ARTIFICIAL SWEETENERS: THEIR ORIGINS AND MECHANISMS.

Our pursuit to tackle one of our senses…a battle between desire and need.

HTTP://WWW.DIETRIFFIC.COM/

ERINA ELISE VAN HORN

SPECIAL TOPICS – SCHOLARLY PAPER SUBMISSION

DR. BRUCE JARVIS

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UNIVERSITY OF MARYLAND

MAY 2009

Animals love the taste of sweet. From humans with their “sweet tooth” down to the

smallest vole hunting for the garden bulbs, it has been said that this craving is due to our

natural instinct to search and find food. (1) Sweetness implies there are calories: calories

to sustain our body through various activities and environments. Most of those calories

come from carbohydrates and lipids (fats and oils). Proteins as well as carbohydrates

contain four calories per gram, whereas fats contain nine calories per gram. (2) The

reason why then we take in more calories from carbohydrates than proteins is due to the

fact that we consume much more in volume of those carbohydrates. Carbohydrates are

molecules that come in a variety of sizes and resulting function. “The smallest

carbohydrates are the simple sugars, also known as monosaccharides and disaccharides.”

(3) The commonly referred to sugar is table sugar, or sucrose, which is a disaccharide.

Sucrose consists of two monosaccharides: glucose and fructose.

Figure 1. Sucrose molecule (3)

Fructose can be found solo in fruits, and glucose will form polysaccharides (also referred

to as complex carbohydrates) e.g. starch found in potatoes, beans and other fibrous

vegetables. (4) Glucose is very important in our diet, as it is the starting block of nearly

all energy derivation/mechanism in mammals.

SUCROSE TO GLUCOSE FOR ENERGY: Gylcolysis is the breakdown sequence of glucose. This sequence of reactions leads to the

transformation of the simple sugars into smaller molecular building blocks, specifically

pyruvate and ATP1 (adenosine triphosphate). For higher organisms, pyruvate is further

metabolized in the presence of oxygen to produce carbon dioxide, water and a significant

amount of additional ATP.

Figure 2. Net reaction of glycolysis (Wiki)

In order for glycolysis to operate effectively there must be the driving force to process the

glucose, meaning there must be an imbalance in the glucose concentration of the

bloodstream. Signals from the pancreas secrete glucagon, which draws glycogen out of

storage. Glycogen is a branched polymerized form of glucose that allows for stable

storage of glucose in the liver (predominantly) or the muscles. Once the glucose is

released from glycogen, it undergoes one further step prior to glycolysis. (2)

One might think that since glucose is our direct source of energy then the more glucose

we have the more energy we would have. For some this is true, however some

individuals must live with the disease diabetes mellitus. In general, diabetes occurs with

hyperglycemia, which is too much glucose in the blood. This is due to one of two factors

– there is not enough insulin to process the glucose or the insulin in the system is no

1 ATP is a powerful donor of phosphate groups to suitable acceptors because of the pyrophosphate nature of the bonds between its three phosphate radicals during in the phosphorylation of glucose. ATP serves as the immediate source of energy for the mechanical work performed by muscle. ATP serves as a link between sources of energy available to a living system and the chemical and mechanical work that is associated with growth, reproduction, and maintenance of living substance. McGraw-Hill Encyclopedia of Science and Technology. The McGraw-Hill Companies, Inc., 2005. Answers.com 23 Apr. 2009. http://www.answers.com/topic/adenosine-triphosphate

longer effective to handle to the amount of glucose present. Diabetics use the glycemic

index to relate a numerical value to a carbohydrate-rich food, based on the average

increase in blood glucose levels occurring after the food is eaten. (5, 6) Glucose

intolerance is not the only sugar-related disease, as there are individuals that cannot

process fructose, but this is less common and treatment is usually similar to prescribed

treatment for diabetics. (7,8)

