Green Tea Suppresses Brain Aging

Abstract

Epidemiological studies have demonstrated that the intake of green tea is effective in reducing the risk of dementia. The most important component of green tea is epigallocatechin gallate (EGCG). Both EGCG and epigallocatechin (EGC) have been suggested to cross the blood-brain barrier to reach the brain parenchyma, but EGCG has been found to be more effective than EGC in promoting neuronal differentiation. It has also been suggested that the products of EGCG decomposition by the intestinal microbiota promote the differentiation of nerve cells and that both EGCG and its degradation products act on nerve cells with a time lag. On the other hand, the free amino acids theanine and arginine contained in green tea have stress-reducing effects. While long-term stress accelerates the aging of the brain, theanine and arginine suppress the aging of the brain due to their anti-stress effect. Since this effect is counteracted by EGCG and caffeine, the ratios between these green tea components are important for the anti-stress action. In this review, we describe how green tea suppresses brain aging, through the activation of nerve cells by both EGCG and its degradation products, and the reductions in stress achieved by theanine and arginine.

Keywords: arginine; brain aging; catechin; epigallocatechin gallate; green tea; stress reduction; theanine.

Figure 1

The structure of green tea catechins.

Figure 2

The structure of EGCG metabolites.

Figure 3

The neurite outgrowth of human SH-SY5Y cells treated with EGCG and its metabolites. A cell suspension (2.5 × 10⁴ cells/well) was plated in a 24-well plate. Samples dissolved in 0.01% DMSO were added to the culture medium to produce a final concentration of 0.05 μM, and these were cultured for 72 h at 37 ℃. Each value represents the mean ± SEM (neurite length, n = 103–107). Asterisks represent significant differences relative to the control (* p < 0.05, Bonferroni’s t-test) [17,48].

Figure 4

The hypothesis that EGCG and its degradation products act on the brain with a time lag.

Figure 5

Confrontational housing as a psychosocial stress loading scenario. To create confrontational housing, a standard polycarbonate cage was divided into two identical subunits with a stainless-steel partition (single housing). Two mice were housed in the partitioned cage for one week (for a short-term experiment) or one month (for a long-term experiment). Then, the partition was removed, and, subsequently, the two mice co-inhabited the same cage (confrontational housing). Group-housed mice were housed in groups of six as a control housing scenario. These mice ingested water with or without theanine (6 mg/kg).

Figure 6

The effect of confrontational housing on the lifespan of SAMP10 mice. Mice were housed alone for one month before confrontational housing conditions were induced. Mice that died early were observed, and the maximum survival time was approximately 20 months (closed square). In contrast, group-housed control mice began to die at around 12 months of age, with a maximum survival time of 26 months (open circles) (n = 12) [77].

Figure 7

The time course of the neocortex volume in SAMP10 (a) and ddY mice (b) between the control (open circle) and theanine (closed circle) groups under confrontational housing conditions. The mice were dissected after 0, 1, 2, 4, and 6 months of confrontational housing and subjected to measurement for ex vivo MR (n = 3–12; *, p < 0.05) [81].

Figure 8

Expression levels of Lcn2 and Npas4 mRNA in the hippocampus of SAMP10 and ddY mice. Mice consumed theanine (closed bar) or normal tap water (control, open bar) ad libitum. After single housing for one month, hippocampal samples were obtained from mice housed confrontationally after 0 days, 3 days, and 1 month. Values are expressed as the means ± SEM (n = 3–6, * p < 0.05) [81].

Figure 9

The relationship between stress and brain aging.

Figure 10

The amount of theanine, and the molar ratio of caffeine and EGCG to theanine and arginine, in 76 samples of Matcha marketed in Japan (a) and 67 samples of Matcha marketed outside of Japan (b).