test 2 h

Folate (B9): Deficiency markers & vitamin status

FIGLU ↑ in blood or urine

Homocysteine ↑

low Serum folate

low RBC folate

reflects long-term folate stores and is the best indicator of folate status; Vitamin B12 deficiency can also increase FIGLU because the methyl folate trap causes a functional folate deficiency.

Vitamin B12: Deficiency markers & vitamin status

B12 deficiency can cause

high homocysteine

high methylmalonic acid/methylmalonyl-CoA. High MMA is specific for B12 deficiency

Vitamin B6: Deficiency markers & vitamin status

B6 deficiency can cause

low plasma PLP

high homocysteine.

PLP is the main active form of B6

Vitamin A: Vitamin status markers

low Plasma retinol

low Retinol-binding protein (RBP)

Plasma retinol usually does not decrease until liver vitamin A stores become depleted.

Vitamin E: Vitamin status markers

Plasma α-tocopherol is the primary marker of vitamin E status

increased red blood cell hemolysis indicates deficiency

Vitamin K: Vitamin status markers

prolonged clotting time

bleeding

high prothrombin time

pivka protein

Methionine cycle, SAM generation & methyl folate trap

THF is converted to 5,10-methylene THF; MTHFR converts 5,10-methylene THF → 5-methyl THF; 5-methyl THF donates a methyl group to vitamin B12; Methionine synthase transfers the methyl group from B12 to homocysteine, forming methionine; Methionine combines with ATP to form S-adenosylmethionine (SAM), the universal methyl donor used for DNA, RNA, protein, lipid, neurotransmitter, and myelin methylation; After donating its methyl group, SAM becomes SAH then homocysteine; In vitamin B12 deficiency, 5-methyl THF cannot donate its methyl group and becomes trapped (methyl folate trap), THF cannot be regenerated, DNA synthesis decreases, and a functional folate deficiency develops.

FPG (Folylpolyglutamate Synthetase): Function

Adds glutamate residues to folate inside cells; Converts folate to the polyglutamate form; Traps folate inside cells for storage and metabolism.

MTHFR (Methylenetetrahydrofolate Reductase): Function

Converts 5,10-methylene THF → 5-methyl THF; Produces the folate form required for methionine synthesis and SAM production; Reduced activity increases homocysteine.

DHFR (Dihydrofolate Reductase): Function

Converts dihydrofolate (DHF) → tetrahydrofolate (THF); Activates folate for DNA synthesis and one-carbon metabolism.

LRAT (Lecithin Retinol Acyltransferase): Function

Transfers fatty acids to retinol to form retinyl esters (mainly retinyl palmitate); Main esterification enzyme in the intestine, liver, and retina; Allows vitamin A to be stored and packaged into chylomicrons. oai_citation:4‡L11- Vitamin A-fs.pptx

Transducin: Function

G-protein involved in vision; Activated when light converts 11-cis retinal to all-trans retinal in rhodopsin; Activates phosphodiesterase to begin the visual signal.

Parathyroid Hormone (PTH): Function

Increases blood calcium; Stimulates calcium release from bone; Increases calcium reabsorption in the kidney; Stimulates activation of vitamin D, increasing intestinal calcium absorption.

Folate (B9): Absorption, factors affecting absorption & deficiency

Dietary folate is mainly polyglutamate and must be converted to monoglutamate before absorption by folate hydrolase (glutamate carboxypeptidase), a zinc-dependent enzyme; PCFT transports folate from intestinal lumen → enterocyte; MRP3 and MRP5 transport folate from enterocyte → blood; RFC transports folate from blood → body cells; Folate receptors transport folate into certain tissues; Absorption decreases with zinc deficiency, alcohol, malabsorption, inflammatory bowel disease, gastric bypass, methotrexate, phenytoin, and sulfasalazine; Pregnancy increases folate requirements; Vitamin B12 deficiency causes a functional folate deficiency because of the methyl folate trap.

Vitamin B6: Absorption, deficiency & treatment

Phosphorylated B6 vitamers must be dephosphorylated before absorption by alkaline phosphatase, a zinc-dependent enzyme; Absorbed mainly by passive diffusion in the jejunum (~75% absorbed); Liver converts B6 to PLP, the major active coenzyme form; FMN-dependent oxidase requires riboflavin to produce PLP; Risk factors include alcohol, poor diet, malabsorption, zinc deficiency, riboflavin deficiency, and certain medications; Deficiency symptoms include seborrheic dermatitis, glossitis, cheilosis, depression, confusion, neuropathy, seizures, and microcytic anemia; Drug-induced deficiency (ex. isoniazid) is treated with pyridoxine supplementation. oai_citation:5‡L10-Vit B6.pptx

Folate (B9): Functions & why it is essential

Functions: one-carbon transfer reactions, DNA synthesis, RNA synthesis, purine synthesis, thymidylate (dTMP) synthesis, methionine cycle, SAM production, DNA/RNA/protein methylation, red blood cell formation, rapid cell division and growth; Essential because humans cannot synthesize folate and must obtain it from the diet.

