Vitamin B3 - Niacin

Question 1

List dietary sources of niacin

Answer 1

Fish and meats
Riches flours, fortified cereals, whole grains, seeds and legumes
Coffee and tea
Green leafy vegetables
Dairy products
Also synthesized in the liver from tryptophan
- takes 60mg of tryptophan to produce 1mg of niacin
- riboflavin, B6 and iron are required for conversion

Question 2

Describe the digestion niacin

Answer 2

In animals niacin mainly occurs as NAD & NADP but after slaughter it undergoes hydrolysis to free nicotinamide

NAD & NADP are also hydrolyzed to free nicotinamide in the intestinal tract

Question 3

Describe the absorption of niacin

Answer 3

Nicotinamide and niacin are absorbed in:
Stomach - minor - enhanced by HCL
Small intestine - major

Two mechanisms

1. Sodium dependent carrier-mediated diffusion
2. Passive diffusion

*can also be absorbed through the skin - creams

Question 4

Describe the transport of niacin

Answer 4

Transported in The Lord primarily as free nicotinamide

Question 5

Describe the function and mechanism of action of niacin

Answer 5

Over 200 enzymes use NAD or NADP as coenzymes
NAD used in:
- glycolysis
- oxidation of acetyl CoA, FAs & ETOH
NADH transports electrons through intermediates in ETC
NADPH required in:
- synthesis of FAs, cholesterol, steroid hormones, deoxyribonucleotides
- regeneration of glutathione and vitamin C
- conversion of folate to its active form
To detoxify alcohol

Question 6

Describe the metabolism and excretion of niacin

Answer 6

NAD/NADP is degraded into nicotinamide which is then methylated and oxidized in the liver to several metabolites that are excreted in the urine

Question 7

Describe consequences and symptoms of deficiency of niacin

Answer 7

Pellagra
Symptoms:
Dermatitis
- appears on areas exposed to sun a s similar to sunburn at first
Dementia and other neurological manifestations
- H/A, apathy, anxiety, loss of memory, peripheral neuropathy and paralysis of extremities
Diarrhea and other GI manifestations
- glossitis, cheilosis/stomatitis, nausea, vomiting and constipation
Death

Question 8

Who are at increase risk of developing a niacin deficiency?

Answer 8

Alcoholic
Elderly in nursing homes
Low income
Carcinoid syndrome –> tumor cells have increased production of serotonin
Hartnup’s disease –> impaired tryptophan absorption
The use of certain medications, such as anticonvulsants and isoniazid

Question 9

Describe consequences and symptoms of toxicity of niacin

Answer 9

UL = 35mg/d

Side effects seen with doses >1g/d

• vasodilatory effects: flushing, redness, burning, itching, tingling, h/a
• GI effects: acid reflux, nausea, vomiting, loss of appetite in children
• hepatotoxicity:
• • obstructive jaundice, hepatitis, liver failure
• • monitor LFTs (liver function tests)
• • liver function reruns to normal after discontinuation
• • sustained release preparations are more hepatotoxic
• hyperurecemia and gout
• hyperglycemia/hypoglycemia

Flush may be reduced by:

• taking 80-500mg of aspirin (will still feel itchy)
• taking an antihistamine
• taking nicotinamide acid after a meal

Question 10

Describe the assessment of nutrient status of niacin

Answer 10

Measuring urinary metabolites
Erythrocyte NAD concentration
Ratio of NAD to NADPH
Clinical response to supplementation

Question 11

Discuss some of the therapeutic uses (clinical indications) of niacin

Answer 11

Hyperlipidemia
- nicotinamide acid 1-6g/d
- shown to lower TC, TGs, LDL, and increase HDL
Anxiety disorders
- nicotinamide 1.5-6g/d

Best taken in divided doses due to short half life and to reduce side effects