Chapter 8 – Vitamins

8.2 Where Do Vitamins Come From?

Vitamins in Food

Eating a variety of foods from all food groups is the best way to get the vitamins needed for good health. Different foods provide different vitamins, so including a wide range of fruits, vegetables, grains, protein foods, and dairy helps meet vitamin needs (see Figure 8.2). In general, whole foods contain more vitamins than processed foods. For example, a whole baked potato provides more vitamin C and folate than a potato that has been cut into chips and deep-fat fried, because processing and high-heat cooking can reduce vitamin content.

A chart depicts each food group of MyPlate and lists which vitamins and minerals are commonly included in that food group. Fruit: Vitamins include vitamin C, folate, vitamin A (in the form of beta-carotene). Minerals include potassium. Vegetables: Vitamins include vitamins C and K, folate, vitamin A (in the form of beta-carotene). Minerals include potassium and magnesium. Protein: Vitamins include vitamins A, D, and E and the B vitamins. Minerals include iron, zinc, and magnesium. Grains: Vitamins include B vitamins. Minerals include iron, magnesium, and selenium. Dairy: Vitamins A and D (fortified) and vitamin B12. Minerals include calcium, potassium, and phosphorous.
Figure 8.2.  Common vitamins and minerals found in each food group. (Image by Heather Leonard licensed CC BY 4.0.)

 

From Plant to Plate: How Vitamins Develop and Change

Fruits and vegetables often contain their highest levels of vitamins when they are allowed to ripen naturally on the plant. As plants grow and mature, they produce vitamins that support their growth and survival. Vitamin levels can vary with ripeness and even among different parts of the plant. For example, peels and outer leaves often contain more vitamins than inner portions. [1]

After harvest, the way foods are stored, processed, and prepared affects how many vitamins remain. Although processing can reduce some nutrients, methods such as freezing and canning can help preserve foods and may retain vitamins better than fresh produce stored for long periods.

Water-soluble vitamins, including vitamin C and many B vitamins, are especially sensitive to heat, light, water, and oxygen. As shown in Table 8.1, nutrient losses can be minimized by using shorter cooking times, less water, and proper storage. [2]

Table 8. Food Processing Impacts on Nutrient Loss

Method of Processing

Effect on Nutrient Retention

How to Minimize Nutrient Loss

Air exposure and time

 After fruits and vegetables are harvested, they continue to undergo natural metabolic changes. Plant enzymes and exposure to oxygen gradually break down vitamins, so fresh produce slowly loses nutrients and quality as it ages.

Purchase fresh items in quantities that can be used as soon as possible.

Cut up foods only when ready to use.

Buy local produce to reduce transport time and exposure to air.

Temperature

Cooking helps kill bacteria, makes food more appealing, and, in some cases, improves the bioavailability of nutrients. However, prolonged exposure to high temperatures can destroy certain vitamins.

Use fast-cooking methods such as microwaving, steaming, or stir-frying.

Water

Water-soluble vitamins and minerals can leach into the water.

Don’t soak produce in water.

Limit the amount of water used to cook foods (e.g., steam vegetables rather than boil them).

Use cooking water in food preparation.

Canning

High temperatures can destroy water-soluble vitamins, but commercial techniques usually use rapid heating to reduce nutrient loss.

Choose a variety of canned goods that don’t have added sugars or sodium to maximize nutrient density.

Freezing

Freezing does not reduce nutrient content, but if foods are blanched prior to freezing, it may slightly reduce levels of water-soluble vitamins.

Choose a variety of frozen goods that don’t have added sugars, syrups, or sauces to maximize nutrient density.

Refining of grains

Many B vitamins, minerals, and phytochemicals, as well as fiber, are lost when whole grains are refined.

Choose whole grains whenever possible.

One important exception is  beta-carotene , a precursor to vitamin A. Chopping and lightly cooking foods such as carrots and sweet potatoes helps break down plant cell walls, making beta-carotene easier for the body to absorb. As a result, cooked carrots can provide more usable vitamin A than raw carrots. However, excessive heat can damage carotenoids, so lightly cooked vegetables are usually the best choice.

Can the Body Makes Its Own Vitamins?

Although most vitamins must come from the foods we eat, your body can produce limited amounts of certain vitamins. In some cases, your body uses substances it naturally makes to produce vitamins. In other cases, vitamins are formed from precursor compounds found in foods. Beneficial bacteria in the intestinal tract can also produce small amounts of certain vitamins.

Vitamin D is unique because your body can make it from a cholesterol-related compound found naturally in the skin. When ultraviolet (UV) rays from sunlight reach the skin, they trigger a series of reactions that produce vitamin D. The amount your body makes depends on factors such as the season, where you live, skin pigmentation, how much skin is exposed, and how much time you spend outdoors. When sunlight exposure is limited, foods such as fatty fish and fortified milk become important sources of vitamin D. [3]

 

In contrast, vitamin A and niacin are produced from dietary precursors.