So, when a portion of the population suffers from a disease in which sucrose is the

initiating culprit, the treatment choices are to either eliminate the source of glucose or

add/regulate the amount of insulin available to the bloodstream. Most options are a

combination of both. As of 2007 almost 18 million people in the United States have been

diagnosed with the disease – over 5 million are estimated to be undiagnosed. (5)

Restricting the amount of glucose for some can be a daunting task. Americans consume

roughly 140 pounds of refined sugar per year; safe to say we consider ourselves as having

a sweet tooth. (10)

Figure 3. Chart on Sugar Consumption Trend (wiki)

This translates not only in an increase of diabetes but an increase in obesity. Omitting

those sweet treats can be down right miserable to many. So, in lieu of ridding the diet of

sweet, science went looking for a sweet replacement.

SUGAR SUBSTITUTES – SWEETENERS Sugar substitute is the broad term used to describe any substance that replaces sugar

(sucrose) as a sweetener in a product. A sugar substitute could either be from a natural

source or artificially derived. Reasons to replace sucrose vary from availability, product

formulation, cost/financial justifications, or a health/dietary choice. Two types of

alternative sweeteners are available: bulk and intense. Many bulk sweeteners have the

same caloric values as sucrose, but are chosen for financial reasons/motives. (9) Polyols,

or sugar alcohols, lead the list of most common sugar replacements, although many have

a fraction of the sweetness of sucrose, along with some other less desirable

characteristics; the leader being their laxative effects. (11)

Intense sweeteners are primarily made up of artificial sweeteners, synthesized from a

variety of starting materials. These sweeteners are intense due to their sweetness being

hundreds, sometimes thousands times sweeter than that of sucrose, and their mode of

action on the sweet taste bud are all similar. However, there are some clear distinctions

as to their mode of action within the body.

SWEET TASTE: IS THERE A DISTINCTION BETWEEN ARTIFICIAL OR NATURAL? Sweetness is a perception. One of the five primary taste sensations, sweet is probably the

most beloved, and cursed, for its intensity. “The primary tastes gave early humans clues

about what food was good to eat and what was harmful. Sweet foods usually had calories.

Salty foods had important vitamins and minerals. Sour foods could be healthy, like

oranges, or spoiled, like rotten milk. Bitter tastes were often poisonous.”(12) Regardless

of the taste, the tongue functions in the same way. The tongue is covered with papillae -

within the papillae are the taste buds. The taste buds are actually small groups of

epithelial cells forming a complex interactive unit comprised of 50-150 neural receptors.

Figure 4. Tongue anatomy (12)

These receptors are sensitive to the electrophysiological characteristics of sweet

molecules; particularly the highly polar regions of the sugar molecule making for the

strong association between sugars and sweetness. Glucose specifically binds to the

heterodimeric receptors, T1R2 and T1R3, which recognize both natural and synthetic

sweeteners. (13,14) “After this binding there is a series of neural firings in the brain that

evokes memory retention about the sweetness. “A specific taste quality perception is

generated, which allows sugars to be differentiated from other compounds.” (1) It is these

same receptors that molecules of artificial sweeteners are triggering as well. They are

most effective at lower concentrations, which correspond well to packaging due to their

intense perception of sweetness. However, at higher concentrations some molecules can

also bind to another taste receptor, the TRPV1 receptor, which triggers the bitter and

sometimes metallic aftertaste associated with some of the artificial sweeteners. (15,16)

Figure 5. Human T1R2-T1R3 sweet receptor – Binding for most sweet ligands occurs on the

T1R2 unit. Sugars may interact with the large N termini of both T1R2 and T1R3. (17) As further research developments are taken with these artificial, intense sweeteners the

conformation of these molecules are studied to determine which specific characteristics

are utilized in the distinction of sweetness. The first prominent theory dealing with

chemical reception for sweetness proposed that in order to be sweet, a compound must

contain a hydrogen bond donor (AH) and a Lewis base (B) separated by about 0.3

nanometers. “According to this theory, the AH-B unit of a sweetener binds with a

corresponding AH-B unit on the biological sweetness receptor to produce the sensation of

sweetness.”(13) This theory was further refined to include a third site that addressed the

variances in sweetness relative to concentration and how simply containing the functional

groups previously believed linked to sweetness was not enough. The interaction within

the molecule itself and the binding sites were also needed to be involved for activation at

the receptor site. (14,15,17) It is evident that the molecule purported to be sweet must

contain a minimum of two binding sites for the receptor to recognize them as sweet.

