Pro-vitamins are semi-essential nutritional molecules very similar to vitamins; in fact, the root indicates that they are isomers, dimers or precursors of specific essential vitamins; the conversion of provitamins into vitamins occurs thanks to some enzymatic reactions of the organism.
A classic example of provitamins is given by the numerous ones chemical variants of retinol (life); to date, about ten types are known, but the most common are:
- Carotenoids: contained in green and yellow vegetables
- Retinol esters: contained in foods of animal origin
All retinol esters are fat-soluble provitamins bound to other fats, therefore, during digestion they require pancreatic enzymatic hydrolysis (lipase and carboxylester-lipase), and intestinal hydrolysis (retinyl-ester hydrolase present on the enterocyte membrane). The absorption of retinol occurs by facilitated diffusion and depends above all on the quantity of provitamin ingested, on the lipid composition of the meal and on the concentration of bile acids poured into the intestinal lumen; the conversion of provitamin into vitamin A occurs in the intestinal lumen, therefore, in the lymphatic circulation (the first way of transporting lipids) it is already available in the form of an active molecule. Retinol performs very important functions in maintaining visual integrity and cell differentiation.
Other very important precursors are the provitamine D, of which the major exponent is ergosterol (provitamin D2), of mainly fungal origin. These are steroid molecules ubiquitously spread in food; they are converted into vitamin D at the metabolic level, more precisely in the skin. The chemical process involved is photo-dependent, that is, it can occur ONLY after sun exposure to ultraviolet (UV) rays. If the diet is deficient in provitamins or the sun exposure is insufficient, disorders of calcium metabolism and skeletal ossification may occur.
Panthenol, or provitamin B 5, is the inactive molecule of the vitamin pantothenic acid; panthenol is widespread among foods and guaranteeing its supply with food is not a common problem. Being a precursor of pantothenic acid, provitamin B5 is essential for the constitution of coenzyme A, an essential component in the metabolism of carbohydrates, amino acids, fatty acids and steroid compounds. Panthenol is also commonly defined as the "beauty vitamin", thanks to its action in maintaining skin tone, elasticity and hydration, as well as in repairing damaged hair.
Pro-vitamins are numerous and all essential to ensure the necessary intake of vitamins; although not yet in the active form, they still represent an excellent source of molecules essential to the metabolism of the human being.
With the exception of ergosterol, it can be said that despite the function of provitamins subordinates to the enzymatic action of our organism, they undoubtedly represent an extremely important category of nutrients, without which the risk of hypovitaminosis would significantly increase.