AUTHORITY NUTRITION

The Water-Soluble Vitamins: C and B Complex

Written by Atli Arnarson, PhD on November 3, 2017

Vitamins are often categorized based on their solubility.

Most of them dissolve in water and are called water-soluble vitamins. In contrast, there are only four fat-soluble vitamins, which dissolve in oil (liquid fat).

Hands Holding Supplements

Nine water-soluble vitamins are found in the human diet:

Unlike the fat-soluble vitamins, water-soluble vitamins are generally not stored in the body. For this reason, you should try to get them regularly from your diet.

This article provides a detailed overview of the water-soluble vitamins — their functions, health benefits, dietary sources, recommended intake and more.

Thiamine, also known as vitamin B1, was the first water-soluble vitamin to be described scientifically.

Types

Many forms of thiamine exist, including:

  • Thiamine pyrophosphate: Also known as thiamine diphosphate, thiamine pyrophosphate is the most abundant form of thiamine in your body. It is also the main form found in whole foods.
  • Thiamine triphosphate: This form is found in animal-sourced foods, but is less abundant than thiamine pyrophosphate. It is believed to represent less than 10% of the total thiamine found in animal tissues.
  • Thiamine mononitrate: A synthetic form of thiamine often added to animal feed or processed food.
  • Thiamine hydrochloride: The standard, synthetic form of thiamine used in supplements.

Role and Function

Like the other B vitamins, thiamine serves as a coenzyme in the body. This applies to all its active forms, but thiamine pyrophosphate is the most important one.

Coenzymes are small compounds that help enzymes trigger chemical reactions that otherwise wouldn’t happen on their own.

Thiamine is involved in many essential chemical reactions. For instance, it helps convert nutrients into energy and supports sugar formation.

Dietary Sources

The richest dietary sources of thiamine include nuts, seeds, whole grains, liver and pork.

The chart below shows the thiamine content of some of the best sources (1).

Chart 1 Thiamine

In contrast, fruits, vegetables and dairy products generally do not provide much thiamine.

Recommended Intake

The table below shows the recommended daily allowance (RDA) for thiamine.

The RDA for infants hasn’t been established. Instead, the table shows the adequate intake, marked with an asterisk. The adequate intake is like the RDA, but based on weaker evidence.

RDA (mg/day)
Infants0–6 months0.2*
7–12 months0.3*
Children1–3 years0.5
4–8 years0.6
9–13 years0.9
Women14–18 years1.0
19+ years1.1
Men14+ years1.2
Pregnancy1.4
Lactation1.4

*Adequate intake

Deficiency

Deficiency is uncommon, but high blood sugar levels may increase thiamine elimination via urine, raising its requirements and the risk of deficiency. In fact, thiamine levels may be reduced by 75–76% in people with type 1 and type 2 diabetes (2).

People with alcoholism are also at an increased risk for deficiency because of a poor diet and impaired thiamine absorption (3).

Serious deficiency may lead to disorders known as beriberi and Wernicke-Korsakoff syndrome.

These disorders are associated with a range of symptoms, including anorexia, weight loss, impaired neural function, mental problems, muscle weakness and heart enlargement.

Side Effects and Toxicity

Thiamine is considered safe. There are no reports of adverse effects after the intake of high amounts of thiamine from food or supplements.

This is partly because excess thiamine is quickly excreted from the body in urine.

As a result, the tolerable upper intake level for thiamine has not been established. However, this does not rule out possible symptoms of toxicity at very high intakes.

Benefits of Supplements

No good evidence shows that thiamine supplements benefit healthy people who get adequate amounts from their diets.

But for those with high blood sugar levels or a poor thiamine status, high-dose supplements may reduce blood sugar and blood pressure (4, 5).

Additionally, low thiamine intake has been associated with various other disorders, such as glaucoma, depression and fibromyalgia. However, more research is needed before strong conclusions can be made (6, 7, 8).

Summary of Thiamine

Thiamine, also known as vitamin B1, was the first B vitamin to be discovered.

Like the other B vitamins, thiamine acts as a coenzyme. It plays an essential role in many metabolic processes, including those that convert nutrients into energy.

The richest dietary sources of thiamine include liver, pork, seeds and whole-grain cereals. Deficiency is uncommon, but diabetes and excessive alcohol intake increase the risk. Serious deficiency may result in diseases such as beriberi and Wernicke-Korsakoff syndrome.

