Here is the best info I have found that will tell you why you shouldn't eat bread and other unmentionables:
From freeacnebook.com chapter 31:
Moreover, grains, beans, and most vegetables have to be cooked, steamed, or heated in some manner before consumption. The heating originates new substances - like mutagenic HCA and N-nitroso compounds and neurotransmitter-impairing beta-carbolines – while these beta-carbolines are also constipating agents.
Why do grains, beans and most vegetables have to be cooked/heated before consumption?
Because we cannot digest these properly when consumed raw; Fruit bearing trees multiply by the means of their fruits, which contain seeds. To spread their seeds, these fruits are filled with sugars and other nutrients to make them as attractive for consumption as possible. The seeds are mostly indigestible and leave the body with the feces.
Grains, beans and most vegetables, on the other hand, contain all kind of substances that work against human consumption. We have to destroy these substances by heat because humans are not specialized in digesting plants (we do not have a crop or four stomachs, etc.) Unfortunately, not all these substances are entirely destroyed by heat, which is why consuming grains, beans, and most vegetables can cause severe pains and gas in the digestive tract.
New 'disorders' have been 'invented', like "a spastic colon", or "Irritable Bowel Syndrome", etc. because we don't want to admit that we shouldn't eat foods that aren't 'designed' to be consumed by humans; these 'disorders' are only about eating the wrong foods.
Some of the digestion-inhibiting substances:
- Grains and beans contain excessive fiber (six times more fiber than fruits), inhibiting the absorption of clean cholesterol. 
- Grains and beans contain phytic acid, which inhibits absorption of protein, minerals, and trace elements.
- Grains contain many other digestion-inhibitors, like pentosanes, lectins, arabinoxilanes, alkylresorcines, and cyanogenic glycosides.
- Beans (and currants) contain 4 to 140 times more lignin (indigestible aromatic alcohol polymers), than fruit does, on the average. Lignins easily bind to healthy cholesterol  and minerals. 
- Soybeans contain many different substances inhibiting digestion, like phytic acid, lots of polyurones, arabinoxylanes (special pentosanes), toxic lectins and mutagenic  phyto-estrogens. Phyto-estrogens can also impair sex-hormone metabolism.  (See this site)
- Vegetables contain more pentosanes and hexosanes than fruits. These special carbohydrates are only partially decomposed by bacteria in the colon, and inhibit digestion.
Brussels sprouts, beets, parsley, celeriac, kale, broccoli, and peas contain most of these substances.
- Some vegetables contain a substantial quantity of oxalic acid, which binds to minerals easily. Mangold, rhubarb, spinach, purslane, bamboo shoots, and beets contain most of these special carbohydrates.
- Some vegetables (like onions) contain mutagenic  flavonoids or dialkyloligosulphides (like garlic), which inhibit Iodide
- Mushrooms easily absorb harmful cadmium and arsenic, and contain alkaloids, which inhibit digestion and cause gripes.
- Lamb's lettuce contains caffeic acid, which easily binds nutrients.
- Rhubarb can contain mutagenic emodine.
Not all of those substances are entirely destroyed by heat;
- Many toxins, like oxalic acid, glycosides, glycoalkaloids, aflatoxins, sterigmatocystine and quercetine are not destroyed by heat. Saponines, fiber, phytic acid  and tannins are only partly destroyed.
- 5 to 20% of enzyme-inhibitors in soy are not destroyed throughout the preparation process, can cause cramps and can increase CCK (cholecystokinine) secretion, which can be harmful to the pancreas. 
- Heat does not usually destroy the substances in plants that inhibit protein digestion.