You may have noticed that most of what you eat does not go into the body but instead goes out into the toilet. This means that the body only utilizes a fraction of what you eat. The digestive system is made up of several organs that work together to break food down into its simplest parts.
Once you chew and swallow food, it goes on a trip through your oesophagus, stomach, small and large intestine, pancreas, gallbladder, and liver. In the process, the body absorbs important vitamins, carbohydrates, minerals, fats and proteins.
It begins with the mouth, breaking down the food by chewing the food and mixing it with bacteria and enzymes in the saliva.
Once swallowed food travels through the oesophagus. The oesophagus is about 25 centimetres long and moves food from the back of your throat down to your stomach. It takes about two to three seconds for the muscles in the oesophagus to move the food along.
Your stomach is the tub at the end of the tunnel. It’s a stretchy sack shaped like the letter J, the stomach stores and breaks down the food you’ve eaten into a liquid mixture, which is slowly emptied into your small intestine.
The small intestine measures three to five centimetres around and 6.7 metres in length. The small intestine is packed underneath the stomach and uses the liver, pancreas and gallbladder to break your food down even further. The small intestine sucks most of the vitamins, minerals, proteins, carbohydrates and fats out of the food. It takes food more than four hours to work its way through. By the end of the journey, some nutrients have made their way to the liver and the leftovers move on to the large intestine.
The large intestine is about seven to 10 centimetres around and 1.5 metres long. This organ takes all the stuff your body can’t use and flushes it out of your body. Your body gets one last chance to absorb water, minerals and nutrients from this last bit of stuff when it passes through the large intestine, also called the colon. After the colon has absorbed all the goodness it can, the stuff hardens and is moved to the rectum, where it waits for you to go to the toilet.
There are many factors, both dietary and physiological, that influence nutrient bioavailability. Examples include: (1) the physical form of the nutrient within the food structure and the ease with which the nutrient can be released from that structure; (2) the chemical form of the nutrient in a foodstuff and its solubility in the lumen; (3) the presence of proteolytic enzyme inhibitors which reduce the body's ability to digest protein; and (4) the presence of enzymes such as thiaminase which partially hydrolyzes thiamin and makes it less biologically active.
Diet-related factors include:
Physiological factors include:
Certain food constituents have the ability to bind nutrients, thereby rendering them more or less absorbable. For nutrients that are transported into the mucosal cell by means of a carrier-mediated pathway, the degree of binding is an important determinant of bioavailability. If the chelating compound has a higher affinity for the nutrient than the specific carrier molecule, then the net effect is a reduction in bioavailability. Conversely, a weak chelator may act as an absorption promoter by holding the nutrient in a suitably soluble form ready to be taken up into the intestinal mucosal cells. Binding compounds that impair vitamin bioavailability include the protein avidin in egg white, which binds biotin, making it biologically unavailable.
Competitive inhibitors of nutrient metabolism make up another category of dietary factors affecting bioavailability. It has been suggested that minerals with similar chemical properties may compete for common binding sites or carriers. Transition metals such as iron, zinc, and copper are typical examples of competitive inhibitors. This will only take place at high levels of intake when the sum of ionic species present at the site of absorption exceed the critical threshold relating to the absorption kinetics of the minerals in question.
The bioavailability of lipid-soluble nutrients (including carotenoids, tocopherols, and other fat-soluble vitamins) is markedly influenced by their physicochemical availability for incorporation into mixed micelles during digestion. The release of lipid-soluble components is thought to occur primarily upon ingestion and initial digestion within the stomach. The factors which may influence their release are: localization within the food matrix; physical break-up of the food; chemical/enzymatic breakdown of the food during ingestion and initial digestion; and the presence of a suitable lipid phase in the form of emulsion droplets or free lipid.
The absorbability of a nutrient also depends upon certain physiological factors, such as the composition and volume of gastric and intestinal secretions. The absorption and utilization clearly depend upon a number of host-related variables. Most of these are key participants in the body's homeostatic regulatory mechanisms such as nutritional status, developmental state, gastric and intestinal secretions, mucosal cell regulation, and gut microflora.
The digestive system must employ restrictive strategies to keep the bloodstream clean. Loosely speaking it is a targeted system designed to identify what it requires and as a result most of what enters the stomach will simply not be accepted into the bloodstream. This 7 tests of fire means that the many capsules in the vitamin section of the supermarket go out into the toilet and most medical pills have bioavailability issues to circumvent. Another way to look at it is the Shape-O Toy, the stages of digestion lead to a shape that must fit the shape in the ball, if the final product differs it will not enter the bloodstream.
Bioavailability can be measured and experimented upon to improve bioavailability. Food, cooking and recipes must understand bioavailability to be nutritious. For example some vitamins are water-soluble, boiling certain foods in hot water and then throwing the hot water out and serving the food means the vitamins went out of the food into the water and into the drain and what is served is less those vitamins. The best way to eat a certain food? That is a mystery. I have not measured the best way to prepare and eat foods.