High-dose thiamine supplements do not seem to have any adverse effects and the tolerable upper intake level hasn’t been established. However, supplements do not appear to have any benefits for those who get adequate amounts from their diets.

Riboflavin is the only water-soluble vitamin used as a food coloring. In fact, it is named for its color — the Latin word flavus means “yellow.”

Types

In addition to riboflavin, dietary substances known as flavoproteins release riboflavin during digestion.

Two of the most common flavoproteins are flavin adenine dinucleotide and flavin mononucleotide. They are found in a wide range of foods.

Role and Function

Riboflavin functions as a coenzyme in various chemical reactions.

Like thiamine, it is involved in the conversion of nutrients into energy. It is also required in the conversion of vitamin B6 to its active form, and in the conversion of tryptophan to niacin (vitamin B3).

Dietary Sources

The chart below shows the riboflavin content of some of its richest dietary sources (1).

Chart 2 Riboflavin

Yeast extract spread is also exceptionally rich in riboflavin, containing around 18 mg in every 100 grams. Other good sources of riboflavin include eggs, leafy vegetables, broccoli, milk, legumes, mushrooms and meat.

Additionally, riboflavin is often added to processed breakfast cereals and is used as a yellow-orange food coloring.

Recommended Intake

The table below shows the RDA or adequate intake for riboflavin. These values represent the daily intake sufficient to meet the requirements of most people.

RDA (mg/day)
Infants0–6 months0.3*
7–12 months0.4*
Children1–3 years0.5
4–8 years0.6
9–13 years0.9
Women14–18 years1.0
19+ years1.1
Men14+ years1.3
Pregnancy1.4
Lactation1.6

*Adequate intake

Deficiency

Riboflavin deficiency is very rare in developed countries. However, a poor diet, old age, lung diseases and alcoholism may increase the risk.

Severe deficiency results in a condition known as ariboflavinosis, which is characterized by a sore throat, inflamed tongue, anemia, as well as skin and eye problems.

It also impairs the metabolism of vitamin B6 and the conversion of tryptophan to niacin.

Side Effects and Toxicity

High intake of dietary or supplemental riboflavin has no known effects of toxicity.

Absorption becomes less efficient at higher doses. Also, very small amounts are stored in body tissues and excess riboflavin is flushed out of the body with urine.

As a result, the safe upper intake level of riboflavin has not been established.

Benefits of Supplements

In most cases, riboflavin supplements do not have any benefits for people who already get enough from food.

Yet, low-dose riboflavin supplements may potentially reduce blood pressure and lower the risk of heart disease in people who are genetically predisposed to them. It’s thought to do this by decreasing high homocysteine levels in those with two copies of the gene MTHFR 677TT (9, 10, 11).

Higher doses of riboflavin, such as 200 mg twice a day, may also reduce migraines (12, 13).

Summary of Riboflavin

Riboflavin, also known vitamin B2, is a coenzyme with various essential functions. For instance, it is required for converting nutrients to energy.

Found in various foods, its richest sources include liver, meat, dairy products, eggs, leafy vegetables, almonds and legumes.

Deficiency is virtually unknown among healthy people in Western countries, although diseases and poor lifestyle habits may increase the risk.

High-dose riboflavin supplements are not known to have any adverse effects, but they usually only benefit those who are deficient. However, evidence suggests they may reduce migraines or lower the risk of heart disease in genetically susceptible people.

Niacin, also known as vitamin B3, is the only B vitamin your body can produce from another nutrient — the amino acid tryptophan.

Types

Niacin is a group of related nutrients. The most common forms are:

  • Nicotinic acid: The most common form in supplements. Also found in both plant- and animal-sourced foods. High-dose nicotinic acid supplements may cause a condition called niacin flush.
  • Nicotinamide (niacinamide): Found in supplements and foods.

The compound nicotinamide riboside also has vitamin B3 activity. It is found in trace amounts in whey protein and baker’s yeast (14, 15, 16).

Role and Function

All dietary forms of niacin are eventually converted into nicotinamide adenine dinucleotide (NAD+) or nicotinamide adenine dinucleotide phosphate (NADP+), which act as coenzymes.

Like the other B vitamins, it functions as a coenzyme in the body, playing an essential role in cellular function and acting as an antioxidant.

