Vitamin B complex: food sources, functions, side effects and daily dosing

Vitamin B complex: food sources, functions, side effects and daily dosing
April 1, 2014 3:41 AM

Vitamins are essential components of the diet we eat, that much we all know. Further, vitamins are divided into two groups depending upon the ability of human body to store them: fat soluble and water soluble vitamins. Fat soluble vitamins can be stored in body in an adequate amount. Water soluble vitamins, on the other hand, are excreted from the body and they need to be taken from diet on regular basis. Of all the water soluble vitamins that we know today, majority belongs to the family of vitamin B.


When we say vitamin B, we’re not talking about a single vitamin. Vitamin B is actually a group of vitamins and is thus called vitamin B complex. All vitamins belonging to this group are critically important in the metabolism taking place inside human body.

Following is the list of B vitamins present in human body:

  • Vitmain B1 (Thiamine)
  • Vitmain B2 (Riboflavin)
  • Vitamin B3 (Niacin or Nicotanic acid)
  • Vitamin B5 (Pantothenic acid)
  • Vitamin B6 (Pyridoxine)
  • Vitmain B7 (Biotin)
  • Vitamin B9 (Folic acid)
  • Vitamin B12 (Cobalamin)

The purpose of this article is to educate the readers about various types of B vitamins, their food sources and functions in human body, side effects if taken in excess and daily dosage as per recommendations.

Vitamin B1 (Thiamine)

Vitmain B1, thiamine or thio-vitamine is sulfur containing water soluble vitamin belonging to the vitamin B complex. The most important derivate of this vitamin in human body is phosphate derivate called thiamine pyrophosphate (TPP). In this form thiamine functions as a co-enzyme for several important enzymes involved in the metabolism of amino acids and sugars.

Dietary sources

This vitamin is used by a variety of unicellular and multi-cellular organism including yeasts, bacteria, plants and animals (including human). But only yeasts, bacteria and plants can synthesize this vitamin on their own. Animals (including humans) need to consume this vitamin on regular basis. The food sources of this vitamin include:

  • Yeast and yeast extracts
  • Pork
  • Cereals like wheat
  • Oatmeal
  • Flaxseeds
  • Brown rice
  • Whole grain rye
  • Kale
  • Cauliflower
  • Asparagus
  • Potatoes
  • Pasta
  • Legumes
  • Oranges
  • Liver
  • Egg
  • Watermelons


Thiamine doesn’t exist alone. It is always found inside the body in the form of its derivates and the functions of thiamine are actually the functions of its derivates.

Important derivates of thiamine include thiamine monophosphate (ThMP), thiamine diphosphate (ThDP), thiamine triphosphate (ThTP), adenosine thiamine diphosphate (AThDP) and adenosine thiamine triphosphate (AThTP).

The functions of each derivate are discussed in detail as follows:

Thiamine diphosphate (ThDP). Thiamine monophosphate doesn’t play an important physiological role. However, the functions of thiamine diphosphate are important. Thiamine diphosphate is synthesized from thiamine pyrophosphate as per following reaction. Thiamine + ATP → ThDP + AMP. ThDP acts as a co-enzyme for several important enzymes.

Most important enzymes and their functions are as following:

  • Pyruvate dehydogenase and alpha ketoglutarate dehydrogenase: These enzymes play very important role in the carbohydrate metabolism. These enzymes, in mitochondria, play an important role in the generation of ATP. In nervous system pyruvate dehydrogenase plays an important role in the generation of an important neurotransmitter called acetylcholine.
  • Transketolase: This enzyme plays an important role in the synthesis of pentose sugars (five carbon containing sugars).

Less important enzymes in which thiamine diphospahte functions as a co-enzyme are:

  • Branched-chain α-keto acid dehydrogenase
  • 2-hydroxyphytanoyl-CoA lyase

Thiamine triphosphate. It plays an important role in energy metabolism and nerve excitability.

  • Adenosine thiamine triphosphate. This form of thiamine is mostly present in yeast and bacteria and plays less important roles in human body.
  • Adenosine thiamine diphosphate. This is present in humans to a lesser extent but the exact function is still unclear.

Side effects

The excess of everything is bad. Same is true for thiamine. If you eat more thiamine than is recommended then you might have to face following side effects:

  • Thiamine is mostly considered safe as long as it is taken from oral routes. There are only a few cases where anaphylaxis due to excess of thiamine has been reported.
  • Excess of thiamine can cause skin irritation and trigger allergic reactions in some individuals.

Recommended intake of vitamin B1

Recommended daily allowance of 1.4 mg is an accepted value in the most countries. But research has shown that intake of as much as 50mg of thiamine produces no side effects. In fact this increases the acuity of brain function.

