B Complex Vitamins

Vitamin B Complex

(see table below)

Summary

The B complex vitamins are a group of eight essential nutrients that play vital roles in various metabolic processes.

Thiamine (B1) , essential for carbohydrate metabolism and nerve function, is absorbed primarily via active transport in the jejunum involving ThTr1 and ThTr2, though passive diffusion occurs at high concentrations. Deficiency can lead to beriberi or Wernicke-Korsakoff syndrome, and is tested for by measuring blood thiamine levels or erythrocyte transketolase activity. Good sources include pork, whole grains, legumes, and nuts.

Riboflavin (B2) , crucial for energy production and cell growth, is absorbed through both active and passive mechanisms in the small intestine, predominantly the jejunum. Its deficiency, known as ariboflavinosis, is indicated by sore throat, cheilosis, and dermatitis, and can be assessed by measuring blood riboflavin levels or the erythrocyte glutathione reductase activity coefficient. Major sources are milk, eggs, meat, and green leafy vegetables. Riboflavin is a crucial component of FAD/FMN, which act as coenzymes for many oxidoreductases.

Niacin (B3) , vital for energy metabolism and DNA repair, is absorbed in the small intestine via active transport at low concentrations and passive diffusion at high concentrations. Pellagra, characterized by dermatitis, diarrhea, and dementia, is a sign of niacin deficiency, which can be diagnosed by measuring urine N1-methylnicotinamide excretion. Niacin is a key component of NAD+/NADP+, which serve as coenzymes for numerous dehydrogenases. Common food sources are meat, poultry, fish, and peanuts.

Pantothenic acid (B5) , essential for energy metabolism and fatty acid synthesis, is absorbed through active transport in the small intestine via the sodium-dependent multivitamin transporter (SMVT). Deficiency is rare and difficult to test for reliably, though urinary pantothenic acid excretion is sometimes used. Pantothenic acid is widely available in foods such as meat, eggs, and vegetables.

Pyridoxine (B6) , critical for amino acid metabolism and neurotransmitter synthesis, is primarily absorbed through passive diffusion in the jejunum. A deficiency can cause seizures, peripheral neuropathy, and anemia. It can be tested by measuring plasma pyridoxal 5'-phosphate (PLP) levels or erythrocyte transaminase activity. Pyridoxine, which functions as the pyridoxal phosphate (PLP) coenzyme, is essential to many transaminases and decarboxylases. Good sources include meat, poultry, fish, and bananas.

Biotin (B7) , vital for carbohydrate, fat, and protein metabolism, is absorbed via active transport in the small intestine using the SMVT transporter. A rare deficiency can cause dermatitis, hair loss, and neurological symptoms and can be tested by measuring plasma or serum biotin levels. As a key coenzyme for carboxylases, biotin can be found in eggs, liver, nuts, seeds, and salmon.

Folate (B9) , crucial for DNA synthesis and cell division, is absorbed through both active transport (PCFT transporter) at low concentrations and passive diffusion at higher concentrations in the jejunum. Its deficiency can lead to megaloblastic anemia and neural tube defects during fetal development. Folate deficiency is diagnosed by measuring serum or red blood cell folate levels, or plasma homocysteine levels. Food sources include dark green leafy vegetables, legumes, and fruits. As tetrahydrofolate (THF), folate is critical in single carbon transfer reactions.

Cobalamin (B12) , essential for nerve function and red blood cell formation, has a complex absorption process involving binding to intrinsic factor (IF) in the stomach, followed by receptor-mediated endocytosis of the IF-B12 complex in the ileum. Its deficiency can cause megaloblastic anemia and neurological problems and is detected by measuring serum B12 levels or serum methylmalonic acid (MMA) levels. Found primarily in animal products, cobalamin plays an essential role as a coenzyme for methylmalonyl-CoA mutase and methionine synthase.

Proper B vitamin status relies on adequate intake and efficient absorption, which can be affected by various factors.

