Sweet Tooths and Sugar Substitutes — A Scientific Analysis
By Carol M. Meerschaert, RD
Today’s Dietitian
Vol. 8 No. 5 P. 34

Ever wonder about the biology of the sweet tooth? Read up on the science of sweetness and see how various sugar substitutes stack up to the real thing.

I had the pleasure of attending the Oldways Preservation and Exchange Trust “Managing Sweetness” conference in October 2004. The meeting was convened to take a long, hard look at sweetness—why people are drawn to sweet tastes and how health professionals can help clients manage sweets in the face of obesity and overweight. The first line of the consensus statement issued at the conference stated that the “biological preference for sweetness is universal.”1

How does this universal taste sensation work? What is new in the science of sweeteners? How can nutrition professionals help clients satisfy their sweet tooth and stay within an appropriate range of caloric intake?

Science of Sweetness
A spoonful of sugar does more than help the medicine go down. Sugar, that simple disaccharide with only 15 calories per teaspoon, has become a bad word like fat was in the ’90s or cholesterol was in the ’80s. Sugar is the ultimate functional food. Foods such as confections, frozen desserts, and baked goods cannot be made without sugar. Sugar tenderizes by inhibiting the formation of gluten in wheat-containing foods. It is one of the oldest food preservatives as it steals water from bacteria—think jelly and fruit preserves. This water-grabbing capacity also keeps food moist. Without sugar, our foods would be boringly white, as sugar is key in the Maillard browning reaction. In frozen desserts, sugar performs the key function of freezing-point depression. It also stabilizes egg foams, strengthens cell texture in fruits and vegetables during cooking, adds flavor, increases volume when creamed into fat, and balances acidic foods. Since our clients clearly need to balance their energy needs, perhaps they should be reminded that the song is a spoonful of sugar, not a truckload.

The term artificial sweetener is used a great deal but is inappropriate in a clinical setting. What is natural? Sugar cane? White sugar? Maple syrup? All must be processed before they can be consumed. There are essentially two categories of sweetness: those that have roughly 4 calories per gram and those that are low calorie. The correct classification is then nutritive or nonnutritive, meaning that the sweetener offers calories or does not offer calories, respectively. This may seem odd, considering we are often trying to help clients select foods with fewer calories. Some sweeteners are called intense sweeteners. This is also a misnomer as no substance elicits higher-intensity sweet taste than sucrose. The more precise term is high potency, meaning that the sweetener offers more sweet taste per gram but the sweet effect in the mouth is the same with a smaller amount of the sweetener.

How Do We Taste?
A taste bud is a cluster of approximately 100 elongated cells arranged much like the segments of an orange.2 Some cells in the taste buds have projections that push out on top and make contact with saliva. Three major nerve systems (chorda tympani, glossopharyngeal, and supra petrosal) carry information from the taste buds to the brain. Each nerve is a bundle of approximately 5,000 axons coded to carry one taste, such as sweet or sour.

Taste bud cells live for roughly 10 days, after which time they are replaced. The individual taste bud cells are taste specific (eg, sweet sensitive, sour sensitive). Some people have nearly 100 times as many taste buds as others. The pattern of taste papillae on the human tongue is unique (just like a fingerprint). Sensitivity to tastes (sweet, sour, bitter) is proportional to the number of taste buds a person has. Low taste bud numbers correlate with obesity and alcoholism. The classic textbook taste quality map for the tongue is wrong; there is good sensitivity to all the primary tastes from all parts of the tongue.

When a sweet-tasting molecule is in saliva, it binds to the sweet taste bud in what is called the Venus Flytrap, closing that trap and triggering a signal to the brain that sweet has been tasted. Many researchers are now investigating what compounds can fit in this Venus Flytrap of the taste bud and elicit sweet taste sensation to develop new food ingredients. But simply tasting sweet is not enough. A sweetener must be safe, stable, water soluble, have a reasonable cost, and, most importantly, taste as close as possible to that sucrose taste we all know and love.

One aspect of sweetness examined when looking for the ultimate sweetener is called the temporal profile. This examines the time it takes for the compound to signal the brain that there is something sweet and also how long it takes for that sweet taste to disappear. It takes approximately four seconds for a 10% sucrose solution to indicate sweet to the brain. Ten seconds later, that taste is gone.

