Exhaustive Guide to Vitamin Physiology, Metabolism, and Nutritional Pathology, and Dietary Sources

DEFINITION AND ESSENTIAL NATURE OF VITAMINS

Vitamins are defined as micronutrients that are absolutely essential for the maintenance of human health. These substances are organic compounds that the human body requires in relatively small quantities to perform a diverse array of vital functions. Unlike macronutrients, vitamins do not provide energy directly to the body. Instead, their importance lies in their roles as biological catalysts. Specifically, they serve as cofactors for enzymes, which are proteins that facilitate chemical reactions. Furthermore, vitamins act as antioxidants, a function that protects the body's cells from oxidative damage and prevents the onset of various chronic diseases.

CLASSIFICATION OF VITAMINS BY SOLUBILITY

Vitamins are broadly categorized into two main groups based on their solubility: hydrosoluble vitamins and liposoluble vitamins. Hydrosoluble vitamins are those that dissolve in water. Because they are water-soluble, they are easily absorbed by the body but are not stored in significant quantities. This lack of storage capacity necessitates a daily intake of these nutrients through the diet. This group specifically includes all vitamins within the $B$ complex—which consists of $B_1$, $B_2$, $B_3$, $B_5$, $B_6$, $B_8$, $B_9$, and $B_{12}$—as well as Vitamin $C$.

Liposoluble vitamins are those that dissolve in fats. These vitamins are absorbed into the body alongside dietary fats and are subsequently stored in internal organs, primarily the liver, and in fatty tissues. Because they can be accumulated in the body over time, they do not necessarily require the same frequency of daily intake as water-soluble vitamins. The vitamins that belong to this category are Vitamin $A$, Vitamin $D$, Vitamin $E$, and Vitamin $K$.

PHYSIOLOGICAL DIFFERENCES IN ABSORPTION, STORAGE, AND EXCRETION

The chemical nature of vitamins dictates how they move through and leave the human body. Hydrosoluble vitamins are absorbed directly into the intestine. In contrast, liposoluble vitamins require the presence of dietary fats to be adequately absorbed by the digestive system. In terms of storage, hydrosoluble vitamins are not retained by the body, making regular consumption a necessity for maintaining healthy levels. Liposoluble vitamins, however, can be stored long-term within the liver and adipose (fat) tissue.

Excretion pathways also differ significantly between the two groups. When the body has an excess of hydrosoluble vitamins, the surplus is eliminated rapidly through the urine. Liposoluble vitamins are not excreted as easily; they are either not excreted at all or are passed through the feces in very small quantities. This difference in excretion means that excessive intake of liposoluble vitamins is more likely to lead to toxicity.

MECHANISMS OF ACTION AND ENZYMATIC FUNCTIONS

One of the primary mechanisms of action for vitamins is their role in enzymatic activity. Many vitamins function as cofactors or coenzymes in critical metabolic reactions, thereby facilitating the work of enzymes. Specifically, Tiamina ($B_1$) in the form of $TPP$ is essential for the decarboxylation of piruvato to transform it into $acetil{\text -}CoA$. Riboflavina ($B_2$), serving as $FAD/FMN$, and Niacina ($B_3$), serving as $NAD/NADP$, are fundamental for electron transfer reactions. Piridoxina ($B_6$), in the form of $PLP$, is vital for amino acid metabolism, including processes like transamination and decarboxylation. Ácido fólico ($B_9$), acting as $THF$, is responsible for the transfer of one-carbon groups.

Beyond enzymatic roles, vitamins operate as antioxidants. Vitamins $C$, $A$, and $E$ are specifically cited for their ability to neutralize free radicals, thereby shielding cells from oxidative stress. Maintaining adequate levels of these vitamins is critical for preventing diseases. Deficiencies or excesses can cause severe adverse effects such as bone malformations, various types of anemia, nervous system disorders, vision impairments, disruptions in cellular reproduction, and failures in the synthesis of neurotransmitters.

THE ROLE OF VITAMINS IN ENERGY METABOLISM AND BIOCHEMICAL PATHWAYS

Vitamins are indispensable for the metabolic pathways that generate energy for the body. In the Krebs Cycle, several $B$ complex vitamins, most notably Niacina and Riboflavina, serve as essential cofactors. In the process of Glycolysis, Tiamina ($B_1$) is a key player in the decarboxylation of pyruvate, which is a required step for the pathway to proceed. Additionally, Vitamin $B_5$, or Ácido pantoténico, plays a crucial role in the synthesis and general metabolism of lipids (fatty acids).

DETAILED PROFILES OF WATER-SOLUBLE VITAMINS

Tiamina ($B_1$) acts as a coenzyme in the decarboxylation of $\alpha$-keto acids and is involved in the Krebs Cycle; it is found in whole grains and pork, and its deficiency leads to Beriberi.

Riboflavina ($B_2$) is a precursor to $FAD/FMN$ used in redox reactions; it is sourced from milk and liver, and its deficiency causes Queilosis.

Niacina ($B_3$) is a precursor of $NAD/NADP$ involved in glycolysis and beta-oxidation; it is found in chicken, tuna, and peanuts, and its deficiency results in Pelagra.

Piridoxina ($B_6$) serves as a coenzyme in transamination for amino acid metabolism; it is found in chicken, bananas, and cereals, and deficiency leads to irritability and convulsions.

Ácido fólico ($B_9$) is a coenzyme in nucleotide synthesis and DNA metabolism; it is sourced from green leafy vegetables and its deficiency causes megaloblastic anemia.

Cobalamina ($B_{12}$) is a coenzyme in folate and homocysteine metabolism and DNA synthesis; it is found exclusively in animal products, and deficiency results in megaloblastic anemia or neuropathy.

Finally, Ácido ascórbico (Vitamin $C$) acts as an antioxidant and is required for collagen synthesis; it is found in citrus fruits and peppers, and its deficiency causes Escorbuto.

DETAILED PROFILES OF FAT-SOLUBLE VITAMINS

Vitamin $A$ is essential for vision and gene expression, specifically acting as Retinal in the visual cycle; it is found in liver and carrots, and while deficiency causes night blindness (ceguera nocturna), toxicity can lead to hepatotoxicity.

Vitamin $D$ regulates calcium and phosphorus for bone mineral homeostasis; it is obtained from fish and sunlight, with deficiency leading to Raquitismo or osteomalacia and toxicity causing hypercalcemia.

Vitamin $E$ functions as a membrane antioxidant to prevent lipid peroxidation; it is found in vegetable oils, and while deficiency causes neuropathy, toxicity can lead to hemorrhages.

Vitamin $K$ is a coenzyme in the carboxylation of proteins required for blood coagulation; it is sourced from green vegetables, and deficiency leads to bleeding (sangrados), though toxicity cases are considered very rare.