Discover Which Vitamins Are Synthesized in Our Body

December 18, 2025 •

3 min read

Many people assume all vitamins must be sourced from our diet, but research confirms the human body can actually produce a handful of these essential nutrients. This article will reveal which vitamins are synthesized in our body and the specific processes involved, highlighting the roles of sunlight, the liver, and the gut microbiome.

Quick Summary

The human body can synthesize certain vitamins, including vitamin D from sun exposure, and others like K2 and biotin with the help of gut bacteria. Niacin can also be made from the amino acid tryptophan.

Key Points

Vitamin D is produced in the skin: The body synthesizes vitamin D3 from cholesterol when exposed to UVB radiation from sunlight.
Gut bacteria produce certain vitamins: Intestinal flora can produce important vitamins like biotin (B7) and vitamin K2, though absorption can be poor.
Niacin is converted from tryptophan: The liver can convert the amino acid tryptophan into niacin (B3), but this process is inefficient.
Provitamin A is converted from carotenoids: Humans can convert beta-carotene, found in plants, into active vitamin A.
Vitamins like B12 and C are not synthesized: Essential vitamins such as B12 and C cannot be produced by the human body and must be obtained from the diet.
Endogenous production depends on external factors: The body's ability to produce vitamins is not self-sustaining and relies on external triggers like sunlight or cofactors from diet.

The Body as a Biochemical Factory

While a significant portion of our vitamin intake comes directly from food, humans possess fascinating biochemical pathways to produce some vitamins internally. This endogenous production, while not always sufficient to meet all needs, plays a critical role in overall health and depends on factors like sun exposure and the gut microbiome.

Vitamin D: The Sunshine Vitamin

Vitamin D is a well-known vitamin our body can synthesize. It acts more like a hormone and is produced in the skin when exposed to UVB radiation, converting a cholesterol precursor (7-dehydrocholesterol) into previtamin D3, which then forms vitamin D3. This D3 is activated in the liver and kidneys to its biologically active form, calcitriol. Factors like season, latitude, and skin type affect synthesis efficiency.

Gut Bacteria: Our Internal Vitamin Producers

Our gut microbiome includes bacteria that synthesize certain vitamins we can absorb.

The synthesis of vitamin K2 and biotin

Vitamin K2 (Menaquinone): Produced by bacteria in the large intestine, such as Bacteroides and Lactobacillus species. However, dietary sources are crucial due to potentially poor absorption of bacterially synthesized K2.
Biotin (Vitamin B7): Also synthesized by gut bacteria, although dietary intake is still necessary to meet requirements. Biotin is vital for metabolic processes involving fatty acids, amino acids, and glucose.

Niacin (Vitamin B3): A Conversion from an Amino Acid

The body can synthesize niacin from the amino acid tryptophan, primarily in the liver, through the kynurenine pathway. This process is inefficient and requires cofactors like vitamin B6, riboflavin, and iron. About 60mg of tryptophan converts to 1mg of niacin, leading to the use of Niacin Equivalents (NE).

Conversion of Provitamins: Vitamin A

While the body doesn't synthesize vitamin A directly, it converts provitamin A carotenoids, like beta-carotene from plants, into the active form. This conversion rate varies between individuals.

Comparison of Vitamins by Source

Below is a comparison of vitamins based on how they are obtained.


Vitamin	Synthesis Pathway in the Body	Primary Source	Dependency on Exogenous Source
Vitamin D	Skin production via UVB light exposure, followed by liver and kidney activation	Sun exposure, fortified foods, fatty fish	Moderate (varies with sun exposure and lifestyle)
Vitamin K2	Gut bacteria metabolism	Fermented foods, animal products, gut bacteria	High (bacterial synthesis may be insufficient)
Biotin (B7)	Gut bacteria metabolism	Eggs, nuts, seeds, certain vegetables	High (bacterial synthesis may be insufficient)
Niacin (B3)	Converted from the amino acid tryptophan in the liver	Meat, poultry, fish, fortified grains	Low-to-Moderate (conversion is inefficient)
Vitamin C	Not synthesized by humans	Citrus fruits, berries, peppers	Absolute (cannot be produced)
Vitamin B12	Produced by bacteria, but not absorbable from gut production	Animal products, fortified foods, supplements	Absolute (cannot be produced)

Conclusion

The human body can synthesize certain vitamins, including vitamin D, vitamin K2, and biotin, often with the help of gut bacteria or external factors like sunlight. The body also converts tryptophan to niacin and provitamin A carotenoids to vitamin A. However, most vitamins must come from the diet, emphasizing the need for balanced nutrition and appropriate sun exposure for overall health.

Frequently Asked Questions

No, the human body cannot produce all the vitamins it needs. While it can synthesize some, such as vitamin D, biotin, and vitamin K2, it relies entirely on dietary sources for many others, including vitamin C and vitamin B12.

When sunlight's ultraviolet B (UVB) rays hit the skin, they convert a cholesterol precursor, 7-dehydrocholesterol, into previtamin D3. This compound then isomerizes into vitamin D3, which is later activated by the liver and kidneys.

While bacteria in your large intestine produce vitamin K2, it's generally considered insufficient to meet all of the body's requirements. Dietary sources like fermented foods and animal products remain crucial for ensuring adequate levels.

The human body relies on dietary sources like eggs, nuts, seeds, and certain vegetables for its biotin supply. While gut bacteria can produce some biotin, it may not be absorbed efficiently enough to cover all needs.

No, the conversion of tryptophan to niacin is not very efficient. The process depends on other vitamins, like B6 and riboflavin, and requires a significant amount of tryptophan to produce a small amount of niacin.

No, humans cannot produce vitamin C due to a genetic mutation that occurred during evolution. This makes us completely dependent on dietary sources, such as citrus fruits and leafy greens, for our vitamin C intake.

We obtain vitamin A in two ways: by consuming preformed vitamin A from animal products or by converting provitamin A carotenoids, like beta-carotene, from plant-based foods.