Vitmain B2 (Riboflavin)

Riboflavin is an important vitamin that acts as a constituent for the co-enzyme of flavor-protein family like FAD and FMN.

Food sources

A large amount of vitamin B2 is present in several food sources including:

  • Almonds
  • Yeast
  • Legumes
  • Mushrooms
  • Milk
  • Cheese
  • Liver
  • Kidneys
  • Leaf vegetables
  • What barn
  • Meat


Riboflavin is an integral component of co-enzymes of flavoprotein family including FAD and FMN. These co-enzymes assist the function of several enzymes involved in the oxidation-reduction reactions of body, as seen in the generation of ATP inside the mitochondria.

Side effects

Riboflavin is non-toxic as long as it is taken from oral route. However, toxic doses of this vitamin can be administered via injection. More than needed intake of riboflavin produces minor side effects including orange coloration of urine, diarrhea and increase in urine output.

Recommended intake

  • The recommended daily intake of riboflavin for males and females is 1.3mg/day and 1.1mg/day.
  • The daily requirement of this vitamin increase slightly during pregnancy and pregnant women are recommended to eat at least 1.4 to 1.6mg of this vitamin each day.
  • For infant the RDA of riboflavin is 0.3-0.4mg/day.
  • For children this value is 0.6-0.9mg/day.

Vitamin B3 (Niacin)

Vitamin B3 is present in different forms in human body like niacin and nicotinic acid.

Food sources of vitamin B3

Major food sources of this water soluble vitamin are:

  • Liver, heart and kidney
  • Chicken meat
  • Beef
  • Sea food
  • Eggs
  • Avocados
  • Tomatoes
  • Dates
  • Lead vegetables
  • Broccoli
  • Carrots
  • Sweet potatoes
  • Asparagus
  • Nuts
  • Legumes
  • Whole grain
  • Mushrooms
  • Yeast and yeast extracts
  • Tofu
  • Peanut butter


Following are the major functions of niacin:

  • Niacin is further composed of two important structures: nicotinamide and nicotinic acid. Niacin is present in the form of two important co-enzyme forms: nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). Both these co-enzymes function in metabolic pathways.
  • NAD carries hydrogen and their electrons during several metabolic reactions as in electron transport chain.
  • NADP is a co-enzyme that catalyzes the generation of nucleic acid.

Side effects

Following are the side effects seen with the intake of niacin:

  • Even when taken within the normal limits, niacin can trigger several unwanted side effects. These side effects include mostly irritation and skin complications including skin flushing, itching, skin rashes and dry skin. This condition might progress to develop eczema.
  • A major side effect seen with more than recommended intake of niacin is hepatic failure.
  • High dose of niacin can trigger an elevated level of blood sugar. This condition is particularly cumbersome in case of diabetes mellitus.
  • Niacin can also increase the amount of uric acid inside the body. This condition is particularly pronounced in patients that are already suffering from diseases of uric acid excess like gout.
  • Some studies have shown that more than recommended intake of niacin can trigger birth defects in children.
  • Niacin can also cause several complications of vision and eyes including maculopathy, blurred vision and blindness.

Recommended intake

  • The recommended daily allowance of niacin for children is 2-12mg/day.
  • Women need a daily intake of as much as 14mg/day of niacin.
  • An average man needs as much as 18mg/day of niacin.
  • The amount of niacin pregnant and breast feeding females need is slightly more than normal i.e. 18mg/day.

Vitamin B5 (Pantothenic acid)

Pantothenic acid is a water soluble vitamin important in the synthesis of some important co-enzymes, and the metabolism and synthesis of fats, carbohydrates and proteins.

Food sources

Food sources of this vitamin include:

  • Meat is the most important dietary source of this vitamin and is rich in mutton, beef and chicken.
  • Whole grains also contain high amount of this vitamin but milling removes much of pantothenic acid.
  • Broccoli and avocados are important vegetable sources of this vitamin.
  • Wheat, rice and peanut butter are some other important plant sources.
  • Yeast and mushrooms also contain high amount of vitamin B5.
  • Sea food is also an important food source.

Functions of pantothenic acid (vitamin B5)

Following are the most important functions of this vitamin:

  • Pantothenic acid is very important in the synthesis of co-enzyme A. Co-enzyme A play several important roles like:
    • It functions as an acyl carrier in compounds like acetyl-CoA. o It also functions to generate energy from pyruvate.
    • Co-A also plays an important role in the synthesis of biologically important molecules like fatty acids, acetylcholine and cholesterol.
  • Pantothenic acid is also required in the transduction of cellular signals through acylation and acetylation.

Side effects

The side effects of this vitamin are unlikely, even with the intake of high doses. That’s why no upper limit for this vitamin has been established. However, some points must be noted here:

  • Some important side effects of high intake of vitamin B5 include intestinal distress and diarrhea.
  • The side effects of this vitamin, however, are more severe in patients suffering from manic disorders where it can lead to the worsening of the symptoms.