Table 1

Name	Number	Function (with Enzyme/Coenzyme)	Clinical Manifestations of Deficiency	Common Sources	Site of Absorption (mechanism)	Method of testing
Thiamine	B1	Carbohydrate metabolism; nerve function (Thiamine Pyrophosphate (TPP)- Pyruvate Dehydrogenase, Alpha-ketoglutarate dehydrogenase, Transketolase)	Beriberi (peripheral neuropathy, heart failure, edema), Wernicke-Korsakoff syndrome (confusion, ataxia, vision changes, memory impairment)	Pork, whole grains, legumes, nuts, seeds, fortified cereals	Jejunum: Primarily active transport via ThTr1 and ThTr2 (thiamine transporters encoded by SLC19A2 and SLC19A3 genes). High concentrations allow for passive diffusion.	* Blood thiamine levels (may not always reflect tissue stores). * Erythrocyte Transketolase Activity (ETKA) - Measures the activity of transketolase, an enzyme that requires thiamine. Increased activity after thiamine is added indicates deficiency. * Urine thiamine excretion
Riboflavin	B2	Energy production; cell growth; metabolism of fats, drugs, and steroids (FAD/FMN- Many oxidoreductases)	Ariboflavinosis (sore throat, cheilosis (cracks at the corners of the mouth), glossitis (inflamed tongue), dermatitis, anemia)	Milk, eggs, meat (especially liver), green leafy vegetables, fortified cereals	Small Intestine (Primarily Jejunum): Both active transport (saturable carrier-mediated) and passive diffusion. Active transport becomes less efficient at high concentrations.	* Blood riboflavin levels. * Erythrocyte Glutathione Reductase Activity Coefficient (EGRAC) - Measures the activity of glutathione reductase, which requires FAD. Increased activity after FAD is added suggests deficiency. * Urine riboflavin excretion.
Niacin	B3	Energy metabolism; DNA repair; cell signaling; antioxidant (NAD+/NADP+- Many dehydrogenases)	Pellagra (the "3 Ds": dermatitis, diarrhea, dementia); also glossitis	Meat, poultry, fish, peanuts, whole grains, fortified cereals	Small Intestine: Both active transport (Na+-dependent) at low concentrations and passive diffusion at high concentrations.	* Urine N1-methylnicotinamide (NMN) excretion - A metabolite of niacin. Low levels suggest deficiency. * Plasma or serum niacin levels (less commonly used).
Pantothenic Acid	B5	Energy metabolism; synthesis of coenzyme A (CoA), involved in fatty acid metabolism.	Very rare (widespread in foods); may include fatigue, headache, insomnia, abdominal distress, nausea, vomiting, numbness, paresthesia.	Widely distributed in foods: Meat, poultry, fish, eggs, mushrooms, avocados, broccoli, sweet potatoes, whole grains	Small Intestine: Primarily active transport via the sodium-dependent multivitamin transporter (SMVT).	* No widely available or reliable test. Urinary pantothenic acid excretion may be used, but reference ranges vary and are not well-established. Deficiency is rare.
Pyridoxine	B6	Amino acid metabolism; neurotransmitter synthesis; red blood cell formation; immune function. (Pyridoxal Phosphate (PLP)- Transaminases, decarboxylases)	Seizures (especially in infants), peripheral neuropathy, dermatitis, anemia, cheilosis, glossitis, depression, confusion, weakened immune system.	Meat, poultry, fish, bananas, potatoes, fortified cereals	Jejunum: Primarily passive diffusion. Some evidence for carrier-mediated transport as well, but passive diffusion is the dominant mechanism.	* Plasma pyridoxal 5'-phosphate (PLP) - The most common and reliable measure. * Erythrocyte Transaminase Activity - Measures transaminase activity before and after adding PLP; an increase suggests deficiency. * Urine excretion of pyridoxic acid
Biotin	B7	Carbohydrate, fat, and protein metabolism; gene regulation (Biotin- Carboxylases: Pyruvate carboxylase, Acetyl-CoA carboxylase, Propionyl-CoA carboxylase, Beta-methylcrotonyl-CoA carboxylase)	Very rare (usually due to genetic defects or excessive raw egg consumption); dermatitis, hair loss, neurological symptoms (depression, lethargy, seizures), conjunctivitis.	Eggs, liver, nuts, seeds, salmon, avocados	Small Intestine: Active transport via the sodium-dependent multivitamin transporter (SMVT).	* Plasma or serum biotin levels (most common). * Urine biotin excretion (less common).
Folate(Folic Acid)	B9	DNA synthesis; cell division; red blood cell formation (Tetrahydrofolate (THF)- involved in one-carbon transfer reactions)	Megaloblastic anemia, neural tube defects (in developing fetuses), fatigue, weakness, irritability, diarrhea, glossitis, impaired immune function	Dark green leafy vegetables, legumes, fruits (especially citrus), fortified grains, liver	Jejunum: Both active transport (Proton-coupled folate transporter - PCFT) at low concentrations and passive diffusion at higher concentrations. The PCFT is crucial for efficient absorption.	* Serum or red blood cell (RBC) folate levels - RBC folate is considered a better indicator of long-term folate status. * Plasma homocysteine levels - Elevated homocysteine can indicate folate deficiency, but is not specific. * DNA analysis using leukocytes. Measures the uptake of deoxyuridine into DNA of lymphocytes to assess folate status in DNA synthesis.
Cobalamin	B12	Nerve function; DNA synthesis; red blood cell formation (Cobalamin- Methylmalonyl-CoA Mutase, Methionine Synthase)	Megaloblastic anemia, neurological problems (peripheral neuropathy, balance problems, cognitive impairment), fatigue, weakness, glossitis	Meat, poultry, fish, eggs, dairy products, fortified foods (e.g., some cereals, plant-based milks)Note: B12 is primarily found in animal products.	Ileum: Requires a complex process: 1. B12 binds to Intrinsic Factor (IF) in the stomach. 2. IF-B12 complex is absorbed in the ileum via receptor-mediated endocytosis (a form of active transport) involving cubilin receptor. Passive diffusion of small amounts also occurs throughout the small intestine.	* Serum vitamin B12 levels. * Serum methylmalonic acid (MMA) levels - Elevated MMA is a more sensitive indicator of B12 deficiency, especially early deficiency. * Serum homocysteine levels - Elevated homocysteine can indicate B12 deficiency, but is not specific. * Schilling test (historically used, now less common) assesses B12 absorption.

