What are vitamins?

What is a vitamin?

The Merck Manual defines vitamins as organic substances important in regulating the body’s functions. ^[2] Unlike fats, carbohydrates, and proteins, vitamins are not an energy source but substances that assist in the body’s chemical reactions. The body either cannot produce vitamins at all or cannot produce them in amounts that meet the requirement; for this reason most are obtained from food.

The shared feature of vitamins is that their deficiency leads to specific diseases. The identification of these diseases also determined the history of the discovery of vitamins.

What is the difference between vitamins and minerals?

Vitamins are organic substances. Minerals are inorganic elements such as iron, zinc, magnesium, calcium, selenium, iodine, and potassium; they are obtained from food and have roles in areas such as bone structure, fluid balance, blood production, and thyroid hormone. On this site, vitamins and basic minerals are collected in the same menu because both are handled under the principle “corrected if deficient, benefit of excess unproven if sufficient”; but they are separate groups chemically and in terms of deficiency pictures.

How many vitamins are there?

According to the Merck Manual, vitamins are divided into two groups: fat-soluble (A, D, E, and K) and water-soluble (B vitamins and vitamin C). The B vitamins are biotin, folate, niacin, pantothenic acid, riboflavin (B2), thiamin (B1), B6, and B12. ^[2] That makes thirteen vitamins in total. The 2017 dietary reference values summary report of EFSA, the EU’s food safety authority, also brings these vitamins together among nutrients in a single document. ^[4]

How did the concept of vitamins arise?

According to the history by Piro and colleagues, the story of the discovery of vitamins is the story of deficiency diseases. Casimir Funk investigated the human counterpart of the factors Eijkman had shown in animals and identified the nutritional factors whose deficiency led to “deficiency diseases”; these diseases were beriberi, scurvy, rickets, and pellagra. Funk named these factors “vitamine” in 1911 (“vita” = life, “amine” = a nitrogenous substance necessary for life); the name was accepted by the scientific community in 1912. ^[1]

The practical consequence of this history is that vitamins were defined through “a disease-related deficiency of the body.” When the deficiency is corrected, the disease recedes; however, this does not mean that an additional amount in the absence of deficiency would improve health further. The Merck Manual also states that the intakes that prevent classic diseases (such as scurvy and beriberi) may not always be sufficient for optimal health, and that this area is controversial and open to research. ^[2]

How is the daily vitamin requirement determined?

Organizations use a family of values rather than a single number. The Dietary Reference Intakes (DRI) framework of the US and Canada defines the following categories: ^[3]

• EAR (estimated average requirement): The average daily intake level estimated to meet the requirement of half of healthy people in a specific age and sex group.
• RDA (recommended dietary allowance): The level that meets the requirement of about 97.5% of healthy people in the same group; it is set two standard deviations above the EAR.
• AI (adequate intake): When data are too scarce to set an RDA, the level based on observed intakes of apparently healthy people and considered adequate.
• UL (tolerable upper intake level): The highest average daily intake not expected to pose a health risk for almost everyone in the general population.

EFSA uses the same concepts under its own names: population reference intake (PRI), average requirement (AR), adequate intake (AI), and upper limit (UL). According to EFSA, AI is the value set from the observed mean intake of apparently healthy people when a PRI cannot be established, that is, when the average requirement cannot be determined; UL is the highest total daily intake from all sources, taken chronically, at which no adverse effect on health is expected. ^[4]

Why do US, EU, and Türkiye values differ?

Because each organization uses its own expert panel, its own data review, and its own criteria of adequacy. TÜBER (Türkiye Dietary Guidelines) 2022 reports that the energy and nutrient reference values used for Türkiye were determined by taking into account EFSA’s scientific opinions for the EU and the Institute of Medicine / Food and Nutrition Board reports for the US. ^[5] TÜBER also states that the EAR value is used to estimate the frequency of inadequate intake in a population, while the RDA/PRI value is not recommended for this purpose. ^[5]

A concrete example of the difference is vitamin B6. According to the US (National Academies) values reported by the Merck Manual, the upper limit for B6 is 100 mg/day. ^[2] EFSA’s 2023 assessment, on the other hand, sets the upper limit for adults at 12 mg/day. ^[8] How the two values were derived and the reason for the difference are explained with sources in the vitamin B6 guide. For this reason, whenever a value is given on this site, the organization it comes from is always stated.

Is more vitamin better?

