From: dst@hardy.u.washington.edu (Dale Tanigawa)
Newsgroups: rec.food.cooking,sci.chem,sci.bio
Subject: Re: Left-handed sugar
Date: 28 Dec 92 10:45:52 GMT
Article-I.D.: shelley.1hmlt0INN4o3
Organization: Care Bear Dismemberment Society
NNTP-Posting-Host: hardy.u.washington.edu

In article <1hklpgINNrub@transfer.stratus.com> det@bigbootay.sw.stratus.com (David Toland) writes:
>In article <72463@cup.portal.com> mmm@cup.portal.com (Mark Robert Thorson) writes:
>>Please excuse the cross-posting into the sci newsgroups, but we've been
>>having a discussion in rec.food.cooking about the supposed existence
>>of left-handed sugars which are indigestible hence calorie-free.
>>I contend that if L-sugars were calorie-free, they probably wouldn't
>>be sweet, either.  Can someone in the sci groups clarify this?
>
>Fructose (fruit sugar) is the levorotatory form of dextrose.  It is
>both sweet and digestible.  It is true that d- and l- forms of chems
>will react to form different products with other chiral (assymmetric)
>compounds, but the various sugars seem to be digestible nonetheless.
>
>I'm not a biochemist, just a (former) organic and electrochemist.
>Perhaps achiral reactions break down the sugars to achiral components,
>or perhaps there is more than one respiratory process involved.  Could
>a biochemist out there clarify further?

Except for a very few exceptions in a very few species, all biologically
active sugars are D- sugars.  Fructose can be made D- or L- form, and the
fructose referred to as fruit sugar is the D- form in biochemistry.  I
don't know if organic chemistry uses different nomenclature, but it appears
it might be so.  Another poster stated that common sugar is a 50-50 blend
of d- and l- sugars in a racemic mixture.  I think this poster may actually
be referring to the alpha- and beta- internal hemiacetals that many sugars
form in solution, rapidly interconverting between the two configurations.
Sugars do not interconvert between D- and L- forms like this.  All the
sugar that you are likely to ever eat is a D- sugar.

L- sugars are indigestible since the chiral molecule doesn't fit into the
enzymes in your body and there is no provision for turning an L- sugar 
into a D- sugar.  Whether it would taste sweet I don't know, it's not
my field of interest.  L- sugars are a bad replacement for D- sugars for
several reasons.  If a particular L- sugar were incapable of being absorbed
in the intestine, anybody eating it would suffer the same symptoms as a
lactose-intolerant person.  The sugar would osmotically "pull" water into
the intestine, causing bloating, pain, and diarrhea.  If the sugar were
able to pass into the bloodstream, it would likely poison the eater.  In
sufficient concentration, sugar is a poison, which is why your body packages
it away in a non-harmful form as soon as it can and parcels it out as you
need it.  Sugar is a common method of preservation in the kitchen---its
preservative effect is why fruits are packed in syrup---no bug could survive
such a high sugar concentration.  Your body would be unable to regulate the
blood level of L- sugars because it has no mechanisms for dealing with them.

I can see where the confusion arises---someone looking at a biochemical
sugar chart would be likely to assume that mannose and sorbose are L- sugars
because they are stereochemically related to glucose and fructose.
Biochemists often leave the D- part off, because it is commonly understood
that it belongs.  Many of the "lesser" sugars, such as mannose and sorbose
are useful for diabetics because they aren't glucose, and they decrease the
available calories because the body's machinery for getting energy from 
these sugars is a lot less efficient than the widely-used glucose method.

Not to mention that any recipe that relied on yeast and sugar for rising
would never be able to use L- sugars either.  Yeast can't digest them any
Newsgroups: rec.food.cooking