Why do we need polysaccharides?

[SticksandStones]

Hi, I was reading up on some basic biology when I came across a claim in my book that humans can't get their carbohydrate needs from mono/di-saccharides alone but also require polysaccharides such as starch as well. However, it also says that starch is a polymer of glucose. So my question is, what difference is there between eating 1 unit of starch, and a number of units of glucose equivalent to the number of units of glucose comprising starch?

 

[Andy Resnick]

That sounds like an odd claim (but possibly true)... what bio book are you using?

 

[SticksandStones]

Essential Biology with Physiology (Campbell Reece Simon).

 

[Bio-student]

Do humans have any 'carbohydrate needs'? Sure we have energy needs and carbohydrates are good for energy, but I can't think of why carbohydrates would be needed whether they come as mono, di or polysaccharides.

 

[Andy Resnick]

Do humans have any 'carbohydrate needs'? Sure we have energy needs and carbohydrates are good for energy, but I can't think of why carbohydrates would be needed whether they come as mono, di or polysaccharides.

That's easy to answer: the Krebs cycle.

 

[Bio-student]

That's easy to answer: the Krebs cycle.

I'm not sure I understand - what about the Krebs cycle?

 

[Andy Resnick]

Essential Biology with Physiology (Campbell Reece Simon).

I'm not familiar with that book. I doubt the claim that polysaccharides are a nutritional requirement, since they get broken down into simple sugars via the gut bacteria. Maybe it has to do with the rate glucose is supplied to the body.

 

[Andy Resnick]

I'm not sure I understand - what about the Krebs cycle?

The Krebs cycle is an essential metabolic recation, and uses carbohydrates to generate (ultimately) ATP. Note that carbohydrates are not an enssential part of our diet; but omitting carbohydrates entirely (a ketogenic diet) requires special attention.

 

[Bio-student]

The Krebs cycle is an essential metabolic recation, and uses carbohydrates to generate (ultimately) ATP. Note that carbohydrates are not an enssential part of our diet; but omitting carbohydrates entirely (a ketogenic diet) requires special attention.

Yeah but that's what I said - there is no direct requirement for carbohydrates themselves, only for the energy which carbohydrates (among other nutrients) can provide. Incidentally, though, the molecule fed into the Krebs cycle is acetyl coA which is a product of fatty acid oxidation as well as glycolysis, so Krebs can continue without dietary carbohydrate. Also, any required glucose can be synthesised from glucogenic amino acids (in protein) and glycerol (in fat)

 

[Andy Resnick]

I hear what you are saying, but I'm not going to give up eating all carbohydrates for the rest of my life. :)

 

[Anti]

I’ll take a stab at this.

Could be a number of reasons that your text makes this claim.

1. Availability of mono- and disaccharides

Simple sugars are not as common as polysaccharides so perhaps your text is implying that you need the latter simply because there’s just no significant or adequate source of the former.

2. Storage function of polysaccharides

Stored energy is crucial, and humans store energy as complex carbohydrates such as glycogen. Similar to the above point, you need a lot of monosaccharides to match one molecule of a complex carb. Glycogen is a long-term energy reserve that is crucial to metabolic function. Perhaps your text is suggesting that complex carbs are essential for physiological processes, not that you necessarily need to eat them to live.

3. Metabolism

Complex carbs take longer to metabolise and offer a long-term source of energy. Simple carbs are processed quickly, therefore they enter the bloodstream and are used up rapidly. As such, you could possibly be eating non-stop in order to function without complex carbs. Complex carbs provide more energy per gram (if I recall correctly) and deliver this energy over a longer period of time, hence we don't "hit the wall" when we stop eating.

Of course, I guess if you front-loaded enough glucose you could build your own internal store of complex carbs, which would lead back to option 2, above.

4. Book is wrong

Surprisingly, this happens fairly often. Perhaps it’s a misprint or some statement that isn’t quite accurate when taken literally but as a general rule is a “safe assumption”. This is more common in a math text, to the point where you’d be hard pressed to find any math text that doesn’t have at least one error in it.

Off the top of my head, I honestly can’t think of a definitive reason that explains why your question isn’t feasible. I’m no physiologist and my background is in a totally different field of biology (and I shudder to think of how much I’ve forgotten along the way) but it is a very good question and one that I plan on digging around for an answer to.

 

[Bio-student]

I hear what you are saying, but I'm not going to give up eating all carbohydrates for the rest of my life. :)

Ha, me neither. Whether they're needed biologically or not they're tasty

 

[Eddbio]

Could you quote exactly what it said, maybe it's just badly written, It probably means that sugars aren't found in mono or di-saccarhides in large quantities and we have to eat poly's found in plant material.