ל:ה - הלכות נטילת ידים ב:טו
Adam transgressed the prohibition of eating from the Tree of Knowledge of Good and Evil. The Tree of Knowledge corresponds to secular wisdom, which confuses good and evil. This is the da'as of the Other Side, which by definition is a blemish in Holy da'as. And, the main aspect of this blemish was through the act of eating. Now, the rectification of this blemish is specifically through faith, as in, "and cherish faithfulness" (Tehillim 37:3) like the Rebbe explains in Torah 62. When a Jew does not eat with kedusha he has blemished his da'as, which relates to Hashem's providence, and he falls into the errors of secular wisdom, as is written, "lest you have eaten and will be satisfied and your heart will be lifted up and you will forget Hashem your God, etc. And you will say in your heart that 'my power and the might of my hand earned this wealth." (Devarim 8) This refers to when a person relies completely on natural occurrences, Heaven forbid, that is, for his livelihood, his strength, and the might of his own hands. He never thinks to thank Hashem for any of these things. This relates to a blemish of the hands - a blemish of the 28 knuckles and joints of the hands, which relate to the 28 letters in the first verse of the account of Creation. Hashem watches and attends to everything at all times. He gives strength (כ"ח - 28), literally 28, for Hashem watches and sustains the world through the 28 letters of Creation. Therefore, every Jew must sanctify his food in holiness.