One of its most important roles is to drive a metabolic process known as glycolysis, the extraction of energy from glucose (sugar).

Dietary Sources

Niacin is found in both plants and animals. The chart below shows the niacin content of a few of its best sources (1).

Chart 3 Niacin

Yeast extract spread is exceptionally rich in niacin, providing around 128 mg in every 100 grams.

Other good sources include fish, chicken, eggs, dairy products and mushrooms. Niacin is also added to breakfast cereals and flour.

Additionally, your body can synthesize niacin from the amino acid tryptophan. Scientists have estimated that 60 mg of tryptophan can be used to create 1 mg of niacin (17).

Recommended Intake

The table below shows the RDA or adequate intake for niacin. These values are the estimated amount of niacin that most people (97.5%) need to get from their diets every day.

It also shows the tolerable upper intake limit (UL), which is the highest daily intake considered safe for most people.

RDA (mg/day)UL (mg/day)
Infants0–6 months2*-
7–12 months4*-
Children1–3 years610
4–8 years815
9–13 years1220
Women14+ years1430
Men14+ years1630
Pregnancy1830–35
Lactation1730–35

*Adequate intake

Deficiency

Niacin deficiency, known as pellagra, is uncommon in developed countries.

The main symptoms of pellagra include inflamed skin, mouth sores, diarrhea, insomnia and dementia. Like all deficiency diseases, it is fatal without treatment.

Fortunately, you can easily get all the niacin you need from a varied diet.

Deficiency is much more common in developing countries where people commonly follow diets that lack diversity.

Cereal grains are especially low in available niacin, since most of it is bound to fiber in the form of niacytin.

However, your body can synthesize it from the amino acid tryptophan. As a result, severe niacin deficiency can often be avoided on a high-protein diet (17).

Side Effects and Toxicity

Naturally occurring niacin from food does not appear to have any adverse effects.

However, high supplemental doses of niacin may cause niacin flush, nausea, vomiting, stomach irritation and liver damage.

Niacin flush is a side effect of immediate-release nicotinic acid supplements. It is characterized by a flush in the face, neck, arms and chest (18, 19).

Liver damage is associated with the long-term use of very high doses (3–9 grams per day) of sustained-release or slow-release nicotinic acid (20, 21, 22).

Additionally, taking niacin supplements for a long time may increase insulin resistance and raise blood sugar levels (23, 24).

Nicotinic acid may also increase the circulating levels of uric acid, worsening symptoms in people who are predisposed to gout (25).

Benefits of Supplements

Nicotinic acid supplements at doses ranging from 1,300–2,000 mg per day are commonly used to normalize blood lipid levels (26, 27).

They bring down high levels of “bad” low-density-lipoprotein (LDL) cholesterol, while raising levels of “good” high-density-lipoprotein (HDL) cholesterol when they are low. Triglyceride levels may also drop in those taking supplements.

Some studies also suggest nicotinic acid reduces heart disease risk, but its benefits are controversial and study results have been inconsistent (28, 29).

Preliminary evidence also indicates that niacin supplements may improve cognition, but further studies are needed before strong claims can be made (30).

Summary of Niacin

Niacin, also known as vitamin B3, is a group of two related compounds — niacinamide and nicotinic acid. They serve many vital functions in the body.

Niacin is found in many different foods, such as liver, fish, poultry, eggs, dairy products, sunflower seeds and peanuts, to name few. Additionally, it is commonly added to processed food like flour and breakfast cereals.

Deficiency is rare in Western nations. People who eat low-protein diets that lack diversity are at an increased risk.

High-dose nicotinic acid supplements are commonly used to normalize blood lipid levels, although some scientists doubt the vitamin’s benefits for heart health.

But supplements may also have some negative side effects, such as liver damage, reduced insulin sensitivity and niacin flush.

Pantothenic acid is found in virtually all food. Appropriately, its name is derived from the Greek word pantothen, which means “from every side.”

Types

There are multiple forms of pantothenic acid or compounds that release the active form of the vitamin when digested. In addition to free pantothenic acid, these include:

  • Coenzyme A: A common source of this vitamin in foods. It releases pantothenic acid in the digestive tract.
  • Acyl carrier protein: Like coenzyme A, acyl carrier protein is found in foods and releases pantothenic acid during digestion.
  • Calcium pantothenate: The most common form of pantothenic acid in supplements.
  • Panthenol: Another form of pantothenic acid often used in supplements.

