Table 2.
Summary of the studies—outcomes.
| References | Stress effects | Sleep effects |
|---|---|---|
| Hinton et al. (2019) | - In both stress groups (low and high) both teas (regular oolong and GABA oolong) tea increased average RR intervals. However, GABA oolong (vs. regular oolong) had a greater significant influence—bigger increase on HRV (a change in in the RR interval) in high (vs. low) stressed individuals. - Immediate stress questionnaire, TP, LF, HF, LF/HF—not significant | - Not measured |
| Yoshida et al. (2015) | - Improved calmness and worry scores in GABA (vs. control) group at the 4th week of treatment - Trends for reduced cortisol and increased adiponectin in GABA (vs. control) group at the 8th (final) week of treatment - ACTH: not significant | - A trend for improved feelings of awakening in GABA (vs. control) group at the 4th and 10th weeks of treatment - VAS sleepiness score- not significant |
| Okada et al. (2000) | - Not measured | - Improved rate for sleep disturbance scores (derived from Kupperman Index) in GABA (vs. control) condition after 4 weeks of use−68% of improvement |
| Fujibayashi et al. (2008) | - Increased TP in GABA condition 30 and 60 min after ingestion (vs. baseline) - Increased HF in GABA condition 30 min after ingestion (vs. baseline) - TP and HF: no difference between GABA and control conditions - LF: not significant | - Not measured |
| Yamatsu et al. (2015) | - Reduced CgA in GABA coffee (vs. water and coffee) condition 30 min after administration | - Not measured |
| Kanehira et al. (2011) | - Lower cortisol and CgA after ingestion of both 25 and 50 mg GABA (vs. control) only in chronic fatigue group - Tension/anxiety score—not significant | - Not measured |
| Nakamura et al. (2009) |
Experiment 1 - Lower LF/HF and higher HF power values in GABA (vs. control) condition 6.5–9.5 min after the task (= 36.5–39.5 min after ingestion) - Higher HF power values in GABA (vs. control) condition 12–15 min after the task (= 42–45 min after ingestion) Experiment 2 - Higher CgA values (due to stress task) in placebo condition 30 and 50 min (vs. baseline) after ingestion. Not observed in GABA condition |
- Not measured |
| Okita et al. (2009) | - Increased LF/HF ratio and HR in control condition 20 and 40 min after intake but this increase was not observed for GABA condition - Stroke volume, cardiac output, HF and LF power, SBP, DBP, MBP—not significant | - Not measured |
| Yamatsu et al. (2016) | - Not measured | - Improved feelings upon awakening score, reduced sleep latency and increased total Non-REM (N1, N2, N3) sleep time in GABA (vs. control) condition after treatment - Trends for improved PSQI, sleep satisfaction, and ease of falling asleep scores and increased light Non-REM (N1, N2) sleep time and sleep efficiency in GABA (vs. control) condition after treatment - Deep Non-REM (N3-SWS), sleep latency and time, REM sleep time, awakening frequency, and delta wave power—not significant |
| Yamatsu et al. (2015) | - Not measured | - A trend for reduced sleep latency in GABA (vs. control) condition after treatment - PSQI total, sleep satisfaction, feeling of awakening, ease of falling asleep, deep Non-REM (N3-SWS) sleep latency, REM sleep time, Non-REM sleep time, awakening frequency and delta wave power—not significant |
| Yoto et al. (2012) | - Alpha and beta waves decreased from 20 to 60 min after intake (due to stress task) but 30 min after GABA intake, this decrease diminished in GABA (vs. control) condition - Tension/anxiety, arousal, and relaxation scores—not significant | - Not measured |
| Abdou et al. (2006) |
Experiment 1 - Increased changes for alpha waves (%) in GABA (vs. placebo) condition - Increased changes for alpha/beta ratio (%) in GABA (vs. placebo and theanine) condition - A trend for reduced changes for beta waves (%) in GABA vs. placebo condition Experiment 2 Decreased IgA in control (vs. GABA) group at the middle and end of the bridge |
- Not measured |
| Byun et al. (2018) | - Not measured | - Reduced sleep latency in GABA (vs. control) group after treatment - Decreased ISI, PSQI total, PSQI-sleep quality and PSQI-total sleep time scores in GABA group (pre vs. post-treatment)—but no between group differences - PSQI-sleep latency, PSQI-sleep efficiency, N1(%), N2(%), N3(%), REM (%), WASO(min), REM-sleep latency, arousal index, AHI, RDI—not significant |
| Yamatsu et al. (2013) | - Increased cortisol in placebo group after 2 and 4 weeks but not in GABA group | - Improved onset and maintenance of sleep, drowsiness in the morning, and recovering from fatigue scores in GABA group after 4 weeks but no placebo vs. GABA group analyses done |
HRV, heart rate variability; ACTH, adrenocorticotropic hormone; HF, high frequency; LF, low frequency; CgA, chromogranin A; SBP, systolic blood pressure; DBP, diastolic blood pressure; MBP, mean blood pressure; REM, rapid eye movement; PSQI, Pittsburgh sleep quality index; ISI, insomnia severity index; N1, Stage 1 Non-REM sleep; N2, Stage 2 Non-REM sleep; N3-Stage 3 Non-REM sleep; WASO, wake after sleep onset; AHI, apnoea-hypopnea index; RDI, respiratory distress index.