Why mix the ingredients all of which were common in Israel's diet according to (2Sa17:27-29)? The severity of the 18 month siege of Jerusalem would make it necessary to mix all available grain to derive sufficient material to bake into bread. Some modern references record that "spelt" is a species of hardy wheat regarded as inferior, since the hulled grains could not be freely threshed and which in some regions of the world is grown for feeding livestock. To be sure the final mixed grain produced a grade of flour inferior to pure wheat or barley flour.

Cooper notes that “Wheat” and “barley” were the most important and widely used grains in the ancient Near East. “Beans” and “lentils” also were staple products usually not mixed with grain or used for flour. These four items, wheat and barley, beans and lentils, often are listed in pairs. “Millet” is mentioned only here in the OT but was used in Mesopotamia. “Spelt” sometimes was planted as a border to wheat or barley. " (Vol. 17: Ezekiel. The New American Commentary. Nashville: Broadman & Holman Publishers.)

As alluded to above, the final siege of Jerusalem began in January 588 B.C. and lasted 18 months, during which the supplies of food became scarce. In times of scarcity in order to make the supplies last longer it was customary to mix several coarser grains (picture is of lentil) with the finer less readily available grains such as wheat and barley and grind these together to make flour. "The outcome of this mixture would be a coarse, unpalatable bread, not unlike that to which the population of Paris was reduced in the siege of 1870-71. This was to be the prophet’s food, as it was to be that of the people of Jerusalem during ...siege." (Pulpit Commentary)

"If you live near a health-food store, see if they carry "Ezekiel Bread." It is made according to the recipe that God gave Ezekiel. While it may not be palatable to those who are accustomed to refined sugars and flours, it is interesting to note that when made according to God's recipe it produces a complete protein. God, the Creator of all things, knew exactly how to illustrate the reality of famine without jeopardizing the health of His obedient servant, Ezekiel!" (Ezekiel Study Notes - Path-light)

YOU SHALL EAT IT ACCORDING TO THE NUMBER OF THE DAYS THAT YOU LIE ON YOUR SIDE, THREE HUNDRED AND NINETY DAYS:

"Shekel" - One shekel weighed  0.40 ounces. Shekel is the standard monetary unit in modern Israel and in Ezekiel's day also referred to coins which were weighed rather than counted,  the weighing being a necessity because of a widespread practice of shaving some silver from shekels to make more shekels! If a little silver was shaved off each of two hundred shekels, for example, one could create an extra ten or twenty shekels. Weighing shekels was an attempt to stop this practice and standardize currency. So here we find that "Twenty shekels" equates with about 8 ounces of this unusual grain mixture per day. The point is clear that this is not a significant quantity of nutrition for a grown man. In verse 16 God goes on to explain the meaning stating that He (God) was "going to break the staff of bread (or cut off the bread supply) in Jerusalem and they will eat bread by weight and with anxiety, and drink water by measure (water will be rationed) and in horror..." God repeated the warning about famine in chapter 14 explaining to Ezekiel that "if a country sins against Me by committing unfaithfulness and I stretch out My hand against it, destroy its supply of bread, send famine against it, and cut off from it both man and beast, even though these three men, Noah, Daniel, and Job were in its midst, by their own righteousness they could only deliver themselves...." (Ezek14:13 14)

Through the prophet Moses God had warned Israel that "if (they did) not obey (Him) and (did) carry out all (the) commandments" (Lev26:14) He would "break (their) staff of bread" which would be so scarce that "ten women (would) bake...bread in one oven, and they (would) bring back...bread in rationed amounts, so that (they would) eat and not be satisfied." (Lev26:26)

The Pulpit Commentary adds that "The common allowance in England for prison or pauper dietaries gives, I believe from 24 to 32 ounces, Besides other food. And this was to be taken, not as hunger prompted, but at the appointed hour. once a day. The whole scene of the people of the besieged city coming for their daily rations is brought vividly before us."

the sixth part of a him is less than one quart of water per day clearly a minimal amount of water considering what were often elevated temperatures and dry conditions. God explains this sign later declaring that the inhabitants of Jerusalem would "drink water by measure (water will be rationed) and in horror because bread and water will be scarce; and they will be appalled with one another and waste away in their iniquity."  (Ezek 4:16 17)

Jerusalem, like most ancient Middle Eastern cities, had to rely on water supplied from outside the city walls which made her vulnerable during long sieges and necessitated watering rationing. 

YOU SHALL DRINK IT FROM TIME TO TIME:

"Having baked it in their site over human dung"  ("bake it in stools of human excrement, in public view" -WBC) God is indicating that the conditions in Jerusalem will not only be austere but downright revolting as the siege progresses and as supplies of staples and necessities of life dwindle to the point of non-existence.