Role and Function

Pantothenic acid plays a key role in a wide range of metabolic functions.

It is required for the formation of coenzyme A, which is necessary for the synthesis of fatty acids, amino acids, steroid hormones, neurotransmitters and various other important compounds.

Dietary Sources

Pantothenic acid is found in virtually all food.

The chart below shows some of its best dietary sources (1).

Chart 4 Pantothenic Acid

Other rich sources include yeast extract spread, shiitake mushrooms, caviar, kidneys, chicken, beef and egg yolks.

Several plant foods are also good sources. In addition to those mentioned above, these include root vegetables, whole grains, tomatoes and broccoli.

Like many other B vitamins, pantothenic acid is often added to breakfast cereals.

Recommended Intake

The table below shows the adequate intake (AI) of pantothenic acid for most people. The RDA has not been established.

AI (mg/day)
Infants0–6 months1.7
7–12 months1.8
Children1–3 years2
4–8 years3
9–13 years4
Adolescents14–18 years5
Adults19+ years5
Pregnancy6
Lactation7

Deficiency

Pantothenic acid deficiency is rare in industrialized countries. In fact, this vitamin is so widespread in foods that deficiency is virtually unheard of, except in severe malnutrition.

However, its requirements may be higher in people with diabetes and those who regularly consume excessive amounts of alcohol.

Studies in animals show that pantothenic acid deficiency has an adverse impact on most organ systems. It is associated with numerous symptoms, including numbness, irritability, sleep disturbances, restlessness and digestive problems (31).

Side Effects and Toxicity

Pantothenic acid does not appear to have any adverse effects at high doses. The tolerable upper limit has not been established.

However, large doses like 10 grams per day may cause digestive discomfort and diarrhea.

In mice, the lethal dose was estimated to be around 4.5 grams for each pound of body weight (10 grams per kg), an amount equivalent to 318 grams for a 154-pound (70-kg) human (32).

Benefits of Supplements

Studies have not provided any good evidence of benefits from pantothenic acid supplements in people who get adequate amounts from their diets.

While people take supplements to treat various disorders, including arthritis, dry eyes and skin irritation, there is no strong evidence for its effectiveness in the treatment of any of these disorders (33).

Summary of Pantothenic Acid

Pantothenic acid, also known as vitamin B5, plays various important roles in metabolism.

Almost all food contains this vitamin. The best sources include liver, sunflower seeds, mushrooms, root vegetables and whole grains.

Since pantothenic acid is so widespread in foods, deficiency is virtually unknown and is usually only associated with severe malnutrition.

Supplements are safe and do not have any adverse effects. However, very high doses may cause diarrhea and other digestive issues.

Although some people regularly take pantothenic acid supplements, there is currently no strong evidence for their effectiveness in the treatment of diseases in those who get adequate amounts from food.

Vitamin B6 is a group of nutrients that are required for the synthesis of pyridoxal phosphate, a coenzyme involved in more than 100 different metabolic processes.

Types

Like the other B vitamins, vitamin B6 is a family of related compounds, such as:

  • Pyridoxine: This form is found in fruits, vegetables and grains, as well as supplements. Processed foods may also contain added pyridoxine.
  • Pyridoxamine: Used until recently in dietary supplements in the US. However, the FDA now considers pyridoxamine a pharmaceutical drug. Pyridoxamine phosphate is a common form of vitamin B6 in animal-sourced foods.
  • Pyridoxal: Pyridoxal phosphate is the main type of vitamin B6 in animal-sourced foods.

In the liver, all dietary forms of vitamin B6 are converted into pyridoxal 5-phosphate, the active form of the vitamin.

Role and Function

Like other B vitamins, vitamin B6 acts as a coenzyme in numerous chemical reactions.

It is involved in red blood cell formation as well as energy and amino acid metabolism. It is also required for the release of glucose (sugar) from glycogen, the molecule the body uses to store carbs.

Vitamin B6 also supports the formation of white blood cells and helps the body synthesize several neurotransmitters.

Dietary Sources

Vitamin B6 is found in a wide variety of foods. The chart below shows some of its richest sources and their content (1).

Chart 5 Vitamin B6

Other good sources include tuna, pork, turkey, bananas, chickpeas and potatoes. Vitamin B6 is also added to breakfast cereals and soy-based meat substitutes.