Folate (B9): Transporters

PCFT (Proton-Coupled Folate Transporter): intestinal lumen → enterocyte (main folate transporter); MRP3 and MRP5: enterocyte → blood; RFC (Reduced Folate Carrier): blood → body cells; Folate Receptors (FR): receptor-mediated endocytosis into tissues (especially placenta and brain).

Vitamin B12: Transporters

R-protein (Haptocorrin): binds B12 in the stomach; Intrinsic Factor (IF): binds B12 in the duodenum after R-protein is digested; Cubam receptor: transports IF-B12 complex from ileal lumen → enterocyte by receptor-mediated endocytosis; Transcobalamin II: transports B12 from blood → body tissues.

Vitamin A (Retinol): Transporters

Retinol enters the enterocyte mainly by passive diffusion; CRBPII (Cellular Retinol Binding Protein II): binds retinol and retinal inside the enterocyte and directs metabolism; LRAT esterifies retinol to retinyl esters so it can leave the enterocyte; Retinyl esters leave the enterocyte in chylomicrons → lymph → blood; RBP (Retinol Binding Protein): transports retinol in blood; TTR (Transthyretin): binds RBP to prevent kidney filtration; STRA6: transports retinol from blood → body cells.

Carotenoids: Transporters

SR-B1 (Scavenger Receptor Class B Type 1): intestinal lumen → enterocyte; Some carotenoids also enter by passive diffusion; ISX decreases SR-B1 expression when vitamin A levels are high, reducing carotenoid absorption and conversion to retinal.

Vitamin B12: Steps of absorption

1. Stomach acid, pepsin, and proteases release B12 from food proteins; 2. B12 binds R-protein (haptocorrin) in the stomach; 3. Pancreatic enzymes digest R-protein in the duodenum; 4. B12 binds Intrinsic Factor (IF); 5. IF-B12 binds Cubam receptors in the ileum; 6. Complex enters the enterocyte by receptor-mediated endocytosis; 7. B12 binds Transcobalamin II and exits the enterocyte; 8. Transcobalamin II transports B12 through the blood to tissues.

Vitamin B6: Schiff base

PLP (Pyridoxal-5'-phosphate) forms a Schiff base (C=N bond) with amino acids; This stabilizes the amino acid and allows PLP-dependent enzymes to perform transamination, decarboxylation, deamination, transsulfuration, racemization, glycogen breakdown, neurotransmitter synthesis, and heme synthesis.

Folate (B9): Functions & enzymes

Functions: DNA synthesis; RNA synthesis; Purine synthesis; Thymidylate (dTMP) synthesis; Methionine cycle; SAM production; Methylation reactions; Red blood cell formation; Rapid cell division; Major enzymes: DHFR converts DHF → THF; MTHFR converts 5,10-methylene THF → 5-methyl THF; Methionine synthase (with B12) converts homocysteine → methionine.

Vitamin B12: Functions & enzymes

Functions: Methionine synthesis; SAM production; DNA synthesis through folate recycling; Normal nerve/myelin function; Red blood cell formation; Enzymes: Methionine synthase converts homocysteine → methionine; Methylmalonyl-CoA mutase converts methylmalonyl-CoA → succinyl-CoA.

Vitamin B6: Functions & enzymes

Main active coenzymes: PLP and PMP; Functions: More than 100 enzyme reactions mainly involving amino acid metabolism; Transamination; Decarboxylation; Deamination; Transsulfuration; Glycogen breakdown; Gluconeogenesis; Neurotransmitter synthesis; Heme synthesis; Immune function; Homocysteine metabolism; Gene expression; Energy production; PLP forms a Schiff base to stabilize amino acids during these reactions.

Vitamin A: Functions & enzymes

Functions: Vision (rhodopsin synthesis); Cell growth; Cell differentiation; Gene expression; Immune function; Reproduction; Bone development; Maintenance of epithelial tissues; Antioxidant functions of provitamin A carotenoids; Enzymes: BCO1 converts β-carotene → 2 retinal molecules (iron-dependent); RRR converts retinal ↔ retinol; LRAT esterifies retinol for storage; Transducin begins the visual signal after rhodopsin activation.

