The pharmaceutical grade of propylene glycol is of a far lower concentration than the industrial grade. It is commonly used to keep snack foods moist, as it absorbs excess water and maintains moisture. You can recognise it as e-number E1520. It is also used in oral, topical and injectable drugs.
It has also been approved for use in military dietary rations and many tobacco products, and is commonly used in the manufacture of some textiles.
The Material Safety Data Sheet for propylene glycol lists only the industrial grade formulation. It states that propylene glycol can cause skin, liver and kidney damage. It is classed as a hazardous substance.
The FDA has placed pharmaceutical grade propylene glycol on the GRAS list (generally recognised as safe). Although, the negative effects of overdose have been noted, and are looked at below. They allow it’s use in flavourings, drugs, and cosmetics, and as a direct food additive.
The WHO also agrees that it is safe for use. They have recommended the acceptable dietary intake of propylene glycol as 25 mg of propylene glycol for every kilogram (kg) of body weight.
The Cosmetic Ingredient Review Panel reported that there was no carcinogenic risk when using low levels of pharmaceutical grade propylene glycol, namely under 50% concentration in cosmetics.
Empirical studies have tested rats and dogs, feeding them various levels of pharmaceutical grade propylene glycol and reported that they found no links to cancer.
Despite the overall consensus that it should be safe to consume, it is mainly limited to non-food use across Europe.
The following extract taken from the Agency for Toxic Substances and Disease Registry shows that pharmaceutical grade propylene glycol can be metabolised by the liver quite quickly, with a resultant by-product of citric acid.
Absorption of propylene glycol from the gastrointestinal tract is rapid: maximal plasma concentrations in humans occur within 1 hour after ingestion.
Propylene glycol is metabolized in the liver by alcohol dehydrogenase to
• lactic acid, and then
• pyruvic acid
Both of these metabolites are normal constituents of the citric acid cycle and are further metabolized to
• carbon dioxide and
About 45% of an absorbed propylene glycol dose is excreted by the kidneys unchanged or as the glucuronide conjugate.
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