The availability of this vitamin is generally higher in animal-sourced foods, compared to plant foods (34).

Recommended Intake

The table below shows the RDA for vitamin B6. The RDA is the daily intake estimated to be sufficient for most people.

The RDA hasn’t been established for infants, so the adequate intake (AI) is presented instead.

RDA (mg/day)UL (mg/day)
Infants0–6 months0.1*-
7–12 months0.3*-
Children1–3 years0.530
4–8 years0.640
9–13 years1.060
Women14–18 years1.280
19–50 years1.3100
51+ years1.5100
Men14–18 years1.380
19–50 years1.3100
51+ years1.7100
Pregnancy1.980–100
Lactation2.080–100

*Adequate intake

Deficiency

Vitamin B6 deficiency is rare. People with alcoholism are at the greatest risk (35).

The main symptoms include anemia, skin rashes, convulsions, confusion and depression.

Deficiency has also been associated with an increased risk of cancer (36, 37).

Side Effects and Toxicity

Naturally occurring vitamin B6 from food does not seem to have any adverse effects.

In contrast, very large supplemental doses of pyridoxine — 2,000 mg per day or more — are linked to sensory nerve damage and skin lesions (38).

High intake of pyridoxine supplements may also suppress milk production in breastfeeding women (39).

Benefits of Supplements

Large doses of pyridoxine have been used to treat carpal tunnel syndrome and premenstrual syndrome.

However, its benefits are controversial. No strong evidence proves that pyridoxine supplements are an effective treatment for these conditions (40, 41).

Because of the adverse health effects of high-dose pyridoxine supplements, they should only be taken under medical supervision.

Summary of Vitamin B6

Vitamin B6 is a group of nutrients that are required for the formation of pyridoxal phosphate, a coenzyme that plays a vital role in numerous metabolic pathways.

The richest dietary sources are liver, salmon, sunflower seeds and pistachio nuts, to name a few.

Deficiency is rare, although regularly drinking high amounts of alcohol may raise the risk.

High supplemental doses may cause nerve damage and skin lesions, but getting vitamin B6 from food doesn’t appear to have any negative effects.

While adequate vitamin B6 intake is healthy, no good evidence demonstrates that vitamin B6 supplements are useful in the treatment of diseases.

People often take biotin supplements to nourish their hair, nails and skin, although strong evidence for these benefits is lacking. In fact, it was historically called vitamin H after the German word haut, meaning “skin” (42).

Types

Biotin is either found in its free form or bound to proteins.

When proteins that contain biotin are digested they release a compound called biocytin. The digestive enzyme biotinidase then breaks biocytin into free biotin and lysine, an amino acid.

Role and Function

Same as all B vitamins, biotin functions as a coenzyme. It is required for the function of five carboxylases, enzymes involved in several fundamental metabolic processes.

For instance, biotin serves an essential role in fatty acid synthesis, glucose formation and amino acid metabolism.

Dietary Sources

Compared to the other B vitamins, biotin doesn’t have as much research behind its content in food.

Animal-sourced foods rich in biotin include organ meats, fish, meat, egg yolk and dairy products. Good plant sources include legumes, leafy greens, cauliflower, mushrooms and nuts.

Your gut microbiota also produces small amounts of biotin.

Recommended Intake

The table below shows the adequate intake (AI) for biotin. The AI is similar to the RDA, but is based on weaker research.

AI (mcg/day)
Infants0–6 months5
7–12 months6
Children1–3 years8
4–8 years12
9–13 years20
Adolescents14–18 years25
Adults19+ years30
Pregnancy30
Lactation35

Deficiency

Biotin deficiency is relatively uncommon.

The risk is greatest among infants who are fed formula low in biotin, people taking antiepileptic medications, infants with Leiner’s disease or people who are genetically predisposed to deficiency (43, 44).

Untreated biotin deficiency can cause neurological symptoms, such as seizures, mental retardation and loss of muscle coordination (45).

Deficiency has also been reported in animals fed high amounts of raw egg whites. Egg whites contain a protein called avidin, which prevents the absorption of biotin (46).

Side Effects and Toxicity

Biotin does not have any known adverse effects at high doses and the tolerable upper limit has not been established.

Benefits of Supplements

Limited evidence suggests that biotin supplements may improve health in those who otherwise get adequate amounts from their diets.