RESEARCH, REGULATION AND THE FDA – CURRENT APPROVED ARTIFICIAL SWEETENERS: How is an artificial sweetener regulated compared to crop-harvested sugar? The United

States Department of Agriculture, USDA, regulates all crops grown on US soil through

permits issued by the Animal and Plant Health Inspection Service, APHIS. APHIS

requires information on the plant, the origin of new genes, or gene product, and the

purpose for developing the crop. (10) The USDA also monitors imports/exports of crops

to international sites. In North America, sugar is predominately harvested from sugar

beets but also from sugar cane. (10) The Food and Drug Administration (FDA) regulates

artificial sweeteners as food additives and/or “foodstuffs”. The FDA came out of The

Pure Food and Drug Act of 1906, created to regulate foods and drugs meant for human

consumption. The main purpose of the act was to protect the consumer against

mislabeled or adulterated food. (18) In 1958 and 1960 the Food Additives and the Color

Additive Amendments required pre-market approval of new food ingredients and colors

used in foods respectively. The process for market approval is a four-step communication

between the FDA and the research company. These steps, laid out by The Federal, Food,

Drug and Cosmetic Act of 1938, which replaced the 1906 act, include: 1st - a proposal,

2nd - comments that are statements by any consumer or organization question/concern, 3rd

- responses from the agency filing the proposal and 4th - final rule from the FDA.

“Additives included are those specified in the regulations promulgated under the FD&C

Act, under Sections 401 (Food Standards), and 409 (Food Additives).” (18,19) This would

include artificial sweeteners as both food additives - alone or in combination - and

tabletop sweeteners.

Saccharin is the oldest artificial sweetener still in use today. (18) It has had a long

market life full of problems associated with its health safety. Discovered in 1879 by a

couple of scientists at John Hopkins University, it is manufactured from toluene in a

reaction initialized by sulfuric acid and phosphorus pentachloride to make the

sulfonamide, then esterfication closes the second ring to produce the final product.

Figure 6. Synthesis of Saccharin, Remsen and Fahlberg method (wiki)

Saccharin is 200-700X sweeter than sucrose and is seen in a multitude of applications. It

does tend to have a slightly bitter taste and metallic aftertaste, and for this reason is

sometimes combined with other sweeteners (both bulk and intense). Because its

discovery was well before the formation of the FDA, it was grandfathered in under

Federal Food & Drug Act of 1938 as GRAS (Generally Recognized as Safe). (19,20) It

continued to be tested, and although it was repeatedly shown to pass out of the body

unchanged, there were animal studies that a significant percentage of the rats used

developed bladder cancer when fed a diet with higher than normal limits of saccharin.

(21,22) Due to concern over cancer the FDA removed saccharin from GRAS status and

initiated a nationwide ban on the substance. At this time there was no other approved

artificial sweetener, so to protect its use in diabetic applications Congress passed the

“Saccharin Study & Labeling Act” in 1977 preventing it from being banned by the FDA.

