Vitamins and Nutrition

Vitamin A is a key component of rhodopsin, a protein in the eyes that detects light, making it essential for normal vision. Calcium absorption is regulated by vitamin D, not vitamin A.

Retinol is the active, preformed shape of vitamin A found in animal-derived foods. Beta-carotene is a provitamin carotenoid that must be converted into vitamin A by the body, while phylloquinone and alpha-tocopherol are vitamins K and E, respectively.

Once inside the enterocyte, fat-soluble vitamins are packaged into chylomicrons alongside other dietary lipids. These chylomicrons then enter the lymphatic system to be distributed throughout the body. Micelles, on the other hand, facilitate the transport of lipids to the enterocyte membrane but do not form inside the cell.

Micelles are spherical complexes of emulsified fats and bile salts that transport fat-soluble vitamins across the watery environment of the intestinal lumen to the enterocyte surface. Chylomicrons are only formed inside the enterocyte after absorption has occurred.

The vast majority of both water-soluble and fat-soluble vitamins are absorbed in the small intestine, primarily in the duodenum and jejunum. The stomach mainly prepares food for digestion, while the liver stores and processes nutrients after absorption.

Vitamins A, D, E, and K are fat-soluble, meaning they dissolve in organic solvents and are absorbed along with dietary fats. Vitamin B-6, vitamin C, and folate are water-soluble vitamins.

Vitamins are organic compounds because they contain carbon-carbon and carbon-hydrogen bonds. Minerals, by contrast, are inorganic elements.

Vitamin C is a critical cofactor in the synthesis of collagen, a structural protein essential for connective tissues, skin, and blood vessels. Deficiencies in vitamin C lead to scurvy, which is characterized by weakened collagen structures.

Adequate folate intake before and during early pregnancy is critical for proper closure of the neural tube in the developing embryo. This has led to widespread fortification of grain products with folic acid to reduce the incidence of spina bifida and anencephaly.

Folate coenzymes play a central role in transfer reactions of one-carbon groups, which are essential for the synthesis of purines and pyrimidines used in DNA. Without adequate folate, cell division is impaired due to the lack of DNA precursors.

Pyridoxal phosphate (PLP), the active coenzyme form of vitamin B-6, is essential for transamination reactions that transfer amino groups to form nonessential amino acids. It also plays a key role in decarboxylation and other aspects of amino acid metabolism.

Biotin is frequently referred to as the beauty vitamin due to its popular association with maintaining healthy hair, skin, and nails. While it is a coenzyme in carboxylase reactions, its cosmetic marketing has cemented this nickname.

Pantothenic acid is an essential component of Coenzyme A (CoA), which is critical for the metabolism of carbohydrates, proteins, and lipids. CoA plays a central role in the citric acid cycle and fatty acid synthesis.

FAD (flavin adenine dinucleotide) and FMN (flavin mononucleotide) are coenzymes derived from riboflavin (vitamin B2). NAD and NADP are derived from niacin (vitamin B3).

Thiamin (vitamin B1) acts as a coenzyme (thiamin pyrophosphate) in the metabolism of carbohydrates and branched-chain amino acids, thereby facilitating energy production. Calcium absorption is facilitated by vitamin D, membrane protection by vitamin E, and vision by vitamin A.