To understand where I'm coming from, first some commonly distributed (mis?)conceptions:

(Obviously to be taken with a big grain of salt because they are somewhat contradictory)

  • Sugar is bad for your health, because it goes into the blood quickly and thus creates a blood sugar spike upon consumption.
  • Snacking often is bad for your health, because it puts the body in a state of permanent digestion and keeps blood sugar high.
  • Carbs are better than sugar because they release the glucose over time a supposed in a spike.
  • Slow carbohydrate food (food that is supposed to release the glucose extra slowly, due to the chemical composition of the carbohydrates inside, or the physical structure of the food?) is better than carbohydrates, because it releases the glucose even slower.

Now the second one and the other three ones are somewhat contradictory, because if it was solely about the peak glucose levels, snacking should be healthy (which it probably is not), so something's off...

So I'd like to know: What are the biological differences between the digestion of sugar and different types of carbohydrates as constituents of different types of food in humans?


For more context:

I would like to understand what the most important aspects of carbohydrate digestion are, in order to be able to conclude a diet that helps me optimize my health from that.

I am very well aware that:

  • Sugar is carbohydrates.
  • Many carbohydrates will eventually be broken down into sugar.

I am also aware that "carbohydrates can be bad".

What I do not understand is how exactly "carbohydrates are bad".

In particular, I got confronted with three viewpoints:

  • High average blood glucose levels are bad.
  • High peak glucose levels are bad.
  • Snacking is bad.

Which seem to go in different, rather contradictory directions.

Also, Studies partially supporting either viewpoint can be found:

Which leaves the non-biologist asking themselves which is the "major effect" (certainly, there will be some truth to each position, but the question is which one(s) got the "main point"), and if there are any other important effects to be considered, hence this broad question here, so I understand, from a biological standpoint, what happens to the carbohydrates when I eat them, so I can conclude for myself how to adapt my diet for "optimal" health.

  • 2
    $\begingroup$ Sugar is a carbohydrate. If you want scientific answers to your questions, you'll have to start with scientific sources that make these specific claims. Anyone can claim whatever they want is good or bad and write a book about it, that doesn't mean it's right or useful for anything besides selling the book. $\endgroup$
    – Bryan Krause
    Feb 16 at 19:55
  • 1
    $\begingroup$ This site is for students of biology and biology professionals. Its purpose is not to provide textbook information for non-biologists. If you wish to learn about the metabolism of carbohydrates you need to buy a textbook and study it. $\endgroup$
    – David
    Feb 16 at 20:07
  • 1
    $\begingroup$ @BryanKrause I added some studies as references, as expected, you can find studies to argue either way, so who "got it" then : ( $\endgroup$
    – KGM
    Feb 18 at 1:42
  • $\begingroup$ well for a start, cellulose is fiber and is not digestible so yes different carbs are digested in different ways, but I suggest breaking this into 3 different questions since otherwise you are asking for a dissertation on carbohydrate digestion. Also several have already been asked and answered. biology.stackexchange.com/questions/31352/… $\endgroup$
    – John
    Feb 18 at 19:47
  • $\begingroup$ The level of detail of a Wikipedia article would suffice : ) $\endgroup$
    – KGM
    Feb 19 at 1:15

1 Answer 1


Scope of Answer

The original poster provided ample context for his question, which related to health considerations. It was perhaps for this reason, among others, that the question had not received an answer at the time of writing: questions relating to medical or health advice are off-topic here. However, his actual question is primarily biochemical:

What are the biological differences between the digestion of sugar and different types of carbs as constituents of different types of food in humans?

Although this might be answered with a little internet search, I felt it would be hospitable if someone offered him an answer to this — and this only.


The basic sugar unit is a mono-saccharide, those of relevance to this question being hexoses or pentoses, having six or five carbon atoms, respectively.

What in non-technical language is called sugar, refers to a specific molecule, sucrose, which is a disaccharide of covalently-bonded glucose and fructose.


What in non-technical language are referred to as dietary carbohydrates generally refers to the storage polysaccharide of plants such as potato and other root vegetables, rice, and other cereal crops used to make bread. This is a homo-polymer composed solely of glucose units.


Summary of the differences in metabolism

Arising from these definitions, the following differences in metabolism emerge:

  1. Different enzymes (amylase for these polysaccharides, sucrase for saccharose) are used to catalyse the hydrolysis of the linkages between the monomeric units.

Sucrase action

  1. Absorption in the gut is different for glucose and fructose, as is transport into cells.

  2. Both glucose and fructose are (or can be) metabolised to pyruvate. However fructose is first metabolised to fructose 1-phosphate, and only enters glycolysis at the triose phosphate stage. A consequence of this is that it by-passes any regulation that occurs for the metabolism of glucose after conversion to glucose 6-phosphate. (In addition, glucose 6-phosphate has alternative metabolic possibilities not shown).

fructose and glucose metabolism


This provides the framework for considerations of the consequences of dietary intake of sucrose, which are not part of the question and off-topic here, as already mentioned. Despite that, this topic has been addressed in questions in SE Biology, as well as elsewhere on the internet. I try not to offer advice to others (nor to take it from strangers). However my wife, whose advice I am sometimes obliged to take, professes the following:

“All things in moderation, and moderation in all things.”

  • $\begingroup$ Nice answer. Also fits very well with other information I recently acquired. $\endgroup$
    – KGM
    Feb 23 at 7:09

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