[Dialogue] How can an artificial sweetener contain no calories?

[Harry Wainwright]

Have you ever asked this question? 
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 <http://www.sciam.com/> ScientificAmerican.com



How can an artificial sweetener contain no calories?



A. Rivard
Argyle, Minn



Biologist William K. Purves of Harvey Mudd College <http://www.hmc.edu>
offers this explanation: 

To understand how something can taste sweet and yet add no calories to the
diet, we should address two questions. First, what are calories,
nutritionally speaking? Second, what constitutes a sweet taste? 

Calories are a measure of the energy made available when we digest and
metabolize food. The energy drives the replacement of molecules we have
lost, enables us to move, and so forth; we store excess energy as fat. A
substance that we do not metabolize releases no energy- it "has no calories"
and is not a food. 

A sweet taste results from the binding of molecules to specific receptor
proteins in our taste buds. Sweet-taste-sensory cells in the taste buds have
these receptor protein molecules embedded in their plasma membranes. Binding
of a molecule to a receptor protein initiates a cascade of events within the
taste-sensory cell that eventually releases a signaling molecule to an
adjoining sensory neuron, causing the neuron to send impulses to the brain.
Within the brain, these signals derived from the taste bud cause the actual
sensation of sweetness. Other sensory cells, with different receptor
proteins, report on other taste modalities: salty, sour, bitter, and "umami"
(also referred to as glutamate, or "meat"). 

The events that occur between binding by the "sweet receptor" and the
sensation in the brain have nothing to do with whether a molecule can be
metabolized to yield energy and thus "has calories." The only factor in
taste is whether the molecule can bind to the receptor. 

So, what determines this binding ability? In April 2001, two research teams
published independent contributions to answering this question. Both papers
announced and described a protein, dubbed T1r3, which appears to be the
primary receptor for sweet substances. The molecular structure of T1r3 can
be seen here. Like all receptor proteins, T1r3 has a well-defined "pocket"
where smaller molecules may enter and perhaps bind. Binding depends on a
good fit of molecular shape and the presence of groups that interact
chemically to stabilize binding. 

Sucrose, the sugar in the sugar bowl, binds fairly well to T1r3 and hence
leads to a sweet sensation in the brain. Enzymes readily metabolize sucrose,
releasing energy and, if our diet contains excess calories, causing fat
deposition. 

Saccharin, once the most popular artificial sweetener, binds to T1r3 much
more strongly than does sucrose, owing to the differing structures of the
two molecules. Therefore, we sense saccharin as being approximately 300
times as sweet as the same amount of sucrose. Moreover, saccharin passes
through the body without being metabolized and thus has no caloric content. 

Aspartame (NutraSweetTM), currently the most-used artificial sweetener, also
binds to T1r3 more strongly than sucrose does and tastes nearly 200 times as
sweet. Unlike saccharin, however, aspartame is metabolized, yielding
methanol and the amino acids phenylalanine and aspartic acid. Further
metabolism of these products does yield calories, but far fewer than those
obtained from the amount of sucrose required to produce the same sweetening
effect. 

  _____  

Arno F. Spatola is a professor of chemistry and the director of the
Institute for Molecular Diversity and Drug Design <http://www.imd3.org>  at
the University of Louisville, where his current research focuses on
peptides, including artificial sweeteners. He provides the following answer:


Is there any caloric value to artificial sweeteners? How am I able to have
my cake (the sweetness of my food) and eat it too (avoid gaining weight from
excess calories)? The answer to these questions, as in most areas of
science, is that it depends. 

Sweetness is a taste sensation that only requires interaction with receptors
on our tongues. Many sugar substitutes, such as saccharin and acesulfame K
(also known as SunetteTM), do not provide any calories. This means that they
are not metabolized as part of the normal biochemical pathways that yield
energy in the form of adenosine triphosphate, or ATP. In some cases, small
quantities of additives such as lactose are added in order to improve the
flow characteristics or to add bulk to the products. But the quantities of
these added ingredients are so small that they do not represent a
significant amount of energy-producing foodstuffs. 

Answer posted on November 27, 2006



C 1996-2006 Scientific American, Inc. All rights reserved.
Reproduction in whole or in part without permission is prohibited.



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