According to WHO, for an adult of normal BMI should consume 25 gm or 6 teaspoons of sugar daily. This is about 5 % of daily average caloric intake for an adult of normal BMI.
To put things in perspective, a can of regular soda contains about 40gm of sugar (10% of daily caloric intake)!!
For a long time (since 2002), WHO recommended 10% caloric intake from sugars per day but newer draft guidelines recommend only 5%.
This added sugar per day adds to added glucose load on body and if this continues on begins to affect the body system leading to metabolic disorders. If anyone is trying to lose weight, he/she should minimize high glycemic load, i.e. limit sugary foods.