Table 2.

Diet Items, Some of Their Key Chemical Components and Enzymes Required to Break Them Down^*

Diet item	Refractory materials or chemical(s)	Less refractory chemical(s)	Enzyme activitiesa
			1	2	3	4	5	6	7	8	9	10	11
Nectar	Nil	Simple sugars	♦	♦		♦
Milk	Nil	Lactose	♦	♦				♦
Animal flesh	Nil	Glycogen	♦	♦	♦	♦
Insects, zooplankton	Cuticle, chitin	Glycogen, trehalose	♦	♦			♦					♦
Bacteria	Peptidoglycan in G(+) bacterial cell walls	Soluble polysaccharides	♦	♦	♦	♦		♦					♦
Terrestrial plant material (flowers, seeds, fruits, leaves, twigs)	Cellulosesb, lignin, insoluble starchesc	Sucrose, starch	♦	♦	♦	♦		♦	♦	♦
Aquatic/marine plant materials (green and brown, diatoms, seaweeds	Celluloses, mannanes, xylans, agarose	Starch, laminarin and chrysolaminarind	♦	♦	♦	♦		♦	♦	♦	♦
Plant exudates (saps, resins, latexes, gums)	Phenols and terpene derivatives, hemicellulose, other complex β-linked polysaccharides	Sucrose	♦	♦		♦		♦		♦
Fungi and lichens	Chitin, N-acetyl-β-D-glucoaminides, N bound to cell-wall components;		♦	♦				♦	♦			♦
Detritus	Celluloses, lignin, xylans, mannanes,	Starches, α-glucans	♦	♦	♦	♦		♦	♦	♦	♦

^*This table is not comprehensive but lists mainly types of food items discussed in this article. The diet items are ranked (top to bottom) in approximate order of the relative amounts of material in them that is refractory to digestion (low to high). From reference (249), based on multiple sources (248,295).

^a

A diamond symbol represents type of enzyme activity for hydrolysis. Enzyme activities include the following:

• 1.
  Proteases (such as pepsins, trypsins, and chymotrypsins) and peptidases (e.g., carboxypeptidases and aminopeptidases).
• 2.
  Ester bond hydrolases (e.g., lipase and phospholipase).

α-glucosidases:

• 3.
  α-amylases (hydrolyzes starch from plants and glycogen from animals).
• 4.
  α-glucosidases (e.g., maltase [hydrolyzes the oligosaccharides that are formed by amylase], sucrase [hydrolyzes sucrose from plants], oligodisaccharidases).
• 5.
  Trehalase (hydrolyzes trehalose, the principal blood sugar in insects).

β-glucosidases

• 6.
  Lactase.
• 7.
  Cellulase (cellulose is hydrolyzed by the concerted action of three types of cellulases: endocellulases, exocellulases, and β-glucosidases).
• 8.
  Xylanase and pectinase.
• 9.
  Laminarinase.
• 10.
  Chitinases.
• 11.
  Lysozyme [hydrolyzes peptidoglycan in G(+) bacterial cell walls (141)].

^bCellulose and hemicellulose.

^cThe crystalline pattern of starch seems to determine its susceptibility to hydrolysis (139).

^dβ-1,3 glucan storage products (laminarin) (236).