Important Notes:

Interactions: Deficiencies in one B vitamin can sometimes affect the utilization or metabolism of other B vitamins.
Bioavailability: The bioavailability of B vitamins can vary depending on the food source and individual factors (e.g., gut health, medications).
Supplementation: While B complex supplements are available, it's generally best to obtain B vitamins from a balanced diet. Excessive intake of some B vitamins from supplements can have adverse effects. Always consult with a healthcare professional before taking high doses of B vitamins.
Risk Factors: Certain groups are at higher risk of B vitamin deficiencies, including older adults, vegetarians/vegans (particularly for B12), individuals with malabsorption syndromes (e.g., Crohn's disease, celiac disease), alcoholics, and pregnant or breastfeeding women.
Folate vs. Folic Acid: Folate is the naturally occurring form of B9 found in foods. Folic acid is the synthetic form used in supplements and fortified foods. Folic acid is generally more bioavailable than folate.
THF: Tetrahydrofolate is the active coenzyme form of folate

Important Considerations:

Multiple Factors Affecting Absorption: Many factors can influence B vitamin absorption, including gut health, medications, alcohol consumption, age, and genetic variations.
Test Interpretation: Interpretation of deficiency test results should be done in the context of clinical signs and symptoms and other relevant laboratory findings.
Sensitivity and Specificity: Some tests are more sensitive (better at detecting deficiency when it is present) than others. Some are more specific (less likely to give a false positive result).
Functional Assessments: Functional assessments (e.g., Erythrocyte Transketolase Activity) provide information about the functional consequences of a deficiency, rather than just measuring vitamin levels in the blood.