"They eat nothing but fire into their bellies" is a literal translation that produces an effect of rude inelegance which is not in the Arabic words. Even in the matter of food and drinks, the mission of Islam is to avoid the extremes of lawlessness on the one hand and extreme formalism on the other. It has laid down a few simple and very reasonable rules. Their infraction causes loss of health or physical powers in any case. But if there is further a spirit of subjective rebellion or fraud - passing off in the name of religion something which is far from the purpose - the consequences become also moral and spiritual. Then it becomes a sin against Faith and Spirit. Continuing the physical simile, we actually swallow fire into ourselves. Imagine the torments which we should have if we swallowed fire into our physical body! They would be infinitely worse in our spiritual state, and they would go on to the Day of Resurrection, when we shall be deprived even of the words which the Judge speaks to a reasonable culprit, and we shall certainly not win His Grace and Mercy.