The average smoothie served in a cafe contains the equivalent of 13 teaspoons of sugar, a recent study showed.

The Health and Environment Institute of Seoul Metropolitan Government announced Monday the results of its analysis of 93 smoothie drinks from small and medium-sized cafes, where the nutritional content of drinks is not labeled.

The cafes subject to investigation, which ran from April to June, were located near hagwons, or private cram schools, particularly frequented by teenagers.

The average amount of sugar in a smoothie was 52.2 grams. Since a teaspoon of sugar weighs just over 4 grams, that's about 13 teaspoons of sugar -- more than half of the maximum daily intake the Korean government recommends for an adult, which is 100 grams. One smoothie even contained 94.6 grams of sugar.

The World Health Organization recommends 50 grams of sugar per day for a person consuming 2,000 calories from food. For children and adolescents, the maximum recommended intake of sugar is lower, at 25 grams -- less than half the amount in a single drink analyzed in the survey.

Researchers also studied how much the sugar content fell when the person ordering asked for a sugar adjustment. Categorizing sugar levels into three tiers: "regular," "less sweet," and "half sweet," and repeatedly ordering the same drink for three days to account for manufacturer variations, the average sugar content of the "less sweet" drinks was 44.4 grams, about 15 percent less than the original. For the "half-sweetened" option, it was 31.9 grams, a 40 percent reduction.

The researchers cited the need for a "sugar level choice system" that allows consumers to quantify and choose sweetness levels based on standardized recipes. In Singapore, cafes indicate the sugar content on a scale from A to D for the packaged and manufactured beverages under a system dubbed "Nutri-Grade," Thailand also has the "Sweet-Noi" system, which translates to "less sweet" in Thai, which allows customers to choose a quantified sugar level from 0 to 100 percent when ordering beverages.

"In order to reduce sugar intake from high-sugar drinks such as smoothies, we need a consumption environment where people can choose 'less sweet' flavors," said Park Joo-sung, director of the Seoul Institute of Health and Environment.