ABSORPTION is accomplished by one of four mechanisms:
1) Active transport is a process requiring energy (ATP) and a nutrient carrier to move an essential nutrient against a gradient. Most carriers are nutrient-specific, and almost all required nutrients are actively transported. Minerals are the exception, as they require both passive diffusion and carrier-mediated transport. Amino acids are actively transported, but also require a carrier.
2) Passive diffusion is a movement of compounds across the cell membrane so as to equalize the concentration of the substrate on both sides of the membrane. This process applies only to such small molecules as electrolytes, water, and small sugars. It does not apply to such large molecules as starch or large proteins. Almost all the water-soluble nutrients are absorbed in this manner or by osmosis (passive absorption).
3) Facilitated diffusion is a movement of nutrients against a concentration gradient and usually does not require energy, but does require a carrier. Fructose absorption uses a carrier without expending energy pulling water with it upon entering the intestines.
4) Pinocytosis or phagocytosis happens when the absorptive cell engulfs the material. This process is used for fat absorption.
After food is digested, absorption is accomplished with the help of enterocytes (cells lining the gastrointestinal tract). The end products are absorbed mainly in the intestines through the villi. Each villus is connected to the circulatory and lymphatic systems. Glucose, glalactose, and amino acids are absorbed with the help of energy supplied by an enzyme and the sodium ion cofactor. Water-soluble nutrients pass directly into the circulatory system, while fat-soluble materials pass through the lymphatic system before being transported by the blood. Amino acids and peptides are actively absorbed by the absorptive cells of the villi. Peptides are broken down into individual amino acids, which then go to the liver via the portal vein for metabolism. Fatty acids that are water-woluble (fewer than twelve carbons) form triglycerides in the absorptive cells and combine with cholesterol, phospholipids, and similar substances with a protein coat (the compound formed is called a chylomicron). These pass through the lymphatic system before entering the blood stream. Understanding the absorption process gives bacground knowledge and a rationale for nutritional therapy for disorders associated with malabsorptive syndrome and reasons for enterostomies (tube placement for enteral feeding).
Stomach absorbs 20% of the alcohol ingested, as well as some short-chain fatty acids.
Duodenum absorbs Vitamins A and B1, iron, calcium, glycerol, fatty acids, monoglycerides, amino acids, monosaccharides, and disaccharides.
Jejunum absorbs glucose, galactose, amino acids, glycerol and fatty acids, nonoglycerides, diglycerides, dipeptides, copper, zinc, potassium, calcium, magnesium, phosphorus, iodine, iron, fat-soluble Vitamins D, E, and K, most of the B complex, Vitamin C, and the rest of the alcohol.
Ileum absorbs disaccharides, sodium, potassium, chloride, calcium, magnesium, phosphorus, iodine, Vitamins C, D, E, K, B1, B2, B6, B12, and most of the water.
Colon absorbs sodium, potassium, water, acids, gases, some short-chain fatty acids metabolized from plant fibers and undigested starch, and vitamins synthesized by bacteria (biotin and Vitamin K).
Water and Electrolyte Absorption:
Each day, 2-2½ litres of water enters the digestive tract. The salivary, gastric, intestinal, and accessory gland secretions contribute another six to seven litres. Out of that total, only about 150 ml. is lost in fecal wastes. The epithelial cells are continually absorbing dissolved nutrients and ions. As solute concentrations decrease, water moves into the surrounding tissues following the solutes, thereby maintaining osmotic equilibrium. The absorption of sodium and chloride ions is the most important factor promoting water movement. Other ions absorbed in smaller quantities are calcium, potasium, magnesium, iodine, bicarbonate, and iron. Calcium absorption occurs under hormonal control, requiring the presence of parathyroid hormone and calcitriol.
Absorption of Iron:
Ferric iron is less readily absorbed than the ferrous form, but why is not fully understood. Ferric salts are reduced to ferrous salts before absorption, however. Ferric citrate is a highly available iron source that is converted to the ferrous state before absorption. The inorganic salts are readily absorbed directly into the bloodstream, where they are transported by the protein transferrin rather than through the lymphatic system. Iron absorption occurs chiefly in the upper intestine and takes place with the iron in the ferrous form therapeutically given as ferrous sulfate or ferrous gluconate.
Ferrous gluconate is the iron salt of gluconic acid (a natural product of glucose metabolism) that is used by pharmaceutical companies as an iron supplement. Olive growers also use it as an artificial coloring.
Absorbed iron goes to the bone marrow for RBC synthesis, to tissues for cellular oxidation processes, and to the liver/spleen/bone marrow for storage reserve. The rate of absorption varies from individual to individual and is dependent on numerous factors, including antagonistic compounds consumed at the same time or preexisting disorders that interfere with absorption.
Heme iron is bound to protoporphyrin in hemoglobin and myoglobin. Unlike iron salts, the absorption of heme iron is not affected by phosphate or phytic acid nor by ascorbic acid. The heme complex is absorbed intact into the intestinal epthelial cells and only then is the iron split off. Its absorption is more efficient than that of iron occurring as a salt.
Absorption of Vitamins:
The nine water-soluble vitamins function primarily as participants in enzymatic reactions. All but one, Vitamin B12, are easily absorbed by the digestive epithelium, they but cannot be absorbed by the intestinal mucosa unless it has been bound by Intrinsic Factor (a protein secreted by the parietal cells of the stomach). The digestive bacteria (involving billions of cells) residing in the intestinal tract are an important source for several water-soluble vitamins, being renewed every four days.