The process of baking in ashes was as old as the time of Abraham (Gen18:6, cf 1Ki19:6), and continues in Arabia and Syria to the present day. The kneaded dough was rolled into thin flat cakes, and they were placed upon, or hung over, the hot wood embers of the hearth or oven. But in a besieged city the supply of wood for fuel soon fails and the inhabitants would be forced to dried animal dung and then to use of human excrement. The besieged Jews would be forced to use the dried contents of the cesspools of Jerusalem. All this "dung sign" drama was to be before the eyes of the exiles and to show the extreme degree of wretchedness to which the besieged city would be exposed.

"Unclean bread" ("bread all filthy" [DRA],  “defiled bread” [YLT]) which the exiles would be reduced to eat because the practices of preparing food in these pagan countries would be unlikely to produce "clean" food . So taken, the words remind us of the risk of eating unclean, food, which almost inevitably attended the position of the exiles (Hosea 9:3; Daniel 1:8), and which, it may be, Ezekiel had already tell keenly.

Ezekiel's plea reminds one of a similar action of his fellow prophet Daniel who "made up his mind that he would not defile himself with the king's choice food or with the wine which he drank" and like Ezekiel appealed to a higher authority, seeking "permission from the commander of the officials that he might not defile himself." (Da1:8)

In the New Testament we see Peter when commanded to "Arise...kill and eat" (unclean animals) make a similar appeal, declaring "By no means, Lord, for I have never eaten anything unholy and unclean." (Acts 10:14)

FOR FROM MY YOUTH UNTIL NOW I HAVE NEVER EATEN WHAT DIED OF ITSELF OR WAS TORN BY BEASTS, NOR HAS ANY UNCLEAN MEAT EVER ENTERED MY MOUTH: (Lev11:39 11:40) (Lv19:7; Dt14:3; Isa65:4; 66:17)

In these last two verses, we have the explanation of the drama God prescribed for Ezekiel to portray before the exiles.

"Break the staff of bread" "Staff" is used elsewhere to describe a physical rod (e.g., Moses' staff) used for support, etc, and in this verse refers to bread as that which is indispensable for the preservation (support) of life. The psalmist writes that God in order to force Jacob to go to Egypt "called for a famine upon the land. He broke the whole staff of bread" (Ps105:16) so clearly this phrase is associated with famine.  Isaiah says that because of sin "the Lord GOD of hosts is going to remove from Jerusalem and Judah both supply and support, the whole supply of bread, and the whole supply of water." (Isa3:1)

AND THEY WILL EAT BREAD BY WEIGHT AND WITH ANXIETY, AND DRINK WATER BY MEASURE AND IN HORROR: (10 11; 12:18 12:19; Ps60:3; La1:11; 4:10; 5:9)

"Eat bread by weight"

Cooper draws an interesting parallel with the manna in the wilderness: "God had provided manna in the days of wilderness wandering that he might teach them to trust him. They had to learn that “man does not live on bread alone but on every word that comes from the mouth of the Lord” (Dt 8:3). But the reverse also was true. Neglect of the word produced a famine (In Am 8:11 the Lord God says "Behold, days are coming...when I will send a famine on the land, not a famine for bread or a thirst for water, but rather for hearing the words of the LORD." cf Hos 4:6) both of food and of the knowledge of God and spiritual truths." (Vol. 17: Ezekiel. The New American Commentary. Nashville: Broadman & Holman Publishers.)

Craigie adds that "Just as, physically, one must eat to live, so too the spiritual life is dependent on partaking of the bread of life, the Word of God. But for ordinary humans, the first of these truths is more evident than the latter. Hunger will reduce the body to weakness and eventually death; but the well-fed and hungry alike may starve the soul and hardly be aware of what they are doing. The prophet’s symbolic diet reveals the reverse of this ancient truth. A people who for so long had ignored the bread of God’s word, would eventually be reduced to emergency rations for the body. In a sense, the diet symbolized judgment; in a deeper sense, it contained a message, a reminder of the true starvation of the spiritual life which was the root cause of the physical crisis"  (Ezekiel. The Daily study Bible series. Louisville: Westminster John Knox Press.)

"With anxiety...horror" Jeremiah writing the post mortem on ravaged Jerusalem declared "All her people groan seeking bread. They have given their precious things for food to restore their lives themselves." (La1:11) adding that "better are those slain with the sword than those slain with hunger; for they pine away, being stricken for lack of the fruits of the field." (La 4:9) The futility of materialism became evident for those who had more silver and gold than bread. Beloved, where are you laying up your treasure?

The fate predicted for rebellious Israel was prefigured in their first king rebellious Saul, to whom the prophet Samuel decreed that "rebellion is as the sin of divination, and insubordination is as iniquity and idolatry. Because you have rejected the word of the LORD, He has also rejected you from being king." (1Sam15:23) In a similar fashion, Israel had not just neglected God's life giving Word, but by God's own accusation had actually "rejected My ordinances" (Ezek5:6) 

"They will be appalled" ("they fall into a stupor" - NJB; "will look at one another in terror" - NLT; "will look at one another in dismay" - NRS, "They will be shocked at the sight of each other because of the lack of food and water" - GWT) The anxiety and astonishment indicates that the joy of mealtimes would vanish from Jerusalem. (cf Ezek 12:19).