Lipid-soluble vitamins accumulation

Lipid-soluble vitamins accumulate in fatty tissue, leading to toxic effects. Water-soluble vitamins are excreted, drastically reducing the chances of an overdose. 

Vitamins are classified as either lipid soluble or water soluble, as shown in Table 13.2. The lipid-soluble vitamins tend to accumulate in fatty tissue, where they may be stored for years. Adults can remain free of deficiency diseases for... 

A lack of lipid-soluble vitamins can be detrimental, but so can excessive amounts, particularly of vitamins A and D, which can accumulate to dangerous levels. Too much vitamin A causes dry skin, irritability, and headaches. Excessive amounts of vitamin D lead to diarrhea, nausea, and calcification of joints and other body parts. Vitamins E and K are less harmful in large quantities because they are readily metabolized. 

Vitamin A is a lipid-soluble vitamin, which can accumulate in the body. Overdoses of this vitamin can be toxic. 

Hypolipoproteinemias Abetaiipoproteinemia No chylomicrons, VLDL, or LDL are formed because of defect in the loading of apo B with lipid. Rare blood acylglycerols low intestine and liver accumulate acylglycerols. Intestinal malabsorption. Early death avoidable by administration of large doses of fat-soluble vitamins, particularly vitamin E. 

The lung also possesses nonenzymatic antioxidants such as vitamin E, beta-carotene, vitamin C, and uric acid. Vitamin E is lipid-soluble and partitions into lipid membranes, where it is positioned optimally for maximal antioxidant effectiveness. Vitamin E converts superoxide anion, hydroxyl radical, and lipid peroxyl radicals to less reactive oxygen metabolites. Beta-carotene also accumulates in cell membranes and is a metabolic precursor to vitamin A. Furthermore, it can scavenge superoxide anion and react directly with peroxyl-free radicals, thereby serving as an additional lipid-soluble antioxidant. Vitamin C is widely available in both extracellular and intracellular spaces where it can participate in redox reactions. Vitamin C can directly scavenge superoxide and hydroxyl radical. Uric acid formed by the catabolism of purines also has antioxidant properties and primarily scavenges hydroxyl radical and peroxyl radicals from lipid peroxidation. 

Bile has long been attributed an important role in medicine [1]. The effect of an impaired bile flow to the intestine has been known to result in steatorrhea — fat malabsorption — and defective absorption of fat-soluble vitamins, notably vitamin K [2], Thus, it is obvious that bile is important for fat assimilation from the intestine. However, it is equally apparent that when fat absorption after bile obstruction or diversion could be studied by quantitative methods, the malabsorption was found to be only partial [3]. In fact, it has seemed surprising that some 60-70% of a normal fat load is absorbed in man and the experimental animal in the absence of bile in the intestine. The absorption of nonpolar lipids, however, is much less efficient, and cholesterol absorption has been reported to have an absolute requirement for the presence of bile salts [4]. Of the bile components important for fat absorption bile salts have been ascribed the main role although experimental results are accumulating regarding the role of bile phospholipids in the specific uptake of sterols by the intestine [5]. 

Vitamin A deficiency can have drastic consequences, as would be predicted from its importance in vision. Night blindness—and even total blindness—can result, especially in children. On the other hand, an excess of vitamin A can have harmful effects, such as bone fragility. Lipid-soluble compounds are not excreted as readily as water-soluble substances, and excessive amounts of hpid-soluble vitamins can accumulate in adipose tissue. 

Some vitamins (A, D, E, and K) have very nonpolar molecular structures and therefore dissolve only in nonpolar solvents. In the body, the nonpolar solvents are the lipids we have classified as fats, so these vitamins are called fat-soluble. The fat-soluble vitamins have diverse functions in the body, and they act somewhat like hormones (Table 12.3). Care must be taken to avoid overdoses of the fat-soluble vitamins. Toxic effects are known to occur, especially with vitamin A, when excess amounts of these vitanfins accumulate in body tissue. Excesses of water-soluble vitamins are excreted readily through the kidneys and are not normally a problem. 

---