For instance, studies suggest biotin may improve symptoms in people with multiple sclerosis (MS) (47, 48).

Observational studies also indicate that biotin supplements may improve brittle nails in women. However, higher quality studies are needed before any claims can be made (49, 50).

Summary of Biotin

Biotin, also known as vitamin B7, is a coenzyme required for many key metabolic processes.

It is found in a wide range of foods. Good sources include organ meats, egg yolk, meat, legumes, cauliflower, mushrooms and nuts.

Deficiency is uncommon and adverse effects are unknown, even at high supplemental doses. Further studies need to establish the tolerable upper intake level.

Limited evidence supports the use of biotin supplements among people who already get adequate amounts from their diets. However, a few studies suggest they may improve the symptoms of MS and strengthen brittle nails.

Vitamin B9 was first discovered in yeast, but later isolated from spinach leaves. For this reason, it was given the names folic acid or folate, words derived from the Latin word folium, meaning “leaf.”

Types

Vitamin B9 comes in several different forms, including:

  • Folate: A family of vitamin B9 compounds that naturally occurs in foods.
  • Folic acid: A synthetic form commonly added to processed foods or sold as a supplement. Some scientists are concerned that high-dose folic acid supplements may cause harm.
  • L-methylfolate: Also known as 5-methyltetrahydrofolate, L-methylfolate is the active form of vitamin B9 in the body. As a supplement, it is thought to be healthier than folic acid.

Role and Function

Vitamin B9 acts as a coenzyme and is essential for cell growth, DNA formation and amino acid metabolism.

It is very important during periods of rapid cell division and growth, such as in infancy and pregnancy.

Additionally, it is required for the formation of red and white blood cells, so deficiency may lead to anemia.

Dietary Sources

The chart below presents a few foods that are great sources of vitamin B9 (1).

Chart 6 Vitamin B9

Other good sources include leafy greens, legumes, sunflower seeds and asparagus. Yeast extract spread is exceptionally rich in vitamin B9, providing around 3,786 mcg per 100 grams.

Folic acid is also frequently added to processed food products.

Recommended Intake

The table below shows the recommended daily allowance (RDA) for vitamin B9. It also presents the daily tolerable upper limit (UL), which is the amount considered safe for most people.

The RDA for infants hasn’t been established. Instead, the table shows the adequate intake values.

RDA (mcg/day)UL (mcg/day)
Infants0–6 months65*-
7–12 months80*-
Children1–3 years150300
4–8 years200400
9–13 years300600
14–18 years400800
Adults19+ years4001,000
Pregnancy600800–1,000
Lactation500800–1,000

*Adequate intake

Deficiency

Vitamin B9 deficiency rarely occurs on its own. It is usually associated with other nutrient deficiencies and a poor diet.

Anemia is one of the classic symptoms of vitamin B9 deficiency. It is indistinguishable from the anemia associated with vitamin B12 deficiency (51).

Lack of vitamin B9 may also lead to birth defects of the brain or neural chord, collectively known as neural tube defects (52).

Side Effects and Toxicity

No serious adverse effects of high vitamin B9 intake have been reported.

Yet, studies show that high-dose supplements may mask vitamin B12 deficiency. Some even suggest that they may worsen the neurological damage associated with vitamin B12 deficiency (53, 54).

Additionally, some scientists are concerned that a high intake of folic acid — a synthetic form of vitamin B9 — may cause health problems.

Benefits of Supplements

There is not much evidence that folic acid supplements benefit healthy people who are following a balanced diet.

A few studies suggest that supplements may reduce the risk of heart disease, improve blood sugar control and slightly reduce the symptoms of depression (55, 56, 57, 58).

However, the benefits of taking vitamin B9 supplements may only be seen in those who are low in the vitamin to begin with.

Summary of Vitamin B9

Like all other B vitamins, vitamin B9 acts as a coenzyme. It is essential for cell growth and various key metabolic functions.

It is found in both plants and animals. Rich sources include liver, legumes and leafy greens.

Deficiency in vitamin B9 is uncommon. The main symptom is anemia, but in pregnant women, low levels also raise the risk of birth defects. High intake doesn’t have any serious adverse effects.

For those who get enough vitamin B9 from their diet, the benefits of supplements are unclear. But studies suggest they may reduce the risk of heart disease and lower blood sugar levels.