The fat-soluble vitamins enter the duodenum in fat droplets mixed with dietary lipids. The small intestine is able to digest a maximum of 10 grams of fat per hour. The vitamins remain with those lipids when micelles form (a packet of chain molecules forming a supermolecular structure). The fat-soluble vitamins are then absorbed from the micelles, along with the products of lipid digestion. One fat-soluble vitamin, Vitamin K, is also produced by the action of resident bacteria and is absorbed in the colon. Three mechanisms are used for the uptake of vitamins, with the exception being Vitamin C. This is actively absorbed via an energy and sodium dependent transport mechanism.
Difficulties in the absorption of all classes of compounds will result from damage to the accessory glands or in the intestinal mucosa. If the accessory organs are functioning normally but their secretions cannot reach the duodenum, the condition is called biliary obstruction (bile duct blockage) or pancreatic obstruction. Alternatively, the ducts may remain open, but the glandular cells may be damaged and unable to continue normal secretory activities, resulting in pancreatitis. Even with normal enzymes in the lumen, absorption will not occur if the mucosa cannot function properly. A genetic inability to manufacture specific enzymes will result in discrete patterns of malabsorption, with lactose intolerance being a good example. Mucosal damage due to ischemia (an interruption of the blood supply), radiation, exposure, toxic compounds, or infection will affect absorption and will deplete nutrient and fluid reserves as well.
Aging causes changes in the digestion and absorption of nutrients. Many physical changes occur along the digestive tract itself, including:
1) The rate of epithelial stem cell division declines, causing the digestive epithelium to be more susceptible to damage from abrasion, acids, or enzymes resulting in peptic ulcers. In the mouth, esophagus, and anus, the stratified epithelium becomes thinner and more fragile.
2) Smooth muscle tone decreases, meaning that the general motility and peristaltic contractions become weaker, causing constipation. Sagging walls in the colon can produce symptoms of diverticulitis; and straining to eliminate fecal matter adds stress to the less resiliant blood vessel walls, causing hemorrhoids. However, problems are not restricted to the lower digestive tract. Weakening of the muscular sphincters can lead to esophageal reflux, causing frequent bouts of heartburn.
3) Such cumulative damage as the loss of teeth contribute to poor digestion. This can be the result of alcohol and drugs, prescription or non-prescription.
4) In the elderly, cancer rates increase, especially colon and stomach cancers. Oral and pharyngeal cancers are also particularly common in elderly smokers.
5) Changes in the other systems affect the digestive system. For example, the decline in olfactory and gustatory sensitivity leads to a disinterest in food affecting the entire body.
In the aging gastrointestinal tract, the secretory activity decreases. The most marked change is in the stomach, with reduced hydrochloric acid production. Pepsin and gastric mucus secretion also decline. A common disorder in the elderly is pernicious anemia, which results when the production of intrinsic factor ceases. Achlorhydria is a complete lack of gastric acid production. The small intestine decreases in its ability to absorb nutrients. The tolerance of fat is reduced; and where there is a high fat intake, the fat content of the feces increases. Calcium absorption in the elderly is decreased, along with a reduced ability to adapt to low calcium intakes. This contributes to a change in levels of Vitamin D and ultimately affecting bone mass.
Constipation is common because peristaltic action is diminished. This slowing down cannot be attributed solely to age, but also to associated factors. Some of these contributing factors are bland diets caused by a decrease in sensory perception; semistarvation caused by lack of interest or financial worries; the intake of drugs that increases with age, causing more health concerns; and damage to the gut wall as a result of drugs, obstruction, disease, or dehydration.
The total energy production per square meter of body surface falls progressively with advancing age. The average reduction is about 12 calories/m²/hour between the ages of twenty and ninety years. This may be as a result of the loss of metabolizing tissue. Nutritional requirements always increase when there is a reduction in nutritional absorption at any age.
The inappropriate consumption of nutrition supplements can lead to additional malnutrition, toxicity, and serious body malfunctions or failure of such. As the body ages, mechanisms decrease in function. Nutrient storage, conversion, and excretion are altered. Forcing the body to accept additional nutrients can aggravate existing problems or cause new ones to weakened organs or systems. Care must be taken when introducing any new supplement.
Protein deficiency in the elderly is common, mainly for two reasons. One is their budgets do not allow for expensive meats which they have used all their lives. They rarely have any knowledge in the use of vegetable protein that can be substituted for meat, which is not only cheaper, but easier on the aging system. Two, the physical body is undergoing a slowdown in all systems which is not able to digest many of the proteins that it once could. This protein deficiency appears in their structural and muscle weakness, slowed body reactions, in increased infections, loss of acid-base homeostasis, excess bleeding, edema, and poor recovery from injury.
The elderly are prone to dehydration because the sensation of thirst diminishes with age. Many are also afraid of incontinent episodes and restrict their fluid intake even more. The use of medications further diminishes the body’s water reserves. Dehydration brings a drop in blood pressure that is associated with poor circulation since there is not enough fluid to transport substances through the body. Joint stiffness, sunken dry eyes, skin and mouth dryness, sagging and wrinkled skin, diverse brain malfunctions, constipation, urinary stones, and kidney malfunctions also accompany dyhydration. Basically, the body begins to shut down when it can no longer meet the demands of water conservation – much like a drought-sticken parched piece of land. Over a lifetime, no fewer than 100 tons of food pass through the digestive tract. It is little wonder, then, that after such abuse, the system wears down.
*Note: Are you aware of the effect of drugs prescription and over-the-counter on your body’s nutrients? Check here.