There are two forms of niacin: nicotinic acid and nicotinamide (aka niacinamide), that have a carboxylic acid group or amide group, respectively. The structure of nicotinic acid and nicotinamide are shown below.
Figure 10.51 (left) Structure of nicotinic acid1 (right) Figure 10.52 Structure of nicotinamide2
Niacin is important for the production of two cofactors: nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP+). The structure of NAD is shown below; you can clearly see the nicotinamide at the top right of the molecule.
Figure 10.53 Structure of NAD3
NAD is reduced to form NADH, as shown below.
Figure 10.54 Reduction of NAD to NADH4
The structure of NADP+ is exactly the same as NAD, except it has an extra phosphate group off the bottom of the structure, as shown below.
Figure 10.55 Structure of NADP+5
Like NAD, NADP+ can be reduced to NADPH.
Niacin is unique in that it can be synthesized from the amino acid tryptophan as shown below. An intermediate in this synthesis is kynurenine. Many reactions occur between this compound and niacin, and riboflavin and vitamin B6 are required for two of these reactions.
Figure 10.56 Tryptophan can be used to synthesize niacin6
To account for niacin synthesis from tryptophan, niacin equivalents (NE) were created by the DRI committee to account for the amount of niacin in foods as well as their tryptophan content. It takes approximately 60 mg of tryptophan to make 1 mg of niacin. Thus, the conversions to niacin equivalents are:
1 mg Niacin = 1 NE
60 mg Tryptophan = 1 NE
The tryptophan levels of most foods is not known, but a good estimate is that tryptophan is 1% of amino acids in protein7. Thus, let's take peanut butter, smooth style, with salt as an example8.
The peanut butter contains 13.403 mg of niacin and 25.09 g of protein8.
Step 1: Calculate the amount of tryptophan:
25.09 g X 0.01 (the numerical value of 1%) = 0.2509g of tryptophan
Step 2: Convert Grams to Milligrams
0.2509 g X 1000 mg/g = 250.9 mg of tryptophan
Step 3: Calculate NE from tryptophan
250.9 mg of tryptophan/(60 mg of tryptophan/1 NE) = 4.182 NE
Step 4: Add NEs together
13.403 NE (from niacin) + 4.182 (from tryptophan) = 17.585 NE
Most niacin we consume is in the form of nicotinamide and nicotinic acid9, and in general, is well absorbed using an unresolved carrier10. However, in corn, wheat, and certain other cereal products, niacin bioavailability is low. In these foods, some niacin (~70% in corn) is tightly bound, making it unavailable for absorption. Treating the grains with a base frees the niacin and allows it to be absorbed. After absorption nicotinamide is the primary circulating form7,9.
References & Links
- Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill.
- Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
- Said H, Mohammed Z. (2006) Intestinal absorption of water-soluble vitamins: An update. Curr Opin Gastroenterol 22(2): 140-146.