Saccharin
There are now numerous choices for reduced-calorie and noncaloric sweeteners on the market. In 1878, Constantine Fahlberg found that a compound called saccharin had a very potent, sweet taste. Saccharin’s temporal profile is the same as sugar, making it an acceptable-tasting sweetener to most people. Unfortunately, saccharin has a bitter and metallic taste that limits its usefulness. Interestingly, research has found that approximately one third of people are hypersensitive to these off-tastes while the remaining two thirds vary from moderately sensitive to insensitive. If a client seems to reject all notion of nonnutritive sweeteners due to taste, he or she may be in that one third of the population that is hypersensitive to the metallic taste.

In the 1960s, some poorly conducted rat experiments indicated that there was a cancer risk associated with saccharin. Subsequent research has shown this not to be the case, but many consumers remain hesitant toward this sweetener. Current research suggests that this compound is safe. Saccharin is also highly water soluble and stable.

Aspartame
In 1965, James Schlatter, a chemist at G.D. Searle & Company, discovered the compound (L)-Aspartyl-(L)-phenylalanine methyl ester, which offered a sweet taste similar to sucrose without the off-taste associated with saccharin. This compound, aspartame, was the first sweet-tasting member of a structural class of sweeteners generally known as dipeptides. Peptides are oligomers of alpha-amino acids. Aspartame is derived from two natural amino acids: (L)-aspartic acid and (L)-phenylalanine. It is roughly 200 times sweeter than sucrose, so although it has calories because it is made of amino acids and is metabolized, it is considered nonnutritive (noncaloric) due to the very small amounts of calories it contributes per serving of food.

The flavor profile of aspartame is essentially indistinguishable from sucrose; no nonsweet taste attributes are observed. However, the temporal profile of aspartame is different from sucrose. Aspartame exhibits both a slightly delayed onset of taste (five vs. four seconds for sucrose) and an extinguishing time of 19 vs. 14 seconds for sucrose.

In the United States, sweeteners that are not on the GRAS (generally recognized as safe) list can be approved for use in food after passing the Food Additive Petition process, which requires extensive safety studies in test animals as well as studies in humans. One objective of these studies is the determination of the highest dose that may be given without adverse effects. This dose is termed the No Observed Adverse Effect Level (NOAEL). The NOAEL, in the most sensitive animal species evaluated, is then used by the FDA to regulate the level at which the food additive may be used in foods. This level, termed the Acceptable Daily Intake (ADI), is defined as 1/100th of the NOAEL in the most sensitive animal species. These NOAEL and ADI exposure levels are given in milligrams per kilogram of body weight. For example, if the NOAEL of a proposed new sweetener was determined to be 500 milligrams per kilogram in the most sensitive animal species evaluated, an ADI of 5 milligrams per kilogram would be allowed.

The objective is to ensure that the ADI exposure level is not exceeded on a chronic basis. To do this, 90th-percentile, 14-day average food category consumption data are examined. Approval of the new sweetener may then be granted for use in food categories where, in the aggregate, 90th-percentile, 14-day average consumption data on these categories does not exceed the ADI.

Upon evaluation of the safety assessment studies, the FDA approved aspartame in 1981 with an ADI of 20 milligrams per kilogram. This ADI is based on a NOAEL of greater than 2,000 to 4,000 milligrams per kilogram in preclinical studies. In 1983, based on clinical studies, the FDA raised the ADI for aspartame to 50 milligrams per kilogram. No effects were noted in the clinical studies at doses many times greater than approved for human consumption.

When ingested, aspartame is completely broken down to the two naturally occurring amino acids and methanol. The safety of the amino acids (L)-aspartic acid and (L)-phenylalanine is obvious since normal dietary protein provides substantially greater quantities of these nutrients than added aspartame in foods and beverages.