Recommended intake

The recommended daily intake of vitamin B5, as per the recommendations of FDA, is as follows:

  • Infants 0–6 months: 1.7 mg
  • Infants 7–12 months: 1.8 mg
  • Children 1–3 years: 2 mg
  • Children 4–8 years: 3 mg
  • Children 9-13 years: 4 mg
  • Adult men and women 14+ years: 5 mg
  • Pregnant women: 6 mg
  • Breastfeeding women: 7 mg 

Vitamin B6 (Pyridoxine)

Vitamin B6 further consists of a number of important biological compounds including pyridoxine, pyridoxamine, pyridoxal sulfate and pyridoxal. However, the most important of these is pyridoxine and is discussed in detail as follows.

Food sources

Food sources of vitamin B6 include:

  • Heart
  • Kidney
  • Fish
  • Soybeans
  • Blackstrap molasses
  • Avocados
  • Brewer’s yeast
  • Lima beans
  • Raisins
  • Chicken
  • Spinach
  • Bananas
  • Potatoes
  • Green peppers
  • Cauliflower


Vitamin B6 assists in:

  • Balancing the levels of sodium and potassium.
  • Production of new red blood cells.
  • Uplifting the health of cardiovascular system and decrease the amount of homocysteine, an important trigger for cardiovascular disease.
  • Balancing hormonal changes in women.
  • Improving the function of immune system.
  • Improving the health of skin and mucosal surfaces.
  • The production of neurotransmitters like dopamine, serotonin, nor epinephrine and epinephrine.

Side effects

Pyridoxine is most likely safe for majority of people. However, some might experience side effects including:

  • Abdominal distress
  • Nausea
  • Vomiting
  • Insomnia
  • Headache
  • Tingling sensation in the extremities
  • Loss of appetite
  • In the most severe cases the excess of this vitamin can cause brain and nerve disorders.

Recommended intake

The RDA for vitamin B6 is as follows:

  • Infants 0-6 months: 0.1 mg
  • Infants 7-12 months: 0.3 mg
  • Children 1-3: 0.5 mg
  • Children 4-8 years: 0.6 mg
  • Children 9-13 years: 1 mg
  • Males 14-50 years: 1.3 mg
  • Males over 50 years: 1.7 mg
  • Females 14-18 years: 1.2 mg
  • Females 19-50 years: 1.3 mg
  • Females over 50 years: 1.5 mg
  • Pregnant women: 1.9 mg
  • Breast-feeding women: 2 mg

Vitamin B7 (Biotin)

Vitamin B7 is also known by several other names like biotin, vitamin H and co-enzyme R. This is another important vitamin belonging to the family of B vitamin.

Food sources

Major food sources of biotin include:

  • Swiss cheese
  • Raw egg yolk
  • Liver
  • Saskatoon berries
  • Leafy green vegetables.


Some important food sources of vitamin B7 include:

  • Biotin plays a key role in the metabolism of proteins, carbohydrates and fats.
  • It is involved in the generation of some important enzymes. Name of these enzymes along with their function is as follows:
    • Acetyl CoA carboxylase: This is an important enzyme is the synthesis of fatty acids from acetate.
    • Propionyl CoA carboxylase: This enzyme plays an important role in the generation of glucose, a process called gluconeogenesis.
    • β-methylcrotonyl Coa carboxylase: This is another important enzyme involved in the metabolism of an enzyme called leucine.
    • Pyruvate CoA carboxylase: This enzyme is involved in several important metabolic processes including energy metabolism and metabolism of amino acids and cholesterol.

Side effects

No side effects of this vitamin have been reported even when used in high doses.

Recommended intake

The recommended amount (e.g. RDA - mg/day) of vitamin B7 that should be taken on regular basis includes:

  • Infants 0-12 months: 7 mcg
  • Children 1-3 years: 8 mcg
  • Children 4-8 years: 12 mcg
  • Children 9-13 years: 20 mcg
  • Adolescents 14-18 years: 25 mcg
  • Adults over 18 years and pregnant females: 30 mcg
  • Breast-feeding women: 35 mcg

Vitamin B9 (Folic acid)

Vitamin B9 is known by several names like folic acid, vitamin M and vitamin B c. It is another water soluble vitamins belonging to the family of B vitamins. It is present both in natural foods that we consume of regular basis and can also be consumed in the form of supplements. The properties of this vitamin are discussed in detail as follows.

