"To eat gravel" was a Hebrew Lamentations 3:16, and is an Arabic, phrase for getting into trouble. So "bread," got by deceit, tastes sweet at first, but ends by leaving the hunger of the soul unsatisfied. There is a pleasure in the sense of cleverness felt after a hard bargain or a successful fraud, which must be met by bidding men look on the after consequences.

Bread of deceit is sweet - Property acquired by falsehood, speculation, etc., without labor, is pleasant to the unprincipled, slothful man; but there is a curse in it, and the issue will prove it.

Bread of deceit is sweet to a man,.... Which may be understood of sin in general, which is bread to the sinner, he eats it: it is called "the bread of wickedness", Proverbs 4:17; but it is but poor bread, no other than ashes Isaiah 44:20; it is "bread of deceit"; there is a deceitfulness in all sin; it is in appearance fair and pleasant to the eye, like the fruit our first parents ate of; or like the apples of Sodom, of which it is reported that they are very beautiful to look at, but when touched drop into ashes; sin promises pleasure, profit, honour, liberty, peace, and impunity, yet gives neither; but the reverse, pain, loss, shame, servitude distress, and destruction; and yet it is sweet to an unregenerate man, one of a vicious taste, or whose taste remains unchanged; it is natural to him and he takes as much delight in it as in eating and drinking; and especially such sins as are called constitution ones, which he is not easily prevailed upon to part with; wickedness is sweet in his mouth, he rolls it and keeps it as a sweet morsel under his tongue, and forsakes it not, Job 20:12. It may be applied to particular sins, as to adultery, as it is by Jarchi, and with which may be compared Proverbs 9:17; and to riches unlawfully gotten; see Job 20:15; and to the cruel usage and persecution of the people of God, called the bread of wickedness and wine of violence, which wicked men take as much delight in as in eating and drinking, Proverbs 4:17; particularly the cruelty of the church of Rome, who has made herself drunk with the blood of the saints, in which she delights, and will be bitter to her in the end, Revelation 17:6. It may be interpreted of false doctrine; so the doctrine of the Pharisees and Sadducees is signified by leavened bread, Matthew 16:6; this is not true bread, does not strengthen, nourish, and refresh, as the Gospel does, but eats as a canker; it is not solid and substantial, but mere chaff, it is bread of falsehood and lying; false teachers lie in wait to deceive, their doctrines are lies in hypocrisy, and, yet these are sweet unto, and taken down greedily by carnal persons; particularly the doctrine of justification by works: this is the bread some men live on, but it is only husks which swine eat; it is feeding on wind, and filling the belly with east wind, which swells and vainly puffs up the fleshly mind; it is contrary to the, Gospel, and is not of the truth, and will deceive persons that trust to it; and yet it is sweet to a natural man; his own righteousness, and to trust to it, is natural to him; it is his own, and what he has laboured for, and is fond of; it affords room for boasting, and he does not care to part with it;
but afterwards his mouth shall be filled with gravel; with that which will be ungrateful, uncomfortable, and distressing to him; the conscience of a sinner, who has been taking his fill of sin and pleasure in it shall be filled with remorse and distress; and with bitter reflections upon himself; with a dreadful sense of divine wrath, and fearful apprehensions of it now; and destruction and damnation will be his portion hereafter; and this will be the consequence of all false doctrine, and of a man's trusting to his own righteousness and despising Christ's; see 2-Peter 2:1.

Wealth gotten by fraud may be sweet, for the carnal mind takes pleasure in the success of wicked devices; but it will be bitter in the reflection.

Bread . . . sweet--either as unlawfully (Proverbs 9:17) or easily obtained.
mouth . . . gravel--well expresses the pain and grief given at last.

17 Sweet to a man is the bread of deceit;
Yet at last his mouth is full of gravel.
"Bread of deceit" is not deceit itself, as that after which the desire of a man goes forth, and that for which he has a relish (thus, e.g., Immanuel and Hitzig); but that which is not gained by labour, and is not merited. Possession (vid., Proverbs 4:17) or enjoyment (Proverbs 9:17) obtained by deceit is thus called, as לחם כּזבים, Proverbs 23:3, denotes bread; but for him who has a relish for it, it is connected with deceit. Such bread of lies is sweet to a man, because it has come to him without effort, but in the end not only will he have nothing to eat, but his tongue, teeth, and mouth will be injured by small stones; i.e., in the end he will have nothing, and there will remain to him only evil (Fleischer). Or: it changes itself (Job 20:14) at last into gravel, of which his mouth is filled full, as we might say, "it lies at last in his stomach like lead." חצץ is the Arab. ḥaṭny, gravel (Hitzig, grien = gries, coarse sand, grit), R. חץ, scindere. Similarly in Arab. ḥajar, a stone, is used as the image of disappointed expectations, e.g., the adulterer finds a stone, i.e., experiences disappointment.