Vitamin B12 is the only vitamin that contains a metallic element, namely cobalt. For this reason, it is often referred to as cobalamin.

Types

There are four main types of vitamin B12 — cyanocobalamin, hydroxocobalamin, adenosylcobalamin and methylcobalamin (59).

All of them can be found in supplements, although cyanocobalamin is the most common. It is considered ideal for supplements due to its stability, but is only found in trace amounts in food.

Hydroxocobalamin is the most common naturally occurring form of vitamin B12, and is widely found in animal-sourced foods.

The other natural forms methylcobalamin and adenosylcobalamin have become popular as supplements in recent years.

Role and Function

Like all other B vitamins, vitamin B12 acts as a coenzyme.

Adequate intake helps maintain brain function and development, neurological function, and the production of red blood cells.

It is also required for converting protein and fat into energy and is essential for cell division and DNA synthesis.

Dietary Sources

Animal-sourced foods are virtually the only dietary sources of vitamin B12. These include meat, dairy products, seafood and eggs.

The chart below shows some of its richest sources and their content (1).

Chart 7 Vitamin B12

Other rich sources include other types of liver, heart, octopus, oysters, herring and tuna.

However, tempeh and a few algae, such as nori seaweed, may also contain small amounts of vitamin B12. Whether these foods can provide sufficient amounts on their own is a matter of debate (60, 61, 62).

Other algae, like spirulina, contain pseudovitamin B12, a group of compounds that are similar to vitamin B12, but unusable by the body (63).

Recommended Intake

The table below shows the RDA for vitamin B12. As usual, the RDA hasn’t been established for infants, so the adequate intake (AI) is presented instead.

RDA (mcg/day)
Infants0–6 months0.4*
7–12 months0.5*
Children1–3 years0.9
4–8 years1.2
9–13 years1.8
Adolescents14–18 years2.4
Adults19+ years2.4
Pregnancy2.6
Lactation2.8

*Adequate intake

Deficiency

Vitamin B12 is stored in the liver, so even if you aren’t getting enough of it, it may take a long time for deficiency symptoms to develop.

Those who are at the greatest risk of deficiency are those who never or rarely eat animal-sourced foods. This includes vegetarians and vegans (64).

Deficiency may also develop in older people. In fact, many require regular vitamin B12 injections.

Vitamin B12 absorption depends on a protein produced by the stomach called intrinsic factor. As people age, the formation of intrinsic factor may reduce or stop altogether (65).

Other risk groups include those who have had weight loss surgery or suffer from Crohn’s disease or celiac disease (66, 67, 68, 69).

Deficiency may cause various health problems, such as anemia, appetite loss, sore tongue, neurological problems and dementia (70).

Side Effects and Toxicity

Only a small proportion of vitamin B12 can be absorbed from the digestive tract. The amount absorbed depends on the production of intrinsic factor in the stomach.

As a result, no adverse effects have been linked with high intake of vitamin B12 in healthy people. The tolerable upper intake level has not been established.

Benefits of Supplements

While vitamin B12 supplements benefit people at risk of deficiency, less is known about their effects among those who get adequate amounts from their diets.

One small study suggests that taking 1,000 mcg per day may improve verbal learning in people recovering from strokes, but more research is needed (71).

Additionally, injections of hydroxocobalamin are used to treat cyanide poisoning, usually in combination with sodium thiosulfate (72).

Summary of Vitamin B12

Vitamin B12 functions as a coenzyme and plays a vital role in many metabolic pathways. It also helps maintain neurological function and the formation of red blood cells.

It is found in virtually all animal-sourced foods, but is absent from plant foods.

As a result, vegans are at risk of deficiency or poor vitamin B12 status. Older people are also at risk because of impaired absorption. Anemia and impaired neurological function are classic deficiency symptoms.

High supplemental intake doesn’t have any known adverse effects. No strong evidence shows that they have benefits either, at least not in those who get adequate amounts from their diets.

Vitamin C is the only water-soluble vitamin that doesn’t belong to the vitamin B category. It is one of the body’s main antioxidants and is required for collagen synthesis.

Types

Vitamin C comes in two forms, the most common of which is known as ascorbic acid.

An oxidized form of ascorbic acid called dehydroascorbic acid also has vitamin C activity.

