Also called glycerol or glycerine, glycerin is present in all natural lipids (fats), whether animal or vegetable. It can be derived from natural substances by hydrolysis of fats and by fermentation of sugars; it also can be synthetically manufactured.
Glycerin is a skin-identical and skin-repairing ingredient, meaning it is a substance found naturally in skin. In that respect it is one of the many substances in skin that help maintain the outer barrier and prevent dryness or scaling.
Humectants such as glycerin have always raised the question as to whether or not they take too much water from skin. Pure glycerin (100% concentration) on skin is not helpful and can actually be drying, causing blisters if left on too long. So, a major drawback of any humectant (including glycerin) when used in pure form is that it can increase water loss by attracting water from the lower layers of skin (dermis) into the surface layers of skin (epidermis), where the water can easily be lost to the environment—that doesn’t help dry skin or any skin type. For this reason, glycerin and humectants in general are always combined with other ingredients to soften skin. Glycerin combined with other emollients and/or oils is a fundamental cornerstone of most moisturizers.
Research shows that a combination of ingredients, including glycerin, dimethicone, petrolatum, antixoxidants, fatty acids, lecithin, among many others, are excellent for helping skin heal, reducing associated dermatitis, and restoring normal barrier function if used on an ongoing basis. [2,3,4] Research also indicates that the presence of glycerin in the intercellular layer helps other skin lipids do their jobs better. 
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- Fowler Jr J. Efficacy of a skin-protective foam in the treatment of chronic hand dermatitis. Am J Contact Dermat. 2000;11(3):165-9.