As part of the act, any product that contained saccharin had to carry the warning label

that the product was known to cause cancer in lab animals. In 2001, saccharin was finally

recognized not to cause cancer in humans taking in normal limits, and the warning label

was removed. (18,22,23)

Aspartame was discovered in 1965 by a group of scientist working for G.D. Searle, that

later become a Monsanto company, to develop a new anti-ulcer drug based on a

tetrapeptide. (18,24)

+

Figure 7. Phenylalanine and Aspartic Acid to yield Aspartame (wiki)

The intermediate was a dipeptide made from the amino acids, aspartic acid and

phenylalanine that was found accidentally to be intensely sweet. On their own, the

individual amino acids have no sweetness, but when combined made a product 200 times

+H3N H N

sweeter that sucrose. In 1980 it was approved as a food additive alone or in combination

with other sweeteners, followed by in 1991 approval as a tabletop sweetener. Commercial

production of aspartame requires an initial fermentation process to produce the amino

acids using specific strains of bacteria. Aspartic acid is made from the bacteria

Brevibacterium flavum, while the phenylalanine is generated from Corynebacterium

glutamicum. (24) These two materials are combined through amidation to establish the

peptide bond. One of aspartames major health risk comes from the reverse of this process

in the body. Once ingested in the body it is metabolized to methanol and the two amino

acids. (18,25)

Figure 8. Metabolic processing of phenylalanine (wiki)

The extra addition of phenylalanine only bothers those with a specific disorder (a rare

genetic disorder phenylketonuria), which means they cannot handle certain

concentrations of phenylalanine (Phe). Individuals with PKU lack the oxidizing system,

specifically Phe-hydroxylase, that converts excess Phe into tyrosine (Tyr) through

“transamination with α-ketoglutarate to p-hydroxyphenylpyruvate” and then ultimately

into homogentisic acid. (26) Excess Phe accumulated in the blood can lead to “metabolic

acidosis”2, distortions of plasma concentrations of other amino acids which can affect

brain function due to lack of neurotransmitters, in particular serotonin. (26)

Even with the health risks associated with aspartame, it is used in a multitude of food

applications, including tabletop portions, with the exception of baked goods. Aspartame

degrades at temperatures above 90˚F; although there have been recent advances to

encapsulate it in order to protect it under heated conditions.

Acesulfame-K was discovered by accident by Germans Clauss and Jensen in 1967, (27)

while they were conducting research on then new cyclic group, dihydro-oxathiaxinone

dioxide. Approved by the FDA as a tabletop sweetener and food additive since 1988,

acesulfame is 200 times sweeter than sucrose. A major positive trait is its stability under

heat applications. This translates into longer shelf life, which makes it highly suitable in

packaged goods, as well as available for use in baking. (18)

Figure 9. Acesulfame K synthesis from chlorophenol (wikicommons)

Most applications have acesulfame salts in combination with another intense sweetener in

order to mask a minor fault (i.e. temperature degradation or slight metallic taste). (28)

2 Metabolic acidosis is an acid imbalance in the body that results in lack of bicarbonate in the blood to neutralize.

Acesulfame is not metabolized or stored in the body. The FDA continues to support the

use of acesulfame in diabetic and low-calorie foods.

Sucralose made it to the market with FDA approval in 1989. Like other artificial

sweeteners, it was discovered by accident in the laboratory in 1976. (18, 31) It is 600X

sweeter than sucrose and is considered non-caloric because it is not digested in the body.

Because it is such an intense sweetener, in order to formulate a tabletop delivery it is

packaged with bulking agents such as dextrose and malodextrin. This does affect its

performance in baked products. Dextrose and malodextrin are cornstarch derivatives that

do add some caloric value, but per serving it is less than 5 calories, which is the top limit

the FDA places on products that are considered calorie-free foods. (20)

Figure 10. From Sucrose to Sucralose (wiki)

Made from sucrose in a selective multi-step displacement reaction that starts with cane

sugar, three of the hydroxyl groups on the sugar molecule are replaced by three chlorine

atoms. (30) Tate and Lyle of Britain, developed and patented the original sucralose

manufacturing process to make Splenda™. Splenda™ is the internationally recognized

brand of sucralose and is recognized by its trademark yellow packet. Since the end of the

patent term this year, several novel approaches to the production of sucralose have been

suggested. Once such method involves the production of intermediate “glucose-6-acetate

by fermentation of glucose using a strain of Bacillus megaterium followed by conversion

to sucrose-6-acetate as a kinetic product using a specially selected fructosyl transferase

produced by a newly isolated strain of Bacillus subtilis.” (32)