Role and Function

Vitamin C supports many essential body functions, including:

  • Antioxidant defenses: Your body uses antioxidants to protect itself against oxidative stress. Vitamin C is one of its most important antioxidants (73).
  • Collagen formation: Without vitamin C, the body is unable to synthesize collagen, the main protein in connective tissue. As a result, deficiency affects your skin, tendons, ligaments and bones (74).
  • Immune function: Immune cells contain high levels of vitamin C. During an infection, its levels are quickly depleted (75).

Unlike the B vitamins, vitamin C doesn’t act as a coenzyme, although it is a cofactor for prolyl hydroxylase, an enzyme that serves an essential role in the formation of collagen (76).

Dietary Sources

The main dietary sources of vitamin C are fruits and vegetables.

Cooked animal-sourced foods contain virtually no vitamin C, but low amounts can be found in raw liver, eggs, fish roe, meat and fish (77).

The chart below provides examples of some raw fruits and vegetables that are exceptionally rich in vitamin C (1).

Chart 8 Vitamin C

Cooking or drying foods significantly reduces their vitamin C content (78, 79).

Recommended Intake

The recommended daily allowance (RDA) of vitamin C is the estimated amount of the vitamin most people need every day.

The table below also shows the tolerable upper limit (UL), which is the highest level of intake thought to be completely safe for most people.

No RDA has been established for infants. Instead, scientists have estimated their adequate intake, which is similar to the RDA, but based on weaker evidence.

RDA (mg/day)UL (mg/day)
Infants0–6 months40*-
7–12 months50*-
Children1–3 years15400
4–8 years25650
9–13 years451,200
Women14–18 years651,800
19+ years752,000
Men14–18 years751,800
19+ years902,000
Pregnancy80–851,800–2,000
Lactation115–1201,800–2,000

*Adequate intake

Deficiency

Deficiency is rare in Western countries, but may develop in people who follow restrictive diets or eat almost no fruits or vegetables. People with drug addiction or alcoholism are also at greater risk.

It leads to a disease known as scurvy, which is characterized by the breakdown of connective tissue (80).

The first symptoms of deficiency include fatigue and weakness. As scurvy becomes worse, people may experience spotted skin and inflamed gums.

Advanced scurvy may cause loss of teeth, bleeding gums and skin, joint problems, dry eyes, swelling and impaired wound healing. Like all vitamin deficiencies, scurvy is fatal without treatment.

Side Effects and Toxicity

Most people tolerate high doses of vitamin C without any side effects.

However, very high doses exceeding 3 grams per day cause diarrhea, nausea and abdominal cramps. This is because only a limited amount of vitamin C can be absorbed from a single dose.

Taking high-dose supplements over 1,000 mg per day may also increase the risk of kidney stones in predisposed people (81).

Benefits of Supplements

There is mixed evidence that vitamin C supplements benefit people who get adequate amounts from the diet.

However, vitamin C can improve iron absorption from a meal, helping those who are low or deficient in iron (82).

Additionally, one analysis of 29 studies concluded that supplements that provide at least 200 mg of vitamin C per day may help you recover from the common cold (83).

While vitamin C supplements may also help lower blood pressure, there is no evidence that they reduce the risk of heart disease (84, 85).

Studies also suggest vitamin C may reduce the risk of cognitive decline, improve blood vessel function and reduce blood sugar levels, but high-quality studies are needed before definite conclusions can be reached (86, 87, 88).

Summary of Vitamin C

Vitamin C is a powerful antioxidant that’s vital for the maintenance of connective tissue.

The main dietary sources are fruits and vegetables, but low amounts can be acquired from raw animal-sourced foods. Deficiency, known as scurvy, is rare in developed countries.

Most people tolerate high-dose supplements without any adverse effects. However, studies on the benefits of vitamin C supplements have had mixed results, suggesting supplements may not be that useful for those who already get sufficient amounts from their diets.

Most vitamins are water-soluble. These include the eight B vitamins as well as vitamin C.

Their roles in the body range widely, but most function as coenzymes in numerous metabolic pathways.

All the water-soluble vitamins are easy to get from a balanced diet. However, vitamin B12 is only found in substantial amounts in animal-sourced foods. As a result, vegans are at a high risk of deficiency and may need to take supplements or get regular injections.

Keep in mind that your body generally doesn’t store water-soluble vitamins, except for vitamin B12. Optimally, you should get them from your diet every day.

An evidence-based nutrition article from our experts at Authority Nutrition.

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