Vitamin E: Functions

Only α-tocopherol has biological activity; Main function is antioxidant protection of cell membranes by preventing lipid peroxidation; Protects polyunsaturated fatty acids, lipoproteins, and cell membranes from oxidative damage; Helps maintain immune function and normal nerve and muscle function.

Vitamin K: Functions

Functions: Activates vitamin K-dependent proteins by γ-carboxylating glutamate residues; Required for blood clotting, bone mineralization, and prevention of blood vessel calcification; Activates clotting factors II (Prothrombin), VII, IX, and X; Also activates proteins C and S and bone proteins including osteocalcin and Matrix Gla Protein (MGP).

Vitamin B12 vs. Folate Deficiency

Both cause megaloblastic (macrocytic) anemia and elevated homocysteine; Only vitamin B12 deficiency causes neurological symptoms (peripheral neuropathy, numbness, tingling, loss of balance, cognitive changes) because B12 is needed for myelin synthesis; Only vitamin B12 deficiency causes elevated methylmalonic acid (MMA); Folate deficiency causes elevated FIGLU; Vitamin B12 deficiency causes the methyl folate trap, producing a functional folate deficiency; Treating B12 deficiency with folate alone may improve anemia but will not prevent neurological damage.

Vitamin Interactions: Deficiencies causing deficiencies in other vitamins/minerals

Vitamin B12 deficiency causes a functional folate deficiency through the methyl folate trap; Zinc deficiency decreases folate absorption because folate hydrolase requires zinc and decreases vitamin B6 absorption because alkaline phosphatase requires zinc; Riboflavin (B2) deficiency decreases PLP formation because FMN-dependent oxidase requires riboflavin; Iron deficiency decreases β-carotene cleavage because BCO1 is iron-dependent.

Alcohol: Effects on vitamin status

Decreases dietary vitamin intake; Damages intestinal cells and decreases absorption; Decreases liver storage of vitamins; Increases urinary vitamin losses; Decreases activation of several vitamins; Commonly contributes to deficiencies of folate, vitamin B6, vitamin A, and vitamin K.

Folate (B9): Food sources

Leafy green vegetables; Spinach; Broccoli; Asparagus; Brussels sprouts; Legumes; Beans; Lentils; Citrus fruits; Avocados; Liver; Fortified grains and cereals.

Vitamin B12: Food sources

Animal foods only; Meat; Liver; Fish; Shellfish; Poultry; Eggs; Milk; Cheese; Fortified breakfast cereals and fortified plant milks.

Vitamin B6: Food sources

Meat; Poultry; Fish; Potatoes; Bananas; Chickpeas; Whole grains; Nuts; Fortified cereals.

Vitamin A: Food sources

Preformed vitamin A (retinol/retinyl esters): Liver; Fish liver oils; Egg yolks; Dairy products; Provitamin A carotenoids: Carrots; Sweet potatoes; Pumpkin; Spinach; Kale; Dark green leafy vegetables; Orange and yellow fruits and vegetables.

Vitamin E: Food sources

Vegetable oils; Wheat germ oil; Sunflower oil; Safflower oil; Almonds; Sunflower seeds; Hazelnuts; Peanuts; Spinach; Avocados.

Vitamin K: Food sources

Vitamin K1 (Phylloquinone): Green leafy vegetables and plant oils; Vitamin K2 (Menaquinones): Animal foods and fermented foods; Vitamin K3 (Menadione): Synthetic form.

Fat-Soluble Vitamins: Interactions

Vitamins A, D, E, and K all require dietary fat, bile salts, pancreatic enzymes, and micelle formation for normal absorption; Diseases causing fat malabsorption can decrease absorption of all four vitamins; Very high vitamin E intake may interfere with vitamin K-dependent clotting and increase bleeding risk; Vitamin A and D regulate gene expression through RXR-containing nuclear receptors.

Vitamin A: Specialized transport proteins

CRBPII binds retinol and retinal inside enterocytes; LRAT converts retinol to retinyl esters; Chylomicrons transport retinyl esters from intestine through lymph to blood; RBP transports retinol in blood; TTR binds RBP and prevents kidney filtration; STRA6 transports retinol from blood into cells.

