Vitamin B-3; Niacin

 Niacin

Vitamin B-3

Niacin, or vitamin B-3, exists in 2 forms; nicotinic acid (niacin) and nicotinamide (niacinamide). Both forms are used to synthesize the niacin coenzymes: nicotinamide adenine, dinucleotide (NAD+) and nicotinamide adenine dinucleotide phosphate (NADP+). 

Niacin in Foods:

Food sources of Niacin

Niacin can be obtained from foods as the vitamin itself or synthesized in the body from the essential amino acid tryptophan. Poultry, meat and fish provide about 25% of the performed niacin in North American diets. Another 11% comes from enriched bread and bread products. Coffee and tea also contribute a little performed niacin to the other diet. Mushrooms, wheat bran, fish, poultry and peanuts are also rich sources of niacin. Protein-rich foods are also good sources of niacin because they provide tryptophan. Unlike some other water-soluble vitamins, niacin is very heat stable and little is lost in cooking.

In the synthesis of niacin from tryptophan, 60 mg of dietary tryptophan is needed to make about 1mg of niacin. Riboflavin and vitamin B-6 coenzymes also are required. Protein is about 1% tryptophan, so 1 g of protein provides 10 mg of tryptophan. Individuals with an adequate protein intake meet much of their niacin requirement through tryptophan. Nutrient databases often underestimate niacin in the diet because the amount of tryptophan in many foods has not been determined yet.

Niacin Needs and Upper Level:

The niacin RDA for adult men is 16 mg/day; for adult women, it is 14 mg/day. The RDA for niacin is expressed as niacin equivalents to account for performed niacin in foods and niacin is synthesized from tryptophan. Niacin intake is ample in the U.S. with intakes of performed niacin from food averaging 31.2 mg/day for men and 20.8 mg/day for women. These figures underestimate intake because they do not include niacin synthesized from tryptophan, which supplies about half the NE in the diet. The Daily Value for niacin on food and supplement labels is 20 mg. The Upper Level for niacin, 35 mg/day, applies only to niacin supplements and fortified foods.

Absorption, Transport, Storage & Excretion of Niacin:

Nicotinic acid and nicotinamide are readily absorbed from the stomach and the small intestine by active transport and passive diffusion, so generally almost all the niacin that is consumed is absorbed. However, the bioavailability of niacin is low in some grains, especially corn. This is because the niacin is tightly bound to protein; less than 30% can be absorbed. Niacin can be released from the protein and its bioavailability improved by soaking corn in a solution of calcium hydroxide dissolved in water. This practice, done to release the skin from the corn kernels so that dough can be formed, is among indigenous peoples of Latin America, where corn, often in the form of tortillas, is a staple food. This culinary practice brings the added benefit of protection against niacin deficiency. After being absorbed, niacin is transported via the portal vein to the liver, where it is stored or delivered to the body's cells. Niacin is converted to its coenzyme forms in all tissues. Any excess niacin is excreted in the urine.

Functions of Niacin:

Like the coenzyme forms of riboflavin, the coenzyme forms of niacin, NAD+ and NADP+, are active participants in oxidation-reduction reactions. The niacin coenzymes function in at least 200 reactions in cellular metabolic pathways, especially those that produce ATP. NAD+  is required mainly for the catabolism of carbohydrates, proteins, and fats. NAD+ acts as an electron and hydrogen acceptor in glycolysis and the citric acid cycle. Under anaerobic conditions, NAD+ is regenerated when pyruvate is converted to lactate. Under aerobic conditions, NADH +  H+ donates electron and hydrogens to acceptor molecules in the electron transport chain, thereby contributing to ATP synthesis. Alcohol metabolism also requires niacin coenzymes.

 These reactions start with an oxidized form of a niacin coenzyme. However, synthetic pathways in the cell __those that make new compounds __use NADH + H+, the reduced form of the coenzyme. This coenzyme is important in the biochemical pathway for fatty acids have higher concentrations of NADPH + H+ than cells not involved in fatty acid synthesis.

Niacin Deficiency:

Because almost every metabolic pathway uses either NAD+ or NADPH + H+, it is not surprising that a niacin deficiency causes a widespread damage in the body. The niacin deficiency pellagra, once a significant public health problem in the U.S. is now eradicated here, thanks to the enrichment of grains and protein-rich diets. The discovery of how pellagra develops from a poor diet, rather than a bacterial infection, is a fascinating story.

  The first official record of pellagra, made in 1735by Spanish physician Gaspar Casal, called this disease mal de la rosa, or "red sickness". This name referred to the rough, red rash that appears on skin exposed to sunlight, such as the forearms, backs to the hands, face, and neck ( called Casal's necklase ). The name pellagra comes from the Italian pelle, meaning "skin" and agra, meaning "rough". Others symptoms of pellagra include diarrhea and dementia. Thus, pellagra is identified by the 3 Ds: dermatitis, diarrhea, and dementia. Death, the fourth D, can result if the disease is not treated. 

Pellagra disease symptoms

  Pellagra has long been associated with corn-based diets. Although there is no evidence of pellagra among the indigenous populations of North, Central, and South America, where corn has been the staple food in the diet for thousands of years, pellagra outbreaks followed the introduction of corn into Europe and Africa. The main reason for this was that the indigenous peoples of Latin America treated corn with alkali (from lime water or wood ashes ), which released the niacin that is tightly bound to protein. Unfortunately, this practice was not adopted in Europe, Africa, or the U.S. When maize became a staple food, especially among poor people who could afford few other foods, the result was a very low niacin intake, often resulting in pellagra. Scientists have since discovered another reason that corn-based diets can lead to pellagra __corn contains little of the amino acid tryptophan.

Pharmacological use of Niacin:

Niacin, as  nicotine acid, is sometimes prescribed by physicians to increase HDL-cholesterol and lower triglyceride levels. When combined with diet, exercise, and other cholesterol lowering medications, nicotinic acid may reduce the risk of heart attack, but a recent large study does not support this. The dose used, 1 to 2 g daily, is more than 60 times the RDA. The most common side effect is flushing of the skin, but GI tract upset and liver damage also can occur. Although niacin is readily available in dietary supplement form, it must not be used as a substitute for the prescription formulation of niacin, which is carefully prepared to an exact dosage and has controlled time-release.