The apparent strength derived from tea or coffee is merely nervous excitement rather than true nourishment, leading to subsequent languor.

Original passage

Because of these results, many suppose that their tea or coffee is doing them great good. But this is a mistake. Tea and coffee do not nourish the system. Their effect is produced before there has been time for digestion and assimilation, and what seems to be strength is only nervous excitement. When the influence of the stimulant is gone, the unnatural force abates, and the result is a corresponding degree of languor and debility.