Niacin vitamin absorption

Milk is an excellent source of calcium, phosphorus, riboflavin (vitamin B2), thiamine (vitamin Bl) and vitamin B12, and a valuable source of folate, niacin, magnesium and zinc (Food Standards Agency, 2002). In particular, dairy products are an important source of calcium, which is vital for maintaining optimal bone health in humans (Prentice, 2004). The vitamins and minerals it provides are all bioavailable (i.e. available for absorption and use by the body) and thus milk consumption in humans increases the chances of achieving nutritional recommendations for daily vitamins and mineral intake (Bellew et al., 2000). 

Bioavailability of Niacin. Factors which cause a decrease in macm availability include (1) Cooking losses (2) bound form in corn (maize), greens, and seeds is only partially available (3) presence of oral antibiotics (4) diseases which may cause decreased absorption (5) decrease in tiyptophan conveision as in a vitamin B deficiency. Fac.tois that increase availability include (1) alkali treatment of cereals (2) storage in bver and possibly in muscle and kidney tissue and (3) increased intestinal synthesis. 

Statins should be avoided. If absolutely necessary, pravastatin could be used, starting at a low dose and with cautious adjustment according to clinical response. The patient s synthetic liver function should be monitored closely. In the event of the slightest deterioration of function, pravastatin should be stopped immediately. Colestyramine/colestipol should be safe to use but may cause a reduction in vitamin K absorption and increase the risk of a bleed. Constipation might induce encephalopathy. The fibrates should be avoided due to their potential effect on coagulopathy. Ezetimibe should be safe to use alone. Acipimox and niacin are gastric irritants and would be best avoided. 

Some itamirLS are water soluble, while others are fat soluble. This classification is valuable as it indicates whether the vitamin is likely to be absorbed similarly to lipids or like other water-soluble nutrients. The fat-soluble vitamins are A, D, E, and K. The water-soluble vitamins arc ascorbic acid, biotin, folate, niacin, pantothenic acid, riboflavin, thiamin, vitamin B i, and vitamin B 2. The classification is also valuable, as it helps chemists decide on the best way to extract and analyze a particular vitamin in foods and biological tissues. Aside from having some bearing on the path ways of absorption and distribution throughout the body, the question of whether a particular vitamin is fat soluble or water soluble has little or no relevance to its function in the body. 

That nongrowing animals require niacin implies that it is lost from the body either as intact niacin or as a modified or breakdown product of the vitamin. An amount of niacin equivalent to nearly 90% of our daily intake is excreted in the forms of N-methyl-2-p)nidone-5urinary metabolites can be used to assess niacin status. Loss of the normal quantity in the urine each day indicates that the supply in the diet is adequate. In humans, the healthy adult excretes 4 to 6 mg of N-methyl-nicoti-namide per day. An abnormally low level indicates that the dietary intake is not adequate. Measurement of urinary niacin metabolites has proven useful in determining the amoimt of niacin available in a variety of foods. The body s ability to use niacin in different foods may vary even if the foods contain identical quantities of the vitamin. One contributing factor to the low availability of niacin is the occurrence of the vitamin in the "bound form," as mentioned earlier. Excretion of normal levels of pyridone, for example, depends not only on normal absorption of the vitamin from the diet, but also on its conversion to NAD or NADP, followed by catabolism to the metabolite. 

The two commercial forms of the vitamin, niacin and niacinamide, are rapidly absorbed from both the stomach and intestine. As the dose increases, absorption decreases. It is not clear whether there is a feedback mechanism operating or the transport system becomes saturated. Conversion to the coenzyme forms occurs in the cells where NAD and NADP are needed. 

Niacin (nicotinic acid pyridine-3-carboxylic acid) and nicotinamide are precursors of NAD+ and NADP+ (Figure 38-19). Niacin occurs in meat, eggs, yeast, and whole-grain cereals in conjunction with other members of the vitamin B group. Little is known about absorption, transport, and excretion of niacin and its coenzyme forms. A limited amount of niacin can be synthesized in the body from tryptophan, but it is not adequate to meet metabolic needs. 

Hartnup disease, an autosomal recessive trait that interferes with the absorption of tryptophan, and carcinoid syndrome in which the amino acid is preferentially oxidized to 5-hydroxytryptophan and serotonin. Prolonged treatment with the drug isoniazid, which competes with pyridoxal 5 -phosphate (a vitamin Be-derived coenzyme required in the tryptophan-to-niacin pathway), also reduces the conversion of tryptophan to niacin. Oral contraceptives that contain high doses of estrogen increase tryptophan conversion efficiency (Braidman and Rose 1971). 

Niacin is categorized as a vitamin because its precursor, tryptophan, is an essential amino acid, so the human synthesis of niacin is dependent upon diets. Preformed niacin is widely distributed in plant and animal foods. The typical preformed niacin sources in diets are meat and meat products, cereals, dairy products, beverages, and eggs. However, cereals with esterified niacin in complexes have this vitamin imavail-able for absorption, but its bioavailability can be increased by treatment with alkali to hydrolyze the esters. Coffee can be a source of niacin, as nicotinic acid is liberated in coffee by roasting. 

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