B Vitamins Flashcards
Thiamin function
Adequacy
DRI
Function:
- Metabolic coenzyme for AA and CHO
TDP/TPP or thiamin pyrophosphate cofactor for decarboxylation reactions
- Cofactor for pentose synthesis and NADPH
- Membrane and nerve conduction (not cofactor)
Erythrocyte transketolase (TPP dependent) activity in PPP
- Thiamin deficiency risk = > 1.25 ratio (enzyme with and w/o TDP)
- [Thiamin] and thiamin esters in blood
- Urinary thiamin excretion (basal and thiamin loaded)
RDA Sex differences due to size
No UL - no negative effects and decreased absorption above 5mg
Riboflavin function
adequacy and DRI
Riboflavin is absorbed —> FMN —> FAD in cytoplasm and mitochondria for energy production
Redox coenzyme in CAC, ETC, B ox, niacin synthesis, B6 activation, neurotransmitter catabolism, antioxidant enzymes
Involved in thioredoxin system, glutaredoxin antioxidant system and glutathione peroxidase system
Glutathione reductase activity coefficient
RBC [riboflavin]
Urinary excretion relative to dietary intake
> 1.4 risk of deficiency (enzyme w/ and w/o FAD)
RDA
Sex differences due to size
No UL —> no observed toxicity (low absorption and solubility)
Niacin function
Niacin adequacy and DRI
Coenzyme for H transfer in dehydrogenases (generates NAD and NADP+)
Urinary excretion of niacin metabolites
- No adjustment for bioavailability because it is high
Some conversion from tryptophan (variable, 60mg Trp —> 1mg B3)
EAR/RDA in Niacin Equivalents (NE)
Sexes adjusted for size
UL = 35 mg/d (niacin flush due to vasodilation)
Pantothenic acid function
adequacy and DRI
Component of CoA and phosphopantetheine (FA metabolism)
Balance study - adequate intake to balance urinary excretion
AI based on usual intake of 4-7 mg/d no evidence this is not adequate
AI set to 5 mg/d
No UL
Pyridoxine function
Pyridoxine adequacy and DRI
Coenzyme in metabolism of AA, glycogen and sphingoid bases
as PLP (pyridoxal 5’ phosphate) and PMP (pyridoxamine 5’ phosphate)
Maintenance of adequate PLP blood levels
Same for both sexes
RDA for ages 19-50
UL = 100 mg/d —> can cause peripheral neuropathy
Biotin functions
Adequacy
DRI
cofactor in bicarbondate dependent carboxylation reactions
Ex. ACC 1 (acetyl coA —> malonyl coA) and 2 (beta ox), propionyl-coA carboxylase, pyruvate carboxylase, methylcrotonyl-coA carboxylase
AI based on estimates of intake
For all adults and both sexes
No UL
Folate Functions
Folate adequacy and DRI
1) Coenzyme for single carbon transfer in nucleic acid and AA metabolism
2) Synthesis of purine and pyrimidine bases → DNA replication
3) Cofactor for enzymes metabolizing His, Ser, Gly, and Met in active coenzyme form: tetrahydrofolate (THF-4)
4) Ser → Gly (conversion of THF-4 –> 5,10 THF-4)
Erythrocyte folate (more accurate)
Blood [homocysteine] and [folate]
Use Dietary Folate Equivalents (DFEs) to adjust for -50% bioavailability in dietary folate vs. Folic acid (supplement/fortified)
RDA for all = 400 mcg/d of DFEs
400 mcg/d at least 1 month prior to conception, and throughout pregnancy
high risk of NTD women should take 4 mg/d
UL = 1000 mcg/d of DFEs
- Inefficient conversion can overwhelm dihydrofolate reductase (adverse cognition and immune effects)
Cobalamin function
Cobalamin adequacy and DRI
Coenzyme for methyl transfer from homocysteine to methionine
- Methyl-folate trap: insufficient B12 means no methyl transfer with methyl synthase from 5-methyl-THF and increased homocysteine/decreased Met
Conversion of L-methylmalonyl-coenzyme A (coA) —> succinyl coA
Maintenance of hematological status
Normal blood B12 status
- L-methyl-malonic acid in urine indicates deficiency
RDA for all ages and sexes
No UL - low toxicity potential
No sex differences
UL present in
AI
B5, B6, B7, B9, B12
B3, B6, B9 (folate)
B5, B7
