I think that the quote is worded in a bit of an awkward way. I have never heard of it exactly, but I think that its intent is fairly powerful. The idea of "food" for the "spirit body" or the soul is something that is transcendent. It is not contingent on present setting and something that is universal. Yet, we, as human beings, have a tendency to focus on the present and the contingent. In this concentration and obsession, there is a loss of the transcendence. It is within this idea that the second half of the quote holds meaning when it suggests that being chained to the transient and the contextual demands of consciousness allows individual to lose sight to what is important. In our tendency to become slaves to "the powers that feed earth bodies," we tend to sacrifice that which is for the soul and that which is meaningful and important. It is this balance and recognition of such values and to minimize being enslaved that becomes so very important to the nature of human consciousness.
My interpretation of the quote is that people forget that their spiritual bodies need food just as much as their earthly body does. When people forget this they become captive to what their earthly body needs. If they did not forget their spirit bodies they would not be so captive to their earthly bodies.