B vitamins

B vitamins are a group of water-soluble nutrients essential for cell metabolism and the production of red blood cells. Chemically, they are quite varied. A supplement that includes all eight is called a vitamin B complex. These vitamins are often known by both a number and a chemical name—for example, B1 is thiamine, B2 is riboflavin, and B3 is niacin—though some, like pantothenic acid (B5), biotin (B7), and folate (B9), are more frequently referred to by their names.

These vitamins are abundant in protein-rich foods such as fish, poultry, meat, dairy, and eggs, as well as in leafy greens, beans, and peas. Many breakfast cereals, baked goods, and infant formulas are fortified with B vitamins. Each B vitamin works either as a cofactor—usually a coenzyme—for crucial metabolic reactions, or as a precursor needed to build one.

**List of B vitamins** *Note: Some substances once assigned B numbers were later dropped from the list when it was discovered the body produces them or they aren’t essential for life. Numbers like 4, 8, 10, and 11 refer to these former candidates.*

**Sources** Meat, eggs, and dairy are rich in B vitamins. Processed carbohydrates like sugar and white flour contain less than their unprocessed counterparts, which is why many countries—including the United States—require that thiamine, riboflavin, niacin, and folic acid be added back to white flour. This is labeled as “enriched flour.” Particularly high concentrations are found in turkey, tuna, and liver. Other sources include spinach, legumes, whole grains, asparagus, potatoes, bananas, chili peppers, and breakfast cereals.

Vitamin B12 is not readily available from plant foods (though it appears in moderate amounts in some fermented vegetables, seaweeds, and mushrooms, though its bioavailability in those sources is uncertain). This makes B12 deficiency a real concern for vegans. Manufacturers of plant-based products sometimes list B12 content, but this can be misleading: the standard US Pharmacopeia test measures a bacterial response rather than B12 directly. Chemical variants of B12 in plants are active for bacteria but not usable by humans, and this can also cause over-reporting of B12 in other foods.

Dietary supplements are a common way to boost B vitamin intake, and energy drinks often contain large amounts. Because B vitamins are water-soluble, excess amounts are usually flushed out in urine, though absorption and metabolism vary. Elderly people and athletes may need extra B12 and other B vitamins due to absorption issues or higher energy demands. Severe deficiencies, especially of B12, may be treated with injections. Diabetics (both type 1 and type 2) are often advised to supplement thiamine, given the high prevalence of low plasma thiamine and increased clearance linked to the disease. Folate deficiency in early pregnancy is linked to neural tube defects, so women planning to become pregnant are encouraged to increase folate intake through diet or supplements.

**Molecular functions** Several major B vitamins (B2, B3, B5, B9, and B12) serve as precursors for essential biochemical compounds: FAD, NAD+, coenzyme A, tetrahydrofolate, and heme B, respectively. Their structural similarities highlight how they are transformed into these end products, which support essential reactions in humans, animals, and cells. FAD, NAD+, and coenzyme A are all critical for releasing free energy through catabolism, powering cellular and organismal activity. Tetrahydrofolate is needed to synthesize certain amino acids, like glycine. Heme B is the porphyrin molecule that holds iron in hemoglobin, enabling oxygen transport in blood.

**Deficiencies** A lack of certain B vitamins leads to named deficiency diseases, while deficiencies of others cause symptoms that don’t have a specific disease name.

**Side effects** Since excess water-soluble B vitamins are excreted in urine, large doses usually cause only temporary side effects (pyridoxine is an exception). Common effects include restlessness, nausea, and insomnia, and these almost always come from supplements, not food.

**Discovery** *(No additional facts provided in the source.)*

**Related compounds** Several substances were once labeled as vitamins but are no longer considered such. The numbers assigned to them create gaps in the B-complex series (e.g., there is no vitamin B4). Some are essential in other organisms’ diets but not humans; others have no known nutritional value or can be toxic. - Vitamin B4: can refer to choline, adenine, or carnitine. Choline is synthesized by the human body, but no further details are given.