Vegetable Glycerin (VG) is a common, plant-derived compound used across the food, cosmetic, and pharmaceutical industries. Its properties as a solvent, humectant, and sweetener have made it a ubiquitous ingredient in many household products. Due to its diverse applications, especially in technologies involving heating and inhalation, questions have arisen about its potential to cause inflammation. This article explores the scientific evidence regarding VG’s biological effects and the inflammatory responses based on how the substance is encountered.
Defining Vegetable Glycerin
Vegetable glycerin is a simple sugar alcohol compound, chemically known as glycerol. It is typically derived from triglyceride-rich vegetable fats, such as palm, soy, or coconut oils, often as a byproduct of soap or biodiesel manufacturing. The resulting substance is a clear, odorless, viscous liquid with a mild, sweet taste.
Glycerol is classified as a polyol because its molecular structure contains three hydroxyl groups. These groups allow it to form hydrogen bonds, giving it a syrupy consistency and the ability to readily mix with water. This structure also makes it a hygroscopic agent, meaning it attracts and retains moisture from the air. This moisture-retaining property is the basis for its use in moisturizing products and as a preservative.
Routes of Exposure and Application
The potential effect of vegetable glycerin on the body is directly linked to the method by which it enters the system, as different routes trigger distinct biological responses.
The most traditional route is ingestion, where VG is used as a low-calorie sweetener, thickening agent, or humectant in foods and beverages. It is also found in medications like cough syrups and capsules. Another common method of exposure is topical application, as VG is a primary component in many skincare products, lotions, and cosmetics, interacting directly with the skin’s surface layers. A third, and more recent, route is inhalation, where VG is aerosolized by heating elements, such as in electronic cigarette liquids or fog machines.
Systemic Inflammation and Metabolism
When pure vegetable glycerin is ingested, it is generally considered safe and is not associated with systemic inflammatory responses. Glycerol is a naturally occurring intermediate in human metabolism, and the body possesses efficient pathways to process it. Upon consumption, it is primarily metabolized in the liver and kidneys, where an enzyme called glycerol kinase converts it into an intermediate compound.
This intermediate can then be used to synthesize new triglycerides or phospholipids, or it can be converted into glucose. Since circulating glycerol does not readily bind to proteins in the same way as glucose, it does not contribute to the formation of advanced glycation end products, which are linked to chronic inflammation.
For topical applications, VG functions as a humectant that draws water into the outer layer of the skin, improving its barrier function and helping to alleviate dryness. Studies suggest that glycerin may even possess anti-irritant and anti-inflammatory properties when applied to the skin, helping to soothe irritated or wounded areas.
The Effects of Aerosolized Glycerin
The inflammatory concern surrounding vegetable glycerin is almost entirely related to its inhalation when heated and aerosolized. When VG is heated to high temperatures, such as those reached by atomizer coils in electronic cigarettes, it undergoes thermal degradation. This process breaks down the VG molecule and generates various toxic breakdown products, including formaldehyde, acetaldehyde, and the potent irritant acrolein.
Localized Respiratory Response
These thermal degradation products, rather than the pure VG itself, cause a localized inflammatory response in the respiratory tract. Studies on human bronchial epithelial cells exposed to VG aerosols have shown significant increases in inflammatory markers, such as Interleukin-6 and Interleukin-8. Exposure also increases the expression of MUC5AC, a protein associated with mucus hypersecretion, which can impair lung function.
Airway Function Disruption
Inhaled VG aerosols, even without nicotine or flavorings, can disrupt the normal function of ion channels in the airways. This disruption reduces the hydration of the airway surface liquid, leading to a thicker, more concentrated mucus that is harder to clear. Animal models have demonstrated that inhaled VG can enhance neutrophil migration and contribute to fibrotic changes in the lungs. The inflammatory effect of aerosolized VG is highly specific to the respiratory system and is driven by the chemical changes that occur during the heating process.