The general principle is this: if there is a deficiency it should be corrected; if there is no deficiency, the benefit of taking extra has mostly not been shown. There are two reasons for this.

The first is evidence. In its 2022 recommendation, the US Preventive Services Task Force (USPSTF) reports the following for nonpregnant, community-dwelling adults: that the harms of beta-carotene supplementation outweigh the benefits; that vitamin E supplementation provides no net benefit in preventing cardiovascular disease or cancer (moderate-certainty evidence); and that evidence is insufficient to determine the balance of benefits and harms of multivitamins and of single or paired nutrient supplements. The Task Force gives a recommendation against (grade D) the use of beta-carotene and vitamin E supplements to prevent cardiovascular disease or cancer. ^[7] These results were given for nonpregnant, community-dwelling adults for the purpose of preventing cardiovascular disease and cancer; they do not cover pregnant women.

The second is safety. The Merck Manual reports that vitamin toxicity mostly results from “megadoses” of A, D, C, B6, or niacin. ^[2] High amounts of vitamin A in pregnancy can lead to structural abnormalities in the baby; TÜBER gives this as an example. ^[5]

How is a vitamin label read?

In the US, the FDA defines the daily value (DV) as “amounts recommended to be consumed or not to be exceeded”; the percent daily value (%DV) shows how much of this value a serving provides. According to the FDA’s general guide, 5% or less per serving is considered low and 20% or more is considered high. ^[6] This information is for US labels; the label term used in Türkiye is different.

TÜBER 2022 reports that a nutrition declaration is mandatory on packaged foods in Türkiye; that the reference intake percentage is calculated according to the average values set for energy, total fat, saturated fat, sugars, and salt; and that it may optionally appear on the label. ^[5] There are separate rules for food supplements: according to TÜBER, for a vitamin or mineral to be stated as an active ingredient, it must provide at least 15% of the daily reference value; the recommended daily serving amount on the label and statements such as “Do not exceed the recommended daily serving,” “Food supplements cannot replace a normal diet,” and “Not a medicine” are mandatory. ^[5]

There are three points to keep in mind when reading a label. First, %DV is not an upper limit but a reference ratio; the safety ceiling is defined by the UL concept. Second, different chemical forms of the same vitamin (for example, folic acid and folate, retinol and beta-carotene) are written with different units and conversions; these conversions are covered on the relevant guide pages. Third, being below or above one hundred percent does not mean “good” or “bad”; whether a vitamin is needed depends on the person’s situation.

Who needs vitamin supplements?

For most healthy adults, the basic path is a varied and balanced diet. TÜBER states that no food supplement replaces an adequate and balanced diet and that the basic goal should be to meet nutrient requirements from food first. ^[5] The same section says that supplements can be of benefit when requirements cannot be met for various reasons, and lists the groups at risk of deficiency: infants, children, adolescents, pregnant women, older adults, strict vegetarians and vegans, and people with increased requirements or using medicines that affect absorption, metabolism, or excretion. ^[5]

The Merck Manual also reports that in countries with a low rate of food insecurity, vitamin deficiencies mainly result from poverty, extremely restricted or fad diets, medicines, alcohol use disorder, long-term and inadequately supplemented parenteral nutrition, and malabsorption; mild vitamin deficiency is common in frail and institutionalized older adults with protein-energy malnutrition. ^[2]

The situation in Türkiye is documented as follows: according to the results of TBSA (Türkiye Nutrition and Health Survey) 2017, 9.9% of individuals aged 15 and over (6.3% of men and 13.4% of women) use nutrient supplements. ^[5] The Ministry of Health also runs national iron and vitamin D support programs for infants and pregnant women; TÜBER recommends folic acid support starting before pregnancy for every woman planning a pregnancy. ^[5] The details of these programs are covered in the relevant guides.

Two more warnings on supplements appear in TÜBER: iron support is generally not recommended in postmenopausal women and adult men; smokers are advised to avoid beta-carotene supplements. ^[5]

Vitamin and mineral guides

The pages below describe, with sources, the current reference values, blood tests, level of evidence for supplementation, and risks for each vitamin and mineral.

The summary in each row reflects the conclusion of the relevant guide page; the full evidence, study designs, and limitations are on the relevant pages.

Where to go after this guide?

Types of vitamins, the solubility difference, and deficiency symptoms are covered in separate guides; the details of each vitamin and mineral are on the pages in the table above. This guide does not recommend personal doses.