Food sources

Most important food sources of folic acid include:

  • Green leafy vegetables
  • Fruits and fruit juices
  • Peas
  • Beans
  • Nuts
  • Dairy products
  • Poultry products
  • Meat and meat products
  • Eggs
  • Seafood
  • Some types of beers
  • Grains
  • Liver
  • Spinach
  • Yeast and yeast extracts
  • Brussels sprouts
  • Asparagus.


Folic acid is perhaps the most important vitamin belonging to the family of B vitamins. The most important functions performed by this vitamin include:

  • The most important effects of folic acid are associated with pregnancy. Following are the roles performed by folic acid during pregnancy:
    • Folic acid is considered a mandatory supplement during the first trimester of pregnancy. Research has proven that adequate intake of folic acid during the first trimester can significantly reduce the chances of birth defects in infants, especially neural tube defects.
    • Moreover, supplementation of diet with folic acid during the first trimester of pregnancy can reduce the chances of congenital heart anomalies, cleft lip and urinary tract dysfunction in infants.
    • Folic acid during pregnancy significantly reduces the chances of the development of pregnancy induced hypertension.
  • Folic acid aids in oocyte maturation and implantation of embryo.
  • Folic acid also has beneficial effects on male sex life. It promotes the production of new sperms, a process called spermetogenesis.
  • Folic acid significantly reduces the levels of homocystenie, which is the most important marker for several important heart diseases.
  • Research has shown that supplementation of diet with folic acid can significantly reduce the risks of several types of cancers like colorectal cancer.
  • Some researches have shown that supplementation of diet with folic acid can function to limit psychological disorders like depression.
  • Regular intake of folic acid can reduce the risks of macular degeneration.

Side effects

Following are the side effects seen with the excessive intake of folic acid:

  • Abdominal distress
  • Diarrhea
  • Skin irritation and rash
  • Insomnia
  • Irritability
  • Nausea
  • Vomiting
  • Stomach upset
  • Skin reactions
  • Gas
  • Excitability
  • Behavioral changes

Recommended intake

The recommended intake (RDA) of vitamin B9 is as follows:

  • Infants 0-6 months: 65 mcg/day
  • Infants: 7 - 12 months: 80 mcg/day
  • Children 1-3 years: 150 mcg/day, maximum 300 mcg/day
  • Children 4-8 years: 200 mcg/day, maximum 400 mcg/day
  • Children 9-13 years: 300 mcg/day, maximum 600 mcg/day
  • Teenagers 14 - 18 years: 400 mcg/day, maximum 800 mcg/day
  • Adults: 19+ years: 400 mcg/day, maximum 1000 mcg/day
  • Pregnant women under 18 years: 600 mcg/day, maximum 800 mcg/day
  • Pregnant women 19+ years: 600 mcg/day, maximum 1000 mcg/day
  • Breastfeeding women: 600 mcg/day, maximum 800 mcg/day

Vitamin B12 (Cobalamin)

The last vitamin that belongs the family of B vitamins is vitamin B12. It is also known by some other names like cobalamin, cynocobalamin, hydroxycobalmine- depending on the type of substance attached to the central structure. The properties of this vitamin are discussed as follows.

Food sources

The food sources of this vitamin include:

  • Vitamin B12 is unique in a respect that the colonic bacteria can synthesize this vitamin.
  • Animal meat and meat products are an important source. These sources include eggs, meat, milk, milk products and liver.
  • Sea foods are also important food source.
  • Vitamin B12 is not present in plants. That’s why the deficiency of this vitamin usually occurs in individuals that solely depend on plants as a source of their food.


Following are the functions performed by vitamin B12:

  • Vitamin B12 plays a very important role in the cellular metabolism of lipids, proteins, and carbohydrates.
  • It is also critical in the production of red bone marrow.
  • It also helps in the production of nerve sheaths and nerve proteins.
  • It also functions as a co-enzyme in the metabolism of methionine. Elevation of methionine is a marker for heart disease.

Side effects

It has very low chances of toxicity in healthy individuals. It can, however, trigger hematological abnormalities. It can cause weakening of vision. It can cause gastric distress, nausea, vomiting and diarrhea. It can also trigger skin allergies and skin reactions.

Recommended intake

The recommended intake of vitamin B12 (RDA - mcg/day) is as follows:

  • Infants 0-6 months: 0.4 mcg
  • Infants 7-12 months: 0.5 mcg
  • Children 1-3: 0.9 mcg
  • Children 4-8 years: 1.2 mcg
  • Children 9-13 years: 1.8 mcg
  • Older children and adults: 2.4 mcg
  • Pregnant women: 2.6 mcg
  • Breast-feeding women: 2.8 mcg
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Written by: Michal Vilímovský (EN)
Education: Medical student, 3rd Faculty of Medicine, Charles University, Prague, Czech Republic
Published: April 1, 2014 3:41 AM
Next scheduled update: April 1, 2016 3:41 AM
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