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Retinol Subject

Because of the presence of an extended polyene chain, the chemical and physical properties of the retinoids and carotenoids are dominated by this feature. Vitamin A and related substances are yellow compounds which are unstable in the presence of oxygen and light. This decay can be accelerated by heat and trace metals. Retinol is stable to base but is subject to acid-cataly2ed dehydration in the presence of dilute acids to yield anhydrovitamin A [1224-18-8] (16). Retro-vitamin A [16729-22-9] (17) is obtained by treatment of retinol in the presence of concentrated hydrobromic acid. In the case of retinoic acid and retinal, reisomerization is possible after conversion to appropriate derivatives such as the acid chloride or the hydroquinone adduct. Table 1 Hsts the physical properties of -carotene [7235-40-7] and vitamin A. [Pg.96]

More specific methods involve chromatographic separation of the retinoids and carotenoids followed by an appropriate detection method. This subject has been reviewed (57). Typically, hplc techniques are used and are coupled with detection by uv. For the retinoids, fluorescent detection is possible and picogram quantities of retinol in plasma have been measured (58—62). These techniques are particularly powerful for the separation of isomers. Owing to the thermal lability of these compounds, gc methods have also been used but to a lesser extent. Recently, the utiUty of cool-on-column injection methods for these materials has been demonstrated (63). [Pg.102]

A small but variable proportion of the carotenoids with one or two P-ionone rings (mainly P-carotene) are cleaved in the enterocytes to produce retinol (vitamin A). This process is very tightly controlled, so that too much vitamin A is not produced, although the control mechanism is not clear. Some cleavage of P-carotene can also occur in the liver, but this does not account for the turnover of P-carotene in the body. Small amounts of carotenoids are subject to enterohepatic circulation, but this does not account for losses. [Pg.118]

Vitamin A (retinol) and retinoic acid are carotenoid oxidation compounds that are very important for human health. The main functions of retinoids relate to vision and cellular differentiation. With the exception of retinoids, it was only about 10 years ago that other carotenoid oxidation products were first thought to possibly exert biological effects in humans and were implicated in the prevention - or promotion of degenerative diseases. A review on this subject was recently published. ... [Pg.187]

The concentrations of phosphate and glucose were measured on all samples and the urine samples were subject to detailed protein analysis two small proteins, retinol binding globulin and P-2-microglobulin (P2M), received particular attention. Unfortunately, none of the markers alone or in combination proved to be entirely appropriate, but a rising concentration of P2M was able to identify nearly three-quarters of subjects who subsequently showed some degree of renal damage. [Pg.281]

The normal requirement of vitamin A for adults is supplied by an adequate diet. The rational uses of retinol are in the treatment of vitamin A deficiency and as prophylaxis in high-risk subjects during periods of increased requirement, such as infancy, pregnancy, and lactation. Once vitamin A deficiency has been diagnosed, intensive therapy should be instituted. The patient should then be maintained on a proper diet. [Pg.620]

In another study (Moriel et al., 2002), 11 patients with mild essential hypertension were compared with 11 healthy subjects for water- and lipid-soluble antioxidants and the concentrations of nitric oxide derivatives in the plasma. A significant reduction in plasma lycopene was observed in the hypertensive patients compared to the normal subjects. Similar reductions in ascorbate, urate, and (3-carotene were also observed in this study. However, there were no differences in the nitrous oxide derivatives between the two groups. Hypertension and lymphatic circulation impairment are associated with liver cirrhosis. When patients with liver cirrhosis were compared to healthy matched controls, a significant reduction in serum lycopene, other carotenoid antioxidants, retinol, and oc-tocopherol were observed in the cirrhotic patients. Based on these observations, the authors recommend thorough screening for the antioxidants and improved diet in the... [Pg.141]

However, in the skin, vitamin D is also activated by oxidation, and an excess of quenching of this oxidation, due to high concentrations of /3-carotene in the skin, may reduce the amount of vitamin D availability. This is not only important for bone construction, but also because vitamin D represents a powerful natural anticancer agent. This can be an explanation why an increase of lung cancer was found in Finland (country with relative lack of sun), where smoking people and subjects with asbestosis were treated with /3-Carotene and Retinol Efficacy Trial. Concomitantly in ATBC, ischemic episodes were more severe due to the very high dosage of /3-carotene, which can become pro-oxidant. [Pg.222]

