Niacin, absorption

Several different niacin formulations are available niacin immediate-release (IR), niacin sustained-release (SR), and niacin extended-release (ER).28,29 These formulations differ in terms of dissolution and absorption rates, metabolism, efficacy, and side effects. Limitations of niacin IR and SR are flushing and hepatotoxicity, respectively. These differences appear related to the dissolution and absorption rates of niacin formulations and its subsequent metabolism. Niacin IR is available by prescription (Niacor ) as well as a dietary supplement which is not regulated by the FDA.28 Currently, there are no FDA-approved niacin SR products, thus, all SR products are available only as dietary supplements. 

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). 

Ezetimibe/Simvastatin (Vytorin) [Antilipemic/HMG CoA Reductose Inhibitor] Uses H rp cholest olemia Action X Absorption of cholesterol phytost ol w/ HMG-CoA reductase inhibitor Dose 10/10-10/80 mg/d PO w/ cyclosporine or danazol 10/10 mg/d max w/ amio-darone or verapamil 10/20 mg/d max -1- w/ sev e renal insuff Caution [X, -] w/ CYP3A4 inhibitors (Table VI-8), gemfibrozil, niacin >lg/d, danazol, amiodarone, verapamil Contra PRG/lactation livCT Dz, t LFTs Disp Tabs SE HA, GI upset, myalgia, myopathy (muscle pain, weakness, or tendOTiess w/ CK 10 x ULN, rhab-domyolysis), Hep, Infxn Interactions t Risk of myopathy W7 clarithromycin, erythromycin, itraconazole, ketoconazole EMS None OD Sxs unknown symptomatic and supportive... 

Absorption of certain drugs, including those with neutral or cationic charge as well as anions, may be impaired by the resins. These include digitalis glycosides, thiazides, warfarin, tetracycline, thyroxine, iron salts, pravastatin, fluvastatin, folic acid, phenylbutazone, aspirin, and ascorbic acid. Any additional medication (except niacin) should be given 1 hour before or at least 2 hours after the resin to ensure adequate absorption. Colesevelam does not bind digoxin, warfarin, or reductase inhibitors. 

Uses Acute chronic gout Action X Renal tubular absorption of uric acid Dose 100-200 mg PO bid for 1 wk, T PRN to maint of 200—400 mg bid max 800 mg/d take w/ food or antacids plenty of fluids avoid salicylates Caution [C (D if near term), /-] Contra Renal impair, avoid salicylates peptic ulcer blood dyscrasias, near term PRG, allergy Disp Tabs, caps SE N/V, stomach pain, urolithiasis, leukopenia Interactions T Effects OF oral anticoagulants, oral hypoglycemics, MTX X effects W/ ASA, cholestyramine, niacin, salicylates, EtOH X effects OF acetaminophen, theophylline, verapamil EMS T Effects of anticoagulants and oral hypoglycemic X effects of verapamil OD May cause N/V, loss of coordination, dyspnea, Szs symptomatic and supportive... 

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. 

The physiologically active forms of niacin are nicotinic acid, nicotinamide, and their coenzymes (93,96). Niacytin and the niacynogens appear to have limited bioavailability, although more work is needed in this area. The absorption and metabolism of niacin has been reviewed (20,93). 

Any additional medication (except niacin) should be given 1 hour before or at least 2 hours after the resin to ensure adequate absorption. Colesevelam does not bind digoxin, warfarin, or reductase inhibitors. 

A. The infrared absorption spectrum of a mineral oil dispersion of the sample, previously dried at 105° for 1 h, exhibits maxima only at the same wavelengths as those of a similar preparation of USP Niacin Reference Standard. 

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. 

Hartnup disease is a rare genetic condition in which there is a defect of the membrane transport mechanism for tryptophan and other large neutral amino acids. The result is that the intestinal absorption of free tryptophan is impaired, although dipeptide absorption is normal. There is a considerable urinary loss of tryptophan (and other amino acids) as a result of the failure of the normal reabsorption mechanism in the renal tubules - renal aminoaciduria. In addition to neurological signs that can be attributed to a deficit of tryptophan for the synthesis of serotonin in the central nervous system, the patients show clinical signs of pellagra, which respond to the administration of niacin. 

The immediate-release formulation of phytosterols is made by Endurance Products Company, based outside of Portland, Oregon. Its Web site is www.endur.com. This is the same company that makes the best formulation of my favorite method of cholesterol control, niacin. Place orders outside the United States at www. endur.com click on Customer Service page. International Orders. Take two 450 mg tablets at the start of two major meals daily to block the absorption of cholesterol in the foods you eat, inhibit the recycling of bile made of cholesterol, and achieve a cholesterol reduction in your bloodstream of up to 10 percent. 

Hartnup s disease. There is a defect in the epithelial transport of neutral amino acids (e.g., tryptophan) leading to poor absorption and excess excretion of these amino acids. Clinical signs resemble those of niacin deficiency (tryptophan is a precursor of niacin), namely the 3 D s Diarrhea, Dementia, Dermatitis. The condition responds to nicotinamide administration. Fan-coni s syndrome is a more generalized defect in molecular transport, involving a multitude of amino acids, glucose, calcium, phosphate, proteins, and other molecules. There may be decreased growth and rickets. 

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. 

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