Count: 0 — No recognized vitamins start with the letter “S.” Vitamins are a defined group of essential organic compounds (A, the B‑complex vitamins, C, D, E, and K) named by discovery, chemical structure, or historical convention, so none of the established vitamin names begin with S. Note the pattern: vitamin names are often single letters or B‑group numbers, and most modern nutrient names that start with S are minerals, phytochemicals, or supplements rather than vitamins.

Recognize that authoritative bodies (NIH Office of Dietary Supplements, WHO, EFSA) maintain the official list of vitamins used for dietary recommendations and research. Historical and technical naming explains the absence: vitamins were named as they were discovered or classified (for example, B vitamins received numbered designations), not by alphabetic coverage. Substances that start with S — for example selenium (an essential trace mineral), sulfur-containing compounds, sulforaphane (a phytochemical), SAMe (a metabolic compound), or spirulina (an algae supplement) — are important nutrients or supplements but do not meet the biochemical definitions used to call something a vitamin.

Consult the standard vitamin list instead when you need recommendations, benefits, food sources, or RDIs: A, B1 (thiamine), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin), B9 (folate), B12 (cobalamin), C, D2/D3, E, and K1/K2. Check NIH, EFSA, WHO, and USDA FoodData Central for authoritative RDIs and food‑source data. If you meant nutrients or supplements that start with S, focus on minerals (selenium), phytochemicals (sulforaphane), or specific supplements (spirulina, SAMe), and treat those as related but distinct from vitamins.