori implied that drinking GABA tea helped the exclusion of sodium from the body and resulted in reducing blood pressure Besides, the lack of significant difference in body weight among these SHR in all groups, the blood pressure of SHR fed with GABA tea was 25-30 mmHg lower than that of the other groups of SHR. However, after feeding GABA tea for 9 weeks, once feeding GABA tea was replaced with deionized water, the blood pressure of SHR increased again to a hypertensive level On the other hand, those SHR fed originally with deionized water for 9 weeks and then changed to GABA tea had significantly reduced blood pressure (figure 4 4)
This experiment clearly indicates that GABA tea indeed has the function of reducing blood pressure . Furthermore, the authors suggested that daily drinking of GABA tea has to be continued in order to lower the blood pressure; otherwise, the blood pressure can increase again once drinking GABA tea stops, just like taking hypertension medicine The only difference is that GABA tea is a natural product with no side effects In addition, drinking GABA tea also offers the pleasure of drinking tea
For further understanding the effect of GABA tea on humans, 13 spontaneously hypertensive patients diagnosed in the hospital were selected for clinical studies One 3-g bag of GABA tea was provided to each patient per day for 3 months after discontinuing hypertension medicine It was found that the original blood pressures of 180 and 184 mmHg were down to 165 and 140 mmHg, respectively. 113 In addition, drinking GABA tea had no unpleasant feeling or side effect at all . Patients also described a feeling of comfort and being less tired after drinking GABA tea Another test using 15 hypertension patients that were not diagnosed by the hospital, also drinking GABA tea for 3 months, resulted in significant blood pressure reduction in 8 of the 15 participants . They all felt relaxed and well . Therefore, Omori et al .7 suggested that hypertension patients drinking a 3 . 5-g bag of GABA tea every day for 2 months would reduce their blood pressure
In 1995, Abe et al.125 investigated the effects of GABA tea on the blood pressure of 21 Dahl salt-sensitive rats with different ages, 11 months (old) and 5 weeks (young) . After feeding a 4% NaCl diet for 3 weeks, 21 old rats were given water (group W), an ordinary tea solution (group T), or a GABA-rich tea solution (group G) for 4 weeks The average GABA intake was 4 0 mg/rat/day After 4 weeks of treatment, blood pressure significantly decreased in group G (176 ± 4; p < 0 .01), b b
a: SHR are divided into three groups. b: Feeding stuff was exchanged between the control group (fed on ordinary green tea) and the experimental group.
FIGURE 4.4 The effect of GABA tea extract on systolic blood pressure of SHR . (From Omori, M . , Yano, T. , Okamoto, J. , Tsushida, T., Murai, T. , and Higuchi, M . 1987. Effect of anaerobically treated tea (Gabaron tea) on blood pressure of spontaneously hypertensive rats . Nippon Nogeikagaku Kaishi 61:1449-51 . With permission . )
compared with group W (207 ± 9) or group T (193 ± 5 mmHg) . Plasma GABA levels were more elevated in group G (111 ± 54) than in group W (not detectable) or group T (14 ± 8 ng/ml) . For those young rats, the average GABA intake was 1 . 8 mg/rat/day. The results show that the body weight or chow and beverage consumption did not differ significantly among the three groups . After 4 weeks of treatment, although blood pressure was comparable in groups W and T (165 ± 3 vs 164 ± 5 mmHg), it was significantly lower in group G (142 ± 3 mmHg) than in the other groups . Plasma aldosterone concentration increased in group G compared to the other groups Thus, GABA-rich tea seems not only to decrease high blood pressure but also to prevent the development of hypertension in Dahl S rats fed by a high-salt diet . These results indicated that due to the tea drinking habit by people in many countries, the administration of GABA-rich tea might become one of the supportive methods to decrease blood pressure in hypertensive individuals
In order to understand further the reducing blood pressure function of GABA tea and its effects on kidney metabolism, Lin et al. 26 carried out another experiment. Using three different rats, spontaneously hypertensive (SHR), stroke prone (SHRSP), and Dahl S rats, as testing animals, the blood pressure, kidney functions, and biotests of serum and kidney of these three types of rats were evaluated after feeding a 5% NaCl diet with and without GABA tea . The results showed that for rats fed with GABA tea, blood pressure was reduced significantly (table 4 4) In addition,
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