The product is then chlorinated and subsequently deacetylated to yield 4,1,6-trichlo-

4,1,6-trideoxy galactosucrose (sucralose). This process involves fewer steps than are

required for chemical synthesis using trityl chloride, acetic anhydride and methanol. That

process includes the final step using sodium methoxide to deacetylized the intermediate

ester. Sucralose is additionally attractive as a sweetener due to the fact that the body does

not metabolize it. The majority of the ingested amount is excreted unchanged from the

body. (30)

Neotame, N- [N- (3,3-dimethylbutyl)-L-α-aspartyl]-L-phenylalanine 1-methyl ester, is a

synthetic sweetener made by the NutraSweet Company. (34) It is similar to aspartame,

but has an extra branch that prevents its metabolism to produce phenylalanine that as

previously discussed can cause a tolerance problem in some people. This branch, a 3,3-

dimethylbutyl group, blocks the peptide enzyme from catalyzing the reaction, therefore

leaving the Neotame residue intact and generating a minor amount of methanol that the

body absorbs.

Figure 11. Metabolic (de-esterfication) of Neotame (wiki)

The residue is eliminated from the body without any concern for accumulation. For

marketing value, this means it does not require a safety-warning label and consumers can

feel better about the consuming the product. FDA approval was granted in 2002. Another

benefit over aspartame is that neotame does not degrade like aspartame at higher

temperatures, which means that it has a better shelf life. Neotame is used mainly in

manufactured goods and not solely as a tabletop sweetener; this is due to the fact that it is

so much more potent as a sweetener, 8000 times sweeter than sucrose. (34) The average

amount needed to sweeten a beverage is a fraction even of a normal marketed, e.g. only

6mg of neotame are needed to sweeten a 12oz beverage. NutraSweet has caused quite a

marketing uproar with its new “pink” sweetener: a combination of neotame and

acesulfame-K in a pink packet. Traditionally the pink packet has been exclusively

associated with saccharin. In a similar way, consumers associate blue packaging to

aspartame, and yellow to sucralose.

ALTERNATIVE SWEETENERS: Regardless of the studies provided to and provided by the FDA, artificial sweeteners

continue to receive negative press from a variety of public awareness groups. Many of

these groups recommend replacement sweeteners that have a more natural origin and are

not synthesized in chemical laboratories. (31,35) Sugar alcohols, or polyols, are a popular

substitute in baked goods as they are considered a bulk sugar – having similar volume

and texture as sucrose. Polyols are chemical derivatives of sugars that have an alcohol

group (-CH2OH) instead of the aldehyde group (-CHO). (36) Their caloric contribution

is about half that of a traditional carbohydrate and does not promote dental decay. Major

downsides of sugar alcohols stem from its major benefit – the body either slowly or

incompletely metabolizes the molecules. This means that diabetics tolerate sugar

alcohols because a glycemic response is not initiated, but also that the molecules pass out

through the small intestines causing gastric distress. (13,36)

Name Erythritol Glycerol Malitol Sorbitol Xylitol

Sweetness to Sucrose

81% 60% 90% 60% 100%

Figure 12. Table of Common Sugar Alcohols Used as Food Additives (wiki)

“The laxative effect of sugar alcohols is due to its slow absorption rate. When the

number of small molecules sit in the small intestine for a very long time water is drawn

into the intestines. This increase in water directly increases the speed of evacuation of

that water.” (11)

FUTURE SWEETENERS: As consumers continue to purchase prepared foods and food products, laboratory

research into artificial sweeteners and food additives continues to be a strong field of

study. A growing trend is towards organic3 or more natural additives. One such interest is

in a sweetener collected and refined from the plant Stevia rebaudiana. Stevia usage has

been documented in a variety of crude forms in early “tribal” times in Paraguay and more

refined versions more recently in Japan, particularly after the Japanese ban on saccharin

in the 1970s. (37)

