NaturallyCurly.com writes:

You know that love-hate relationship we have with fatty foods? Well, the same holds true for glycerin, according to Marsha Coulton, founder of Curl Junkie.

“If you use too much, it can be too heavy and feel like an oil in your hair,” Coulton says. “If you use too little, you don’t get the effect you’re looking for.”

Just as fats (at least the good ones!) are important in a healthy diet, glycerin is a coveted ingredient when it comes to feeding our curls the moisture they crave. Coulton uses glycerin in both of her curl sprays, but in different proportions. Curl Fuel, which is heavier, has more glycerin. But she cut the glycerin by a third for her Curly Boost spray because too much can weigh down curlies with fine hair.