Vitamin A can be produced from beta-carotene, a red-orange pigment found in plant foods such as carrots, sweet potatoes, and pumpkins. The body converts beta-carotene into active vitamin A, primarily in the small intestine. Animal foods such as milk, eggs, and liver provide preformed vitamin A, known as retinol, which can be used directly by the body.

Niacin can be synthesized in the liver from tryptophan, an amino acid found in protein-rich foods such as turkey, chicken, eggs, dairy products, and fish. However, this conversion is relatively inefficient and depends on adequate protein intake as well as several vitamins involved in the process.

These examples illustrate that although the body can synthesize some vitamins under certain conditions, most vitamins must still be obtained from a balanced, varied diet.

Carrots were not always orange. Early carrots were typically purple or yellow, while orange varieties were later developed because they are rich in beta-carotene, a plant pigment the body converts to vitamin A.[4] Vitamin A supports vision, immune function, and normal growth.

Purple carrots contain anthocyanins, pigments that act as antioxidants and give the carrots their deep purple color. Different carrot colors reflect different beneficial plant compounds, known as phytochemicals. Eating a variety of colorful vegetables can provide a wider range of nutrients and health-promoting compounds.

ARS researchers have selectively bred carrots with pigments that reflect almost all colors of the rainbow. More importantly, though, they're very good for your health." — USDA ARS
[pb_glossary id="3246"]anthocyanin[/pb_glossary] Figure 8.3. A rainbow of carrots. (“Carrots of many colors cutout” by USDA, Public Domain.)

Vitamins Made by Bacteria within the Human Intestines

Some vitamins can be synthesized not by our bodies, but by the helpful bacteria living within us. Bacteria in the gut can make vitamin K and the B vitamins.

Bacteria that colonize the large intestine can synthesize one form of vitamin K, although the total amount made in the large intestine is not clear. [5]

Natto: An excellent source of Vitamin K

Natto is easy to recognize by its sticky, stringy texture and glossy soybeans—features that come from fermentation with Bacillus subtilis.
Figure 8.4.  “Natto mixed” by Kinchan1 licensed CC BY 2.0.

Natto is easy to recognize by its sticky, stringy texture and glossy soybeans—features created during fermentation. This process, carried out by the bacterium Bacillus subtilis, not only gives natto its distinctive texture but also produces large amounts of vitamin K₂ (menaquinone). As a result, natto is one of the richest dietary sources of this important vitamin.

Gut bacteria are also able to make some of the B vitamins, though the amount synthesized of each vitamin is dependent on the composition of each individual’s microbiome.[6]  Dietary choices (e.g., intake of high fiber foods or probiotics) and medication use can alter a person’s microbiome, possibly promoting or inhibiting the production of vitamins in the large intestine.

Review Questions

Attributions

This section is an adaptation of “Sources of Vitamins and Minerals” in Nutrition: Science and Everyday Application, v. 1.0 by Alice Callahan, PhD; Heather Leonard, MEd, RDN; and Tamberly Powell, MS, RDN, licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

  1. Escobedo-Avellaneda Z, Gutierrez-Uribe JA, Valdez-Fragoso A, Torres JA, Welti-Chanes J. Phytochemicals and antioxidant activity of juice, flavedo, albedo and comminuted orange. Journal of Functional Foods. 2014;6:470-481. doi:10.1016/j.jff.2013.11.013 ↵
  2. Devi R. Food processing and impact on nutrition. Research Scholar, Department of Economics, Kurukshetra University, Kurukshetra, Haryana, India. Published 2005. Accessed January 4,  2025. http://saspjournals.com/wp-content/uploads/2015/08/SJAVS-24A304-311.pdf ↵
  3. Cui D, Yu X, Guan Q, et al. Cholesterol metabolism: molecular mechanisms, biological functions, diseases, and therapeutic targets. Mol Biomed. 2025;6(1):72. Published 2025 Oct 9. doi:10.1186/s43556-025-00321-3 ↵
  4. Maron DF. How orange carrots conquered the world. Nature. 2023;622:221. doi:10.1038/d41586-023-03035-0. ↵
  5. Gu Q, Li P. Biosynthesis of vitamins by probiotic bacteria. In: Probiotics and Prebiotics in Human Nutrition and Health. IntechOpen; 2016. Accessed January 4, 2026. https://www.intechopen.com/books/probiotics-and-prebiotics-in-human-nutrition-and-health/biosynthesis-of-vitamins-by-probiotic-bacteria ↵
  6. Yoshii K, Hosomi K, Sawane K, Kunisawa J. Metabolism of dietary and microbial vitamin B family in the regulation of host immunity. Front Nutr. 2019;6:48. doi:10.3389/fnut.2019.00048 ↵
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Introduction to Nutrition and Wellness, 2nd Edition Copyright © 2026 by Janet Colson and Sarah Harris is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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