It is intended to provide compositions whereby oxidative stress caused by active oxygen species, free radicals, etc. <i>in vivo</i> can be lessened to thereby prevent the outbreak or worsening of diseases and yet a high safety can be achieved. It is observed that, by using compositions containing oxidation−type coenzyme Q and/or reduction−type coenzyme Q, urinary 8−hydroxydeoxyguanosine level is lowered in normal and diabetic animals. Also, it is histopathologically confirmed that tissue denaturation caused by oxidative stress can be prevented thereby in diabetic rats. Thus, it is found out that oxidative stress <i>in vivo</i> can be lessened. That is to say, oxidative stress <i>in vivo</i> can be lessened by using compositions containing coenzyme Q as the active ingredient.
