Effect of β-casomorphin-7 on MDA level of liver tissues (mean ± SD, n = 10) of aged mice. #P < 0.05 vs the model control group; ##P < 0.01 vs the model control group; L, low dose (2×10-7 mol·d−1) of β-casomorphin-7 treated; M, moderate dose (1×10-6mol·d−1) of β-casomorphin-7 treated; H, high dose (5×10-6mol·d−1) of β-casomorphin-7 treated.