Vitamin E: Specialized transport proteins

α-TTP (Alpha-Tocopherol Transfer Protein) in the liver selectively binds α-tocopherol and incorporates it into VLDL for release into blood; Without α-TTP, vitamin E deficiency develops even with adequate intake.

Vitamin K: Specialized transport proteins

Vitamin K is transported from the intestine in chylomicrons; It is later transported in VLDL, LDL, and HDL because it is fat soluble.

Vitamin A: Retinoic acid regulation of gene expression

All-trans retinoic acid binds Retinoic Acid Receptors (RAR); 9-cis retinoic acid binds Retinoid X Receptors (RXR); RAR and RXR form heterodimers; The receptor complex binds Retinoic Acid Response Elements (RAREs) on DNA; Co-repressors are released and co-activators are recruited; Gene transcription increases or decreases depending on the target gene; This regulates cell differentiation, growth, development, immune function, and epithelial maintenance.

Vitamin A: ISX gene regulation

High vitamin A increases all-trans retinoic acid; All-trans retinoic acid activates RAR/RXR, increasing ISX gene expression; ISX suppresses BCO1 and SR-B1 expression; Less β-carotene is absorbed and converted to retinal, preventing excessive vitamin A production.

Vitamin A: Night blindness & vision

Vitamin A provides 11-cis retinal, the light-sensitive component of rhodopsin in rod cells; Light converts 11-cis retinal → all-trans retinal, activating rhodopsin; Rhodopsin activates transducin, beginning the visual signal; Without enough vitamin A, rhodopsin cannot regenerate efficiently, causing night blindness and eventually xerophthalmia and blindness.

Vitamin A: Liver metabolism, storage & release

Retinyl esters from chylomicron remnants are taken up by the liver; Retinyl esters are hydrolyzed to retinol; Retinol binds CRBP; LRAT re-esterifies retinol to retinyl esters for storage in hepatic stellate (Ito) cells; When needed, retinyl esters are hydrolyzed back to retinol; Retinol binds RBP, then TTR, and is released into the blood; STRA6 transports retinol from blood into target cells; When vitamin A is high, more retinol is stored as retinyl esters, ISX expression increases, and β-carotene absorption/conversion decrease.

Vitamin A: Functions of carotenoids

Provitamin A carotenoids (β-carotene, α-carotene, β-cryptoxanthin) can be converted to retinal; Act as antioxidants by quenching singlet oxygen and free radicals; Help protect lipids and cell membranes from oxidative damage; May reduce risk of chronic disease; Some carotenoids (lutein and zeaxanthin) protect the retina and macula but are not converted to vitamin A.

Vitamin K: Forms

Vitamin K1 (Phylloquinone): Found mainly in green leafy vegetables and plant oils; Vitamin K2 (Menaquinones): Produced by bacteria and found in animal foods and fermented foods; Vitamin K3 (Menadione): Synthetic form used in animal feeds, not a natural dietary form.

Vitamin K: Blood clotting & anticoagulants

Vitamin K is required for γ-carboxylation of glutamate residues, activating clotting proteins; Activates clotting factors II (Prothrombin), VII, IX, and X, plus proteins C and S; Warfarin inhibits vitamin K epoxide reductase (VKOR), preventing regeneration of active vitamin K and reducing activation of clotting factors, which slows blood clot formation.

Biologically active forms of each vitamin

Folate: THF and its one-carbon derivatives (especially 5-methyl THF); Vitamin B12: Methylcobalamin and Adenosylcobalamin; Vitamin B6: PLP (major active form) and PMP; Vitamin A: Retinal (vision), Retinoic acid (gene regulation), Retinol (transport/storage); Vitamin E: α-Tocopherol; Vitamin K: Reduced vitamin K (hydroquinone form) functions as the active cofactor for γ-carboxylation.

Fat-soluble vitamin absorption

Dietary fat stimulates bile release; Bile salts emulsify lipids; Pancreatic enzymes digest dietary lipids; Fat-soluble vitamins are incorporated into mixed micelles; Micelles deliver vitamins to the brush border; Vitamins enter enterocytes by passive diffusion and/or transport proteins (ex. SR-B1 for carotenoids); Vitamin A is esterified by LRAT; Vitamins are packaged into chylomicrons; Chylomicrons enter lymph then blood; Chylomicron remnants deliver vitamins mainly to the liver.

Folate (B9): Deficiency disease

Megaloblastic (macrocytic) anemia; Fatigue; Weakness; Glossitis; Elevated FIGLU; Elevated homocysteine; Neural tube defects during pregnancy due to impaired DNA synthesis.