Figure 13. Stevioside (L) and Rebaudioside A (R) from Stevia rebaudiana (37)

3 “Organic”. In 1990, Congress passed the Federal Organic Foods Production Act, which called for national organic food guidelines including certification of growers and standards for organic food production, monitoring crops for chemical contamination and livestock for living conditions and screening organic imports. Under standards adopted by the U.S. Agriculture Dept. (USDA) in 2000 and fully effective in 2002, synthetic fertilizers and pesticides and antibiotics may not be used in raising organic foods, and the use of irradiation, biotechnology, and sewer-sludge fertilizer is also banned.

The Stevia plant produces several steviol glycosides4, with stevioside (“stevia”) and

rebaudioside A (“rebiana”) in the highest percentage. Up until recently, the European

Union and the US FDA have not granted companies wishing to use stevia compounds as

a food additive permission, due to concerns that the chemical compounds may cause

mutagenic or reproductive difficulties. For this reason, stevia and rebiana containing

products started appearing in health food stores as “herbal supplements” – as the FDA

has no regulatory rule over these products as they do over food additives. (20) Concerns

over which glycosides were present in the food product and the manufacturing principles

of the supplier kept the FDA suspect of any product containing stevia as a primary

sweetener.

Figure 14. Steviol(37 )

Even though they each contain several glucose molecules, neither stevia nor rebiana are

absorbed into the blood stream and therefore do not affect blood glucose. (37) Both

4 "glycoside." A glycoside is a group of natural occurring molecules in which one or more glucose molecules are attached The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. Answers.com 19 May. 2009. http://www.answers.com/topic/glycoside

hydrolyze to yield the aglycone steviol that is subsequently eliminated through the

kidneys.

Figure 15. Metabolism of Stevia and/or Rebiana to steviol. (37)

Up until recently, reports have been conflicting as to what exactly the affects of

consuming stevia leaves have been on reproductive and genetic health due to

inconsistency with data collection and administration of the tests. A 2008 study lead the

Joint Expert Committee on Food Additives (JECFA) of the FAO/WHO to support the

FDA on approving manufacturing guidelines of stevia and rebiana as a food additive.

Figure 16. Elimination of Steviol. (37) On December 17, 2008 the FDA granted GRAS status to manufacturers Cargill and

Merisant to produce rebauside A according to specified purity guidelines. (38,39) This

allows these companies to use rebiana as a food additive in accordance to FDA ruling.

There has always been an interest in a sugar alternative for persons living with diabetes,

and secondary benefit to an alternative would be for those looking to reduce their caloric

intake. Although, glucose is still necessary for energy production in the body, there is

sufficient quantity achieved through other carbohydrate means. Those that are considered

diabetic sugars are listed as such because they do not activate insulin production from the

pancreas. Not all are a benefit over the simple sugar molecule, and must be evaluated for

their benefits over their detriments. Whether the choice of an artificial sweetener over a

natural/harvest crop sugar is due to medicinal reasons or dietary restriction, there are

good options to support any decision. Artificial sweeteners are proven to be a safe

alternative to sucrose, and a person is given a variety to choose from depending upon the

level of desired sweetness. Products that use artificial sweeteners usually contain more

than one because “certain sweeteners amplify one another”. (13)

The looming question that about most artificial sweeteners is: should we modify our

foods or should our behavior be modified? Can we not just moderate our consumption of

crop sugar? This has becoming increasingly more difficult with more and more pre-

made, pre-packaged food products that have made their way into our homes. Food

manufacturers look for inexpensive ways to bring products to market that consumers will

enjoy. Both bulk and intense sweeteners have filled this job effectively. Our job as

consumers is, and will forever remain, to read the labels and understand what we are

putting into our bodies.

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