In this paper of the WHO, it is claimed that we should limit our consumption of free sugars:


  WHO recommends a reduced intake of free sugars throughout the
  lifecourse. [...]  Free sugars include monosaccharides and
  disaccharides added to foods and beverages by the manufacturer, cook
  or consumer, and sugars naturally present in honey, syrups, fruit
  juices and fruit juice concentrates.


At first, I was wondering if the sugars in a fruit can be different from sugars in a juice. According to the answer to my question here, the sugar molecules are exactly the same in both (in the case of a 100% pure juice).

So, why this recommendation? Why doesn't our body digest sugars from fresh fruit the same way as sugars from juice? To make it simple, my question only concerns 100% pure juice with no added sugars.

I tried to find an answer and, according to the answers of this question, because our body is slow to digest sugars from a fruit, it assimilates it better. Is it right? And, if it is the case, if we eat fiber and starch with our fruit juice, it should be the same as a fresh fruit, right?

Sugars in 100% natural fruit juices are chemically the same as in whole fruits. They mainly include glucose, fructose and sucrose:


Apple nutrition data (expand the carbohydrate section)
Apple juice nutrition data


Sugars in whole fruits are "incorporated" into the fruit, which means the digestive system first needs to physically decompose the fruit and then extract and absorb sugars, which takes some time. In fruit juices, sugars are "free," so they are absorbed quicker than from whole fruits, which results in higher blood glucose levels, which is a risk factor for diabetes type 2 (Defeatdiabetes, Diabetes.co.uk).

Also, fruit juices are liquid, so they pass through the stomach quicker than whole fruits, so they fill the stomach for a shorter time and may be therefore less satiating. This can make you drink more juice than you intended, which can result in unwanted weight gain.

If you eat foods high in fiber along with juice, the fiber will slow down the absorption of sugars from the juice (Nutrients). If you eat foods that contain mainly plain starch (white bread, cookies or rice, or potatoes) along with juice, the starch will be quickly digested and absorbed as glucose and will raise the blood glucose even quicker than the sugar from the juice.

The effect of nutrients from foods on blood glucose level after meals is expressed as glycemic index (GI): the higher the GI, the higher blood glucose (Harvard):


Glucose = 100
Cornflakes (mainly plain starch) = 81
Potato, boiled (mainly plain starch) = 78
White bread (mainly plain starch) = 75
White rice (mainly plain starch) = 73
Sucrose = 65
Honey = 61
Apple juice = 41
Apple: = 36
Kidney beans (high in fiber) = 24
Fructose = 15


In conclusion, natural sugars from whole fruits, fruit juices and artificially added sugars are all digested in the same way, but sugars from juices are absorbed quicker, which can result in higher blood glucose levels. Foods that contain mainly plain starch can raise the blood glucose levels much more than fruits and fruit juices.
