Vitamin Sources, Chemical Conversion, Absorption, and Metabolism
We can obtain vitamins (as well as other nutrients) from 1)our diet
2) bacterial gut flora (small amounts, insufficient) and 3)exogenous sources (ie., pills,
powders, liquids, intravenous, etc.,). Depending on the type of medium consumed, digestion and
release of vitamins and other foodstuffs can either begin in the mouth or the stomach.
FOOD FOR THOUGHT
If a food is being consumed and is being chewed, digestion is beginning in the mouth.
Obviously, grinding food with teeth breaks it down. To help speeden the process,
saliva is liberated in the mouth. Saliva is a mixture of water, enzymes, and a few other substances. As the food
is chewed, saliva mixes with the bolus of food. An enzyme present in the saliva called salivary amylase
(also known as alpha-amylase), helps to start breaking the chemical bonds between complex carbohydrates (sugars)
in the food (if carbohydrates are present), and thus, releases them as free branches, or
free sugars. As food is broken down further from chewing, more of the complexes holding the vitamin are broken down.
This helps to release vitamins for absorption, once the food source reaches the small intestine. Once the food is chewed
enough and swallowed, the bolus enters the stomach. Carbohydrate digestion is temporarily halted by the stomach. The salivary
amylase that was liberated in the mouth to start carbohydrate digestion is deactivated by the low pH (increased acidity)
of the stomach. The stomach creates an acid environment by creating
acid (HCL for convenience) in the stomach, breaks down the amylase, and also helps a bit in digestion.
Although the stomach secretes a highly acidic solution of hydrochloric acid, the stomach remains
unharmed. This is because the stomach wall secretes a protective mucous layer as well. This mucous protects the stomach
wall against the acidic effects of HCL. If mucous production is somehow inhibited, the stomach
lining could be damaged, and this is what happens when we develop an ulcer. Substances such as alcohol, and certain foods
can damage the mucous layer (and mucous-producing cells), giving the HCL a chance to damage the lining and causing an ulcer.
Caffeine (in tea, coffee, soft drinks, etc.) and nicotine, cause the parietal cells to produce more than a normal amount
of HCL, also with a risk of ulceration. The effects are usually reversible, if the diet is revised by decreasing
alcohol intake, quitting smoking, limiting teas, coffees, etc., the ulcer will heal. However, with increased
diet changes and subsequent damage of the lining over the years, it can cause chronic ulcers, and increase the
chance of disease processes such as stomach cancer.
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Aided by its muscular walls, the stomach further mixes the bolus. The food is then squeezed from the stomach into the small intestine. Once the food is in the small intestine, enzymes are liberated form the pancreas to continue digestion. As the food is further broken down, the released vitamins can be absorbed through the barrier of the small intestine. Fats and fat-soluble vitamins are digested with the help of pancreatic enzymes and bile. Bile is a mixture created in hepatocytes (cells) of the liver, and stored in the gallbladder, a sac neatly tucked away, next to the liver. Once a fatty meal enters the small intestine, the gallbladder is stimulated to contract and squeeze out bile. The bile travels from the gallbladder to the small intestine, via a thin duct (tube), just in time to mix with the food present in the small intestine. Bile functions in mixing with fats (and fat soluble vitamins), increasing the surface area and thus helping them to be absorbed through the small intestinal wall at a quicker rate. Once vitamins and other nutrients pass through the walls of the small intestine, they are transported through the blood by transport proteins, other substances, and sometimes by themselves.
Every vitamin (and other nutrient) has its own story of absorption, function or functions, and excretion. We shall discuss each vitamin in detail in order to gain a complete picture, beginning at the nutrient source, and following it throughout the body until it is finally excreted. .