Animals tend to have different metabolic strategies when energy is positive than when it is negative. The metabolic strategy is how much food is digested and used per unit of time such that there are endothermic strategies and ectothermic strategies. Activity of the animal increases the rate of metabolic function above the BMR (for endotherms) or SMR (for ectotherms). The use of energy is distributed amongst activity, homeostasis, growth, and reproduction. Going past or below the metabolic rates usually triggers a change in the metabolic strategy of the animal.

Essential nutrients are those molecular structures necessary for the continued functioning of the human system including vitamins, minerals, fatty acids, and amino acids. Vitamins are essential to metabolic reactions in that they function as coenzymes (carriers) such as the case of ferric acid. Minerals are those compounds formed from elements other than the main ingredients in SPONCH (sulfur, phosphorous, oxygen, nitrogen, carbon, hydrogen) such as sodium chloride. Fatty acids are those molecules which are carboxylic acids that are either saturated or unsaturated and that are required for certain metabolic respiration processes and not produced by the human body (must be obtained through food intake). The essential nutrient amino acids are not constructed in the human body and are considered necessary for the continued health and livelihood of the human body including: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Organisms that filter out tiny organic materials from the water in front of their food intake component (what is essentially a mouth) are called suspension feeders. Substrate feeders are organisms (such as the worm or termite) that eat the dirt or soil through which they burrow. Deposit feeders are like substrate feeders except that they process the material around on the top of the surface. Fluid feeders are those organisms that feed on the fluids of other organisms or plants. Bulk feeders are considered grazing animals such as cows, horses, maybe to some extent humans as well. Substrate feeders have the interesting opportunity to create new pathways to travel down and as they create these pathways they are simultaneously undergoing cellular respiration to process the energy required to continue to process the soil or wood or whatever their particular environment happens to be. Fluid feeders are likely parasitic in nature, at least to some extent, i.e. if an organism lived in the blood stream of another animal it would be using resources that would otherwise be useful to the animal body.

Ingestion, digestion, absorption, elimination are the four stages of food processing, whereby ingestion involves the mechanical and chemical breakdown of the food material into more manageable components much smaller in nature such that then the digestive process can begin to break down the components into their smaller forms (such as “pizza” into the fatty acids embedded in the cheese). The absorption stage is equivalent to the part where the food passes through the animal such as in the gastrointestinal system whereby the intestines begin to absorb the passing food materials. The elimination process is where the materials that have been unused and once the complex lipid molecules have been broken down and no longer used in the system it goes out and the organism is basically done with the materials. The gastrointestinal tracts usually run through the entire organism and the material is ingested in such a way in the abdomen such that the material can be distributed to the rest of the body (the intestines aren't at your toes or in your head!).

The digestive system includes the salivary glands, mouth, esophagus, stomach, liver, pancreas, gallbladder, small and large intestines, and rectum. The salivary glands produces a substance that begins the decomposition of food input materials. The mouth assists in mechanical decomposition of the food by breaking it apart into smaller and smaller components assisted by the chewing action of the jaw and structure of the teeth. The food particles travel down the esophagus and into the stomach, the liver secretes bile, stores the bile in the gallbladder, the bile is used to break down fat that has been taken in, the food travels to the small intestine where most of the food digestion and absorption takes place. The small intestine includes the duodenum, jejunum, and ileum. The pancreas, liver, and gallbladder all secrete digestive juices (such as bile) into the duodenum of the small intestine to break down food. The jejunum is the second part of the small intestine which digests the broken-down food materials. The jejunum is responsible for absorbing digested foods into the human system to distribute to other cells (etc.). The jejunum connects to the large intestine at the cecum (through the ileocecal valve) of the large intestine. The appendix is an offshoot of the cecum. The large intestine removes some liquids and electrolytes from the food materials being processed and eventually ends with the anal canal.

Bile assists in the digestion of fats in the duodenum (remember that bile is produced in the liver and stored in the gallbladder). Numerous digestive enzymes function in the small intestine as hydrolytic enzymes in order to break polysaccharides (in other words: complex carbohydrates) down to monosaccharides (the monomers). Also, numerous enzymes are in the duodenum that have polypeptide substrates meaning that they break apart proteins down into amino acids such as the enzymes trypsin and chymotrypsin. An enzyme called dipeptidase attaches to the lining of the intestine and splits small peptides, carboxypeptidase breaks one amino acid at a time (beginning at the carboxyl group end of some polypeptide chain), and aminopeptidase works in the opposite direction of carboxypeptidase (it starts at the head, not the carboxyl tail). Protein digestion could occur with only aminopeptidase or only carboxypeptidase and not the other, however, the rate of hydrolysis is greatly increased with both proteins present. Nucleases hydrolyze DNA and RNA into their component nucleotides. Lipase hydrolyzes fat molecules. The most complex of these processes is the protein digestion where enzymes specifically target `heads' or `tails' and start breaking off amino acids one by one and can even have some primitive sense of `team work' going on in the duodenum of the small intestine.

The brain triggers the release of gastrointestinal juices (gastric juices) when animals see, hear, or taste food. It is by this hormone-based triggering that the stomach and the intestines do not continuously release enzymes for the degradation of food for digestion when there is no more food remaining in the intestines. The hormone gastrin is released into the circulatory system when food stimulates the stomach wall. This release of gastrin goes through the blood stream and after it completely makes its way back it actually triggers the stomach to release even more gastric juices so that there is continued digestion as long as there are food stuffs still within the stomach which really turns into an example of negative feedback as the acidity of the stomach solution becomes too low. Other hormones called enterogastrones, secreted by the wall of the duodenum, tells the intestinal cells to secrete secretin, which signals the pancrease to release bicarbonate, which then proceeds to neutralize the acid. These hormones are used to stop food from being transferred down further, to secrete special enzymes to degrade food, and generally control the acidity of the digestive contents relative to the general presence of enzymes in the blood stream sort of as a messenger system to influence the intestinal system.

The large intestine serves to remove liquids and electrolytes from the (once-was) food materials and it eventually leads to the excretion of the waste material that leaves the body. One of the other functions of the large intestine is vitamin K metabolism which undergoes oxidative metabolism. Apparently, vitamin K helps prevent coagulation (blood clotting) in the human body.

Omnivores are discontinuous feeders as they process the food that they had digested—the same as carnivores and herbivores. Herbivores eat mostly plant life, carnivores eat animals, and omnivores eat either of the two. Herbivores must have a longer digestive system in order to process the cellulose that they are taking in. Omnivores do not necessarily have to process the cellulose and carnivores get energy from plants by eating herbivores so that they may attain the right energy. The feeding strategies are largely mediated by evolutionary relationships between predator-prey relationships in ecosystems.