Zinc: Essential Functions and Absorption

Zinc is an essential trace element required by all living organisms, playing a pivotal role in numerous physiological and biochemical processes. It is widely distributed in various food sources, including meats, shellfish, dairy products, seeds, nuts, and whole grains. This abundance in the diet ensures that, in normal conditions, a sufficient amount of zinc can be obtained through regular food consumption. However, because virtually no zinc exists in the body as free ions, its bioavailability – or the proportion of the mineral that can be absorbed and used by the body – depends heavily on the digestive process and its efficiency. Factors such as the presence of dietary phytates (found in legumes and whole grains) can inhibit zinc absorption, while animal proteins can enhance its uptake.

The majority of zinc absorption occurs in the small intestine, primarily through a transcellular process, where it is actively transported across the intestinal cells. Among the different sections of the small intestine, the jejunum has been identified as the site with the highest rate of zinc transport. This efficient mechanism ensures that zinc enters the bloodstream and is distributed to various tissues where it performs critical functions.

Zinc is indispensable for a wide array of cellular metabolic processes. It acts as a cofactor for more than 300 enzymes and is involved in the synthesis and degradation of carbohydrates, lipids, proteins, and nucleic acids. This means that zinc is essential for energy production and macromolecule synthesis within cells. It is also crucial for DNA synthesis, which makes it fundamental for growth and development, especially in children and during pregnancy. Moreover, zinc is vital for the maintenance of the immune system. It contributes to both innate and adaptive immune responses, helping the body fend off infections and ensuring a well-regulated immune reaction.

Additionally, zinc supports neurological function and cognitive health. It influences neurotransmitter release and neuronal communication, which are essential for learning, memory, and mood regulation. Furthermore, it plays a key role in reproductive health, contributing to proper hormone function and the development of reproductive organs.
Zinc: Essential Functions and Absorption

Digestion for Nutrient Absorption

In addition to mastication and digestion, the absorption of nutrients in the body involves a complex interplay of various physiological processes. Once the ingested food undergoes mastication and begins its journey through the digestive system, the breakdown of complex substances is facilitated by enzymes and gastric juices. The intricate process of hydrolysis takes place, breaking down starches into monomeric sugars, proteins into amino acids, and triglycerides into fatty acids.

The significance of this digestive breakdown extends beyond the mere conversion of complex compounds into simpler forms. It acts as a pivotal step in the absorption of nutrients, as the body can more effectively assimilate monomeric sugars, amino acids, and fatty acids. However, it is essential to note that certain nutrients, such as specific vitamins and inorganic elements, do not undergo this extensive digestive process and are absorbed differently.

Moreover, the digestive breakdown plays a crucial role in the body's defense mechanism. By breaking down complex molecules, it acts as a barrier against the absorption of large, potentially harmful substances. This selective absorption ensures that only essential nutrients are absorbed, safeguarding the body against potential threats.

As polymeric nutrients like proteins undergo hydrolysis, the associated vitamins and trace elements are released. This release enhances the efficiency of nutrient absorption, as these essential substances become more readily available for uptake by the body. The orchestrated sequence of events during digestion, from mastication to the release of associated elements, underscores the intricate nature of nutrient absorption and highlights the body's remarkable ability to extract essential components from ingested food for optimal health and functioning.
Digestion for Nutrient Absorption

Lactose In Milk

Milk is an emulsion produced by mammary glands which characterize with white color, mild taste and flavor.

The major constituent of milk is water, but milk contains varying levels of lipids, proteins, and carbohydrates which are synthesized within with mammal gland. Their composition is influenced by animal species, environmental conditions, nutrition of animals, their lactation state and others.

Lactose is the principal carbohydrate found in milk. It is virtually unique to milk, having been found elsewhere only in fruits of certain members of the Sapotaceae. Lactose is consumed through milk and other unfermented dairy products, such as ice cream.

Disaccharides lactose in milk imparts sweet profile and human milk has a major concentration of lactose (7.4%) compared to milk of other species. The main role of lactose includes:
*It provides galactose for the synthesis of the nerve structures of the growing infant
*In the intestine, it gets metabolized to lactic acid which eliminates harmful bacteria.

Lactase is a membrane-bound enzyme located in the brush border epithelial cells of the small intestine. Once lactose reaching the small intestine, where the hydrolytic enzyme lactase (β-galactosidase) is located, lactase catalyzes the hydrolysis of lactose into its constituent monosaccharides (glucose and galactose). Only monosaccharides among the carbohydrates are absorbed from the intestines.

Lactose accounts for ~30% of the caloric value of whole milk. Lactose is an important energy source and sometimes it is referred to simply as milk sugar, as it is present in high percentages in milk. Lactose-intolerance individuals have a lactase deficiency; therefore, lactose is not completely catabolized.
Lactose In Milk

The roles and functions of mucosal cells

All cells depend on their external environment for their supply of nutrients. Food in the lumen is not technically inside the body because it has not been absorbed.

