niacin Flashcards
why is niacin needed
as its the other form of redox cofactor
niacin exists in what 2 forms
NAD
NADP
We cant absorb in NAD and NADP true cofactor form
why does niacin need to be mobile
as It participates in reaction, enzyme takes it off and it reacts somewhere else so it needs to be mobile
what forms can be absorb niacin in and explain absorption
in nicotinic acid and nicotinamide form
absorbed as free nicotinic acid in stomach and small intestine or nicotinamide in small intestine
NAD/NADP in food hydrolysed by enzymes
absorption is facilitated (contribution of passive at high concentrations)
there is poor bioavailability of nicacin when
where covalently bound to complex proteins or carbohydrates (e.g. corn, wheat, maize etc.)
bioavailability of niacin can be increased by what
treatment in lime-water (nixtamalization; calcium hydroxide; alkaline)
As alkaline solution releases cofactor. This is done in Mexican foods. Such as in cornflour
Lime water is solution of calcium hydroxide
in food niacin is found in what form, explain
found in cofactor form so has to be hydrolysed into absorbable form of nicotinic acid or nicotinamide
Cofactors are strongly covalently bound to carbs or proteins, so difficult to remove cofactors and make niacin source available
niacin sources
yellowfin tuna
lamb liver
chicken breast
peanuts
pork chop
rib of beef
sunflower seeds
portobello mushrooms
Commonly associated with cereal.
Fruit and veg only have trace amounts of niacin
legumes are good sources of niacin
describe niacin in cereal
Commonly associated with cereal. But the source is bioavailable (as not in absorbable form)
In wholewheat most niacin is bioavailable
what do we need niacin for
NAD: redox reactions
(energy production)so in ATP synthesis
glycolysis (converts glycose into pyruvate)
pyruvate dehydrogenase
TCA cycle
b-oxidation(pathway that degrades fatty acids)
production of lactate
(NADH)
production of excretory metabolite of
vitamin B6 (pyridoxal to pyridoxic acid)
what is NADPH involved with
NADPH: redox reactions
(lipid biosynthesis;
anti-oxidative defence of cells(so protect from antioxidant defence system)
fatty acid synthesis
cholesterol/steroid synthesis
synthesis of deoxyribonucleotides
regeneration of antioxidants
(glutathione, vitamin C and
thioredoxin)
folate metabolism (THF, methyl-THF, methylene THF)
importance of supplying enough tryptophan in diet
we can use to synthesis niacin and not be reliant on diet to supply niacin
what are the nicotinic acid equivalents
= niacin + potential niacin
nicotinamide can be synthesised by what and where
from tryptophan in liver
60 mg tryptophan equivalent to 1 mg of dietary niacin
generally, 10-12.6 mg tryptophan per gram of dietary protein
tryptophan sources
pumpkin and squash seeds
roasted soybeans
reduced fat mozzarella
lamb shoulder
chicken breast
tuna (cooked)
crab (cooked)
eggs
white beans
what are the EAR and RNI for niacin
EAR 5.5 mg/1000 kcal
RNI 6.6 mg/1000 kcal
what are the RNIs for males and females for niacin
RNI males = 17 mg/d
RNI females = 13 mg/d
how much niacin do the median intakes of protein for males and females supply
84 g/d males (17.64 mg niacin equival.)
61.8 g/d females (12.98 mg niacin equival.)
explain the percentage of niacin excretion in the body processes
normally excrete 20% to 30% of niacin intake as N-methylnicotinamide
and 40% to 60% as N-methyl-2-pyridone-5-carboxamide in the urine.
what happens to excretion of pyridone compared to methylnicotinamide in niacin deficiency
excretion of pyridone falls more than methylnicotinamide.
methylnicotinamide ratio useful indicator of what
pyridone: methylnicotinamide ratio useful indicator. Less than one indicative of deficiency.
what happens to, erythrocyte ratio of NAD:NADP, during deficiency
seems to be preserved in favour of NAD in deficiency (indicated by NAD:NADP < 1.0).
niacin deficiency is called what
pellagra
what happens from pellagra
Dermatitis
Dementia
Diarrhoea
Death
Dermatitis similar to sunburn (see Casal’s collar) and also occurs around pressure points (e.g. elbows)
Neurological:
Headache, apathy, memory loss, peripheral neuritis, paralysis of extremities
Gastrointestinal:
Glossitis, stomatitis, nausea, vomiting, diarrhoea
what can pellagra be promoted by
treatment of tuberculosis (e.g. isoniazid)
malabsorption (e.g. IBD)
Hartnup disease – malabsorption of tryptophan absorption
what is used to treat hyperlipo-proteinaemia
large doses of nicotinic acid (6 g/d)
