The safety of consuming indigo depends on whether one is referring to the raw plant material, the isolated chemical dye, or the certified food colorant. Indigo, a color used for millennia, was originally extracted from plants but is now predominantly created through chemical synthesis. The safety profile is complex, changing significantly based on the substance’s source and purification level. Understanding how the blue pigment forms and how regulatory bodies classify its purified form is necessary to determine its suitability for ingestion.
The Botanical Origins and Inherent Safety
The blue pigment indigo is primarily associated with the plant Indigofera tinctoria, historically called True Indigo. The raw plant material does not contain the blue dye itself but a colorless precursor molecule called indican (indoxyl-beta-D-glucoside). The leaves and stems must undergo a specific process to yield the vibrant blue color.
The raw plant material is generally not intended for direct consumption and poses a safety risk. Ingesting unprocessed parts of Indigofera tinctoria can lead to gastrointestinal distress, including nausea, vomiting, and diarrhea. The raw plant contains glycosides and other compounds that can irritate the digestive system. Other historical sources, such as woad (Isatis tinctoria), also contain the indican precursor, and their unprocessed components are similarly limited in safety.
Indigotin: The Chemistry and Toxicity Profile
The characteristic blue color of indigo comes from the chemical compound indigotin (C16H10N2O2). This molecule forms when the colorless precursor indican is released from plant cells and hydrolyzed by enzymes into indoxyl. The indoxyl then spontaneously reacts with oxygen in the air (oxidation) to form the insoluble, bright blue indigotin molecule.
The safety of indigotin is rooted in its chemical properties, particularly its high insolubility in water and lipids. Because it does not readily dissolve in the body’s fluids, indigotin is poorly absorbed through the intestinal wall and largely passes through the digestive tract. Toxicological studies on the pure compound have consistently shown a very low level of acute and chronic toxicity.
Most commercially used indigo today is synthetic, although natural indigo pigment contains both indigotin and its isomer, indirubin. Synthetic indigotin is often manufactured from petroleum products using a chemical process involving compounds like aniline and hydrogen cyanide. Despite the use of harsh chemicals in production, the final purified synthetic indigotin is chemically identical to the natural compound.
The critical distinction for safety lies in the purity of the material, not its source. Textile-grade indigo, used to dye materials like denim, is not manufactured for consumption and may contain various contaminants. These commercial dyes can contain trace amounts of heavy metals or other finishing agents unsafe for human ingestion. Therefore, only specific, highly purified forms are permitted for use in food.
Regulatory Status in Food and Supplements
The purified, synthetic form of indigotin is permitted as a color additive in food, drugs, and cosmetics under strict regulatory oversight. In the United States, this colorant is known as FD&C Blue No. 2; in Europe, it is classified as E132. The approved colorant is technically indigotine, the disulfonated sodium salt of indigotin, which makes the molecule water-soluble and easier to incorporate into food products.
The safety of this food-grade colorant is assured by stringent purity standards established by regulatory bodies like the U.S. Food and Drug Administration. These standards mandate a high concentration of the colorant (typically not less than 85%) and set maximum limits for impurities such as lead, arsenic, and mercury. Furthermore, the acceptable daily intake (ADI) has been established, indicating the amount considered safe for daily consumption over a person’s lifetime.
FD&C Blue No. 2 is commonly used in various processed foods, including baked goods, cereals, snack foods, and candies, to achieve a blue or violet shade. Beyond certified food use, the crude, unpurified extract known as Qing Dai (Indigo Naturalis) is used in Traditional Chinese Medicine (TCM). This traditional preparation contains indigotin and other indigo-related compounds like indirubin, and is used in supplements to treat conditions such as ulcerative colitis.
The use of Qing Dai in supplements is based on traditional practice and differs significantly from the certified food colorant. Because it is a crude preparation, its exact composition and concentration of active compounds can vary, and it may not meet the strict purity standards applied to FD&C Blue No. 2. For this reason, supplements containing Qing Dai should be approached with caution and are typically taken under the guidance of a healthcare professional.