Mucosa is the innermost layer and this layer line the interior of the digestive tract and thus is in direct contact with essential nutrients (in food) available in the external environment. It becomes obligatory, therefore for this mucosal cells to take in all the nutrient essential not only for their own metabolisms but also for that the whole organism.

The mucosal cells are uniquely adapted to perform this primary functions: transporting from the external environment to the internal environment the nutrients essential for all of the cells that comprise the total organism.

In some cases, these cells secret a mucus layer that serves to lubricate the passage and protect the cells. Epithelial cells of mucosa are arranged into folds to increase the surface area. The amount of folding is dependent on the region of the gastrointestinal tract.

Because mucosal cells are in direct contact with churning food and harsh digestive secretions, they live only about two to five days.
The roles and functions of mucosal cells 

Digestion and absorption at infant age

The mechanisms for digesting and absorbing major nutrients are not fully mature in the premature and term infant.

The complex process of digestion/absorption can be optimally effective only when the GI tract and accessory organs are totally develop and fully functioning. The primary function of the GI tract is the digestion and absorption of nutrients.

Not only must the muscular tube (alimentary canal) with it a mucosal lining and endocrine cells be operating efficiently in conjunction with the nervous system, but the accessory organs (pancreas, liver, and gallbladder) with their important digestive secretions also must be physiologically mature.

Normally, the initial breakdown or hydrolysis of carbohydrates depends on both salivary and pancreatic amylase. However, in the infant, carbohydrate hydrolysis is limited by the fact that although salivary amylase is present by 34 weeks’ gestation, secretion of pancreatic amylase does not begin until age 4 to 6 months.

The feeding of infants is based on primarily in degree of maturation of the GI tract and accessory organs. Good examples of the emphasis on GI tract maturity are the care given to the fat in infant formula and the time and sequence of the introduction of various foods into the infant’s diet.

Only those fats possessing an ease used in commercial formulas and the introduction of solid food, beginning with baby cereal usually occurs no earlier than 4 months of age. The absorption coefficient of fats is 90 to 95% during the first week of life and at least 96% at 1.5 months of age.

The infant pancreas, although structurally mature at term, is usable for several months to produce enzymes sufficient for effective digestion.
Digestion and absorption at infant age

Absorption and Transport of Thiamin

The bioavailability of thiamin occurring naturally in foods is believed to be high. Dietary sources of thiamine include: yeasts, pork, sunflower seeds, legumes, chicken, fish, beef, wheat germ, cereal products, lentils, potatoes, rice polishing and nuts.

Occasionally, however, anti-thiamin factor may be present in the diet. For example, thiaminases present in the raw fish catalyze the cleavage of thiamin, thereby destroying its activity.

These thiaminases are thermolabile, however and cooking of fish rendered the enzymes inactive. Other anti-thiamin factors that are thermostable may be found in tea and certain fruits and vegetables such as blueberries, black currents, Brussels sprouts and red cabbage.

Dietary thiamine is absorbed readily from jejunum and proximal ileum and is transported to tissues where it is converted to the active form, thiamine pyrophosphate (TPP).

Absorption of thiamin can be both active and passive, depending upon the amount of the vitamin presented for absorption.

At low physiologic concentrations, thiamin absorption is an active process. This Na+ dependent, carrier mediated absorption occurs primarily in the jejunum but can occur in other portions of the small intestine as well.

When intakes of thiamin are high, the absorption route is predominantly passive. The rate of thiamin absorption is always quite high except in the case of ethanol ingestion and/or folate deficiency.

Thiamine is carried by the portal blood to the liver. In normal adults, 20-30% of plasma thiamine is protein-bound all of which appears to be TPP. The transport of thiamine into erythrocytes seems to be facilitated diffusion process, whereas it enters other cells by an active process.

Thiamine uptake by active transport is highest in the jejunum and ileum, with both passive diffusion and active carrier-mediated transport.

Ethanol ingestion interferes with active transport of thiamin, and folate deficiency prevents the normal duplication of enterocytes, thereby decreasing absorption, both active and passive.

Conversion to the active coenzyme form requires adenosine triphosphate (ATP) and thiamin pyrophosphokinase, an enzyme found in the liver and brain (and perhaps in other tissue as well).

Another form of thiamin (thiamin triphosphate, or ATP) is synthesized in the brain by action of a thiamin diphosphate (ADP) – ATP phosphoryl-transferase.
Absorption and Transport of Thiamin

Diet with calcium-rich foods

A calcium-rich diet is recommended for all individuals a part of a healthful, well balanced diet.

Consuming calcium-rich foods daily will help minimize resorption of calcium from the bones and thus promote dense, strong bones.

Calcium is found salmon (with bones), sardines, seafood and dark green leafy vegetables. Milk and dairy products are the primary source of calcium in the North American diet.

Other calcium-rich foods include almonds, asparagus, blackstrap molasses, brewer’s yeast, broccoli, buttermilk, cabbage, carobs, cheese, collards, dandelion green, dulse, figs, filberts, goat’s milk, kale, kelp, milk, mustard greens, oats, prunes, sesame seeds, soybeans tofu, turnip greens, watercress, whey and yogurt.