During the 1950-60 time period, there was speculation that the ultraviolet sensitivity of animals was due to a secondary absorption band of retinol and retinal designated the p-band. More recent data does not show a good correlation between the peak spectral absorption in the UV band and the peak absorption for these two p-bands. In Section 17.3.3, the absorption in the lens due to these two p-bands is seen to occupy a distinctly different spectral region than that of the UV chromophoie in humans. If the absorption in the U V was due th the presence of these bands in the chromophores of the S-, M- or L-band photoreceptors, the measured sensitivity in the UV band should correlate with the measured sensitivity in at least one of these other bands. Multiple authors have performed these experiments. They have noted the lack of correlation between the UV sensitivity of their subjects and the state of adaptation of the subjects other spectral bands165. [Pg.97]

During the development of vitamin A deficiency in experimental animals, the plasma concentration of RBP falls, while the liver content rises. The administration of retinol to deficient animals results in a considerable release of holo-RBP from the liver. This is a rapid effect on the release of preformed apo-RBP in response to the availability of retinol, rather than an increase in the synthesis of the protein. There is no evidence that retinol controls the synthesis of RB P (Soprano et al., 1982). This provides the basis of the relative dose response (RDR) test for liver stores of vitamin A (Section 2.4.1.3) administration of a test dose of retinol gives a considerably greater increase in plasma retinol, bound to RBP, in deficient subjects than in those with adequate liver reserves, because of the accumulation of apo-RBP in the liver. [Pg.46]

Plasma Retinol Binding Protein Measurement of plasma concentrations of RBP may give some additional information. Indeed, it has been suggested that because retinol is susceptible to oxidation on storage of blood samples, measurement of RBP may be a better indication of the state of vitamin A status. In adequately nourished subjects, about 13% of immunologi-caUy reactive RBP in plasma is present as the apo-protein, whereas in vitamin A-deficient children, the proportion of apo-protein may rise to 50% to 90% of... [Pg.65]

The Relative Dose Response (RDR) Test The RDR test is a test of the ahUity of a dose of vitamin A to raise the plasma concentration of retinol several hours later, after chylomicrons have heen cleared from the circulation. What is being tested is the ahUity of the liver to release retinol into the circulation. In subjects who are retinol deficient, a test dose will produce a large increase in plasma retinol, because of the accumulation of apo-RBP in the liver in deficiency (Section 2.2.3). In those whose problem is due to lack of RBP, then little of the dose will be released into the circulation. An RDR greater than 20% indicates depletion of liver reserves of retinol to less than 70 /rmol per kg (Underwood, 1990). [Pg.66]

Very few direct studies have been performed to determine human vitamin A requirements. In the Sheffield study (Hume and Krebs, 1949), 16 subjects were depleted of vitamin A for 2 years only three subjects showed clear signs of impaired dark adaptation. One of these subjects was repleted with 390 /rg of retinol per day, which resulted in a gradual restoration of dark adaptation the other two subjects received /3-carotene. On this basis, the minimum requirement was presumed to be 390 /rg, and the reference intake was set at 750 /rg. [Pg.66]

Since then, eight more subjects have been studied (Sauberlich et al., 1974 Hodges etal., 1978). From these studies, the reference intake for adult men was set at 1,000 /rg of retinol equivalent, with a minimum physiological requirement of 600 /rg per day. Because the signs of deficiency only resolve slowly, it is possible that depletion/repletion studies overestimate requirements. [Pg.67]

A concern has been raised that phytosterol doses that are effective for cholesterol reduction may impair the absorption and lower blood concentrations of fat-soluble vitamins and antioxidants. A number of studies showed that phytosterols had no effect on plasma concentrations of vitamin D, retinol, or plasma-lipid-standardized alpha-tocopherol. Moreover, the reports of the effect of phytosterols on concentrations of blood carotenoids (lutein, lycopene, and alpha-carotene) are controversial. There seems to be general agreement that phytosterol doses >1 g/d significantly decrease LDL-C standardized beta-carotene concentrations however, it remains to be determined whether a reported 15-20% reduction in beta-carotene due to phytosterol supplementation is associated with adverse health effects. Noakes et al. found that consumption of one or more carotenoid-rich vegetable or fruit servings a day was sufficient to prevent lowering of plasma carotenoid concentrations in 46 subjects with hypercholesterolemia treated with 2.3 g of either sterol or stanol esters. [Pg.133]

Vitamin A has been widely studied in the field of cancer prevention, and studies have shown that smokers who consume more dietary vitamin A from foods have a lower risk for lung cancer (Mayne et al. 1998). Vitamin A is generally given as a dietary supplement in one of two forms, either as retinol (vitamin A) or as [3-carotene, a precursor which is converted by the body to vitamin A. An investigation focusing on dietary intake of vitamin A in asbestos workers (40 subjects) reported that subjects who had developed bronchial metaplasia reported a lower intake of dietary vitamin A than those without the condition (Mayne et al. 1998). [Pg.138]


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See also in sourсe #XX -- [ Pg.1003 ]




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