Vitamin B12: Deficiency disease

Megaloblastic anemia; Elevated homocysteine; Elevated methylmalonic acid (MMA); Peripheral neuropathy; Numbness and tingling; Poor balance; Cognitive changes; Irreversible neurological damage if untreated.

Vitamin B6: Deficiency disease

Microcytic (sideroblastic) anemia due to impaired heme synthesis; Seborrheic dermatitis; Cheilosis; Glossitis; Peripheral neuropathy; Depression; Confusion; Seizures; Elevated homocysteine.

Vitamin A: Deficiency disease

Night blindness; Xerophthalmia; Bitot spots; Keratomalacia; Dry skin; Poor immune function; Increased infection risk; Poor growth; Blindness if severe and prolonged.

Vitamin E: Deficiency disease

Hemolytic anemia due to increased RBC membrane damage; Peripheral neuropathy; Ataxia; Skeletal muscle weakness; Retinal degeneration; Impaired immune function.

Vitamin K: Deficiency disease

Impaired blood clotting; Easy bruising; Prolonged bleeding; Increased prothrombin time (PT); Hemorrhage; Hemorrhagic disease of the newborn in infants with low vitamin K stores.

Folate (B9): Who is at risk for deficiency or needs more?

Pregnant women (higher RDA to prevent neural tube defects); People with poor diets or alcoholism; Individuals with malabsorption disorders (celiac disease, Crohn disease); Patients after gastric bypass; People taking methotrexate, phenytoin, sulfasalazine, or other folate-antagonist drugs; People with vitamin B12 deficiency (functional folate deficiency).

Vitamin B12: Who is at risk for deficiency or needs more?

Older adults with decreased stomach acid; Vegans and strict vegetarians; People with pernicious anemia (lack intrinsic factor); Patients with gastric bypass or ileal disease/resection; Individuals with pancreatic insufficiency; Long-term users of acid-suppressing medications or metformin; Infants of vegan mothers.

Vitamin B6: Who is at risk for deficiency or needs more?

Older adults; Alcohol use disorder; Poor dietary intake; Malabsorption disorders; Patients taking isoniazid, penicillamine, hydralazine, or oral contraceptives; Riboflavin or zinc deficiency can reduce PLP formation.

Vitamin A: Who is at risk for deficiency or needs more?

People with fat-malabsorption disorders (cystic fibrosis, Crohn disease, celiac disease); Liver disease; Pancreatic insufficiency; Children in developing countries; Pregnant and lactating women have increased requirements but should avoid excess preformed vitamin A because it is teratogenic.

Vitamin E: Who is at risk for deficiency or needs more?

People with fat-malabsorption disorders; Premature infants with very low birth weight; Individuals with abetalipoproteinemia; Patients with α-TTP deficiency.

Vitamin K: Who is at risk for deficiency or needs more?

Newborn infants (low liver stores, sterile intestine, low vitamin K in breast milk); People with fat-malabsorption disorders; Long-term antibiotic use; Chronic liver disease; Bile obstruction; Patients taking warfarin require consistent vitamin K intake.

Vitamin A: Retinoic acid regulation of gene expression (step-by-step)

Retinol is oxidized to retinal, then retinal is oxidized to retinoic acid; All-trans retinoic acid binds RAR; 9-cis retinoic acid binds RXR; RAR and RXR form a heterodimer; The complex binds Retinoic Acid Response Elements (RAREs) on DNA; Corepressors are released and coactivators are recruited; RNA polymerase begins transcription; Gene expression changes to regulate cell differentiation, growth, development, immune function, and epithelial maintenance.

Vitamin A: ISX regulation

When vitamin A is high, more all-trans retinoic acid is produced; Retinoic acid activates RAR/RXR, increasing ISX expression; ISX suppresses SR-B1 and BCO1 expression; Less β-carotene is absorbed and less is converted to retinal; This prevents excessive vitamin A production.

Vitamin A: What happens when vitamin A is high?

More retinol is converted to retinyl esters and stored in the liver; ISX expression increases; SR-B1 and BCO1 decrease; β-carotene absorption and conversion decrease; Chronic excess preformed vitamin A can cause hypervitaminosis A with liver damage, headache, bone pain, dry skin, hair loss, and birth defects during pregnancy.

Vitamin K: Risk factors for deficiency

Newborn age; Fat malabsorption; Long-term broad-spectrum antibiotics; Liver disease; Bile duct obstruction; Chronic pancreatic disease; Very low dietary intake.