There are many herbs contains calcium.  This include alfalfa, burdock root, cayenne, chamomile, chickweed, chicory, dandelion, mullein, nettle, oat straw, paprika, parsley, peppermint, plantain, raspberry leaves,  eyebright, fennel seed, fenugreek, flaxseed, hops, horsetails, kelp, lemongrass, red clover, rose hips, shepherd’s purse, violet leaves, yarrow, and yellow dock.

Besides the many supplements or antacids that provide calcium, many food manufacturers have been adding calcium to their foods as well.

Consumption of food with high in protein, fat and/or sugar affects calcium uptake. The average American diets of meats, refined grains and soft drinks leads to increase excretion of calcium.

Consuming alcoholic beverages, coffee, junk foods, excess salt, and/or white flour also leads to the loss of calcium by the body.

Calcium may be poorly absorbed from foods rich in oxalic acid. Oxalic acid can be found in almonds, beet greens, cashews, chard, cocoa, soybeans and spinach. Oxalic acid known as oxalate, is the most potent inhibitor of calcium absorption by binding with it in the intestines and producing insoluble salts that cannot be absorbed.

The normal consumption of foods containing oxalic acids should not pose a problem, but overindulgence in these foods inhibits the absorption of calcium.

Oxalic acid can also combine with calcium to form calcium-oxalate kidney stones. However, that taking magnesium and potassium supplements can prevent the formation of this types of stone.
Diet with calcium-rich foods

Digestion and absorption of carbohydrates

The carbohydrates in the diet are broken down by the enzymes on the mouth, pancreas and intestinal epithelium.

Digestion resumes in the small intestine where more polysaccharide splitting enzymes from the pancreas break the carbohydrate down completely into disaccharides. The enzyme released through the common bile duct into the small intestine.

Then enzymes on the surface of the cells of the small intestine break these into simple sugars or monosaccharaides.  Maltose is split into two glucose molecules; lactose is split into one glucose and one galactose; and sucrose into one glucose and one fructose.

However the great majority of carbohydrates in human meal are digested and absorbed as glucose, if a blood glucose levels are measured before such a meal and at half hourly intervals thereafter, it would show a rise in blood glucose, peaking at about the half hour mark and returning to fasting levels almost as quickly.

If a person were to abstain from carbohydrates for considerable periods say a week, the blood glucose levels would still be normal in spite of a minimal or zero intake.

The active absorption of glucose across the intestinal mucosa is thought to be by phosphorylation in the mucosal cell.

The body’s capacity to maintain blood glucose within specific limits is achieved by a variety of hormones, the two most important of which are insulin and glucagon. Both are secreted by the pancreas into bloodstream, as required.

When the blood glucose level arises, the body adjusts by storing the excess. The frost organ to detect the excess glucose is the pancreas, which releases the hormone insulin in response.
Digestion and absorption of carbohydrates

Absorption and Transport of Vitamin C

The ascorbic acid contained in foods appears to be readily available and absorbed.

Absorption of ascorbic acid in the intestine occurs through a sodium-dependent active transport system. But simple diffusion may also contribute somewhat to uptake of the vitamin.

The reduced and oxidized forms of the vitamin are absorbed by different mechanism of active transport:
*Ascorbic acid uptake by the sodium-dependent vitamin C transporter (SVCT)
*Dehydroascorbic acid uptake by glucose transporters (GLUT)

The transport of ascorbic acid into the ileum is a carrier-mediated process at low mucosal concentrations of ascorbic acid.

Absorption rate can vary from 16% at very high intakes (approximately 12g) to 98% at low intakes.

The degree of absorption as suggested by the urinary excretion of the vitamin appears to be adversely affected by pectin, zinc, copper, and iron.

From the intestinal cells, ascorbic acid diffuses through anion channels into extracellular fluid and enters the plasma by way of the capillaries.

At present it is unknown whether the decreased urinary ascorbic acid caused by the presence of the above three minerals reflects a less efficient absorption or an increased oxidation of the vitamin before it can be absorbed.

The cellular accumulations of vitamin C in humans are mediated by a variety of specific transporters located at the cell membranes and regulated in a cell-specific manner.

Absorbed ascorbic acid is transported in the plasma as a free anion. Normal plasma ascorbic acid concentrations range from about 0.4 to 1.7 mg/dL and it readily equilibrates with the body pool of the vitamin. The size of the pool therefore varies with the intake. Ascorbate moves freely into the cells, but the concentration is much greater in some tissues than in others.

The highest concentration of ascorbic acid are found in the adrenal and pituitary glands (with each possessing approximately 30-50 mg/100 g of wet tissue) as well as in the eyes, brain and white blood cells.

An intermediate level of the vitamin is found in the liver, lungs, pancreas and leukocytes, while smaller amounts occur in the kidneys muscles and red blood cells.

In absolute terms based on total weight, the liver contains the most ascorbic acid. The maximum pool is estimated at about 2g.
Absorption and Transport of Vitamin C