Methanol has received some negative attention. But beverages formulated with aspartame to a sweetness level matching 10% sucrose contain the equivalent of 50 to 60 milligrams per liter of methanol, which is substantially less than the average 140 milligrams per liter content of fruit juices. Because methanol is naturally present in foods and beverages, the body metabolizes methanol and it does not accumulate in the body and thus cannot reach harmful levels. According to the American Dietetic Association (ADA) position paper on sweeteners, “A serving of nonfat milk has six times more phenylalanine and 13 times more aspartic acid than a diet soda sweetened with aspartame and an 8-ounce serving of tomato juice has four times more methanol.”2

Acesulfame
The discovery of acesulfame in 1967 by Karl Clauss and Harold Jensen was accidental, just like saccharin and aspartame. The potassium salt of acesulfame is also referred to as acesulfame-K (AceK) and is found under the brand name Sunette. The temporal profile data on AceK appears quite similar to sucrose. AceK has a flavor profile with bitter and metallic attributes that are equivalent to or stronger than those of saccharin. And just like with saccharin, it has been found that the population is heterogeneous with respect to its sensitivity to the bitter and metallic off-tastes of AceK. Interestingly, the same individuals who are sensitive to saccharin’s off-taste characteristics are also sensitive to those of AceK. AceK is generally used as a component of blends with aspartame and other sweeteners to create a taste that appeals to the maximum number of people.

AceK received its first U.S. approval for tabletop use in 1988 and received approval for all food categories in 1998. An ADI of 15 milligrams per kilogram was established.

Sucralose
In 1976, a collaborative program between Professor Leslie Hough’s laboratory at Queen Elizabeth College in the University of London and scientists at Tate & Lyle discovered that certain sucrose derivatives are sweet. One derivative of sucrose, named sucralose, is the first carbohydrate-based sweetener that is substantially more potent than the simple sugars. Sucralose, known under the brand name Splenda, is made by selectively replacing three hydrogen-oxygen groups on the sugar molecule with three chlorine atoms. Sucralose exhibits a clean, sweet taste equivalent to sucrose. No off-tastes have been observed. Temporal profile data on sucralose are not available, so it is impossible to quantitatively compare it with sucrose in this dimension. In general, however, sucralose’s sweetness appears to linger, somewhat similar to aspartame.

In 1998, the FDA granted approval for the general use of sucralose in foods and beverages. The FDA established an ADI of 5 milligrams per kilogram after review of the safety assessment studies submitted. Sucralose slowly hydrolyzes in acidic food and beverage products to the monosaccharides 4-chloro-4-deoxy-galactose and 1,6-dichloro-dideoxy-fructose. For this reason, safety studies on sucralose and the two degradation products were required. This is currently the most expensive of the alternative sweeteners.

Sugar Alcohols
Another category of sweet substances is the sugar alcohols. Xylitol, maltitol, lactitol, isomalt, and erythritol are examples. They are sometimes referred to in the food trade as Sugar MacroNutrient Substitutes (MNSs). These sweeteners are approved for use following the GRAS affirmation process rather than the Food Additive Petition process. In the United States, specific bioavailable calorie contents have been assigned to each sugar alcohol, while in the European Union, a value of 2.4 calories per gram has been assigned for all Sugar MNSs. Much controversy has developed over the bioavailable calorie issue, and agreement on an absolute number of calories per gram is unlikely.4 The generally accepted bioavailable calorie content in the United States, however, is 3 calories per gram.

A significant limitation for the use of these reduced-calorie compounds is the intestinal discomfort experienced mostly in the form of laxation and flatulence. Another consideration is that they are more expensive than other sweeteners.

Special Populations
While the use of nonnutritive sweeteners used to focus on people with diabetes, the current concern is controlling excess body weight. This concern has many looking for dietary advice on sweeteners, including children and pregnant women. The ADA states, “Taken as a whole, nutritive and FDA-approved nonnutritive sweeteners are safe for children and pregnant women.”5

Because children are small and tend to drink more fluids, they have the highest intake of nutritive and nonnutritive sweeteners when calculated by the amount of sweetener consumed per kilogram bodyweight per day. Estimated intakes of nonnutritive sweeteners in children are below the established ADI for all approved sweeteners. Estimated intake of aspartame is roughly 10% of the ADI, and it may be as high as 60% for AceK. The current trend of blending sweeteners in food products should continue to keep estimated intakes of nonnutritive sweeteners in children well below the ADI, according to the ADA position paper.

Tests on reproductive toxicity are part of the toxicology testing required for FDA approval of sweeteners. The use of aspartame, AceK, and sucralose during pregnancy is considered safe. The ADA states, “The consumption of acesulfame potassium, aspartame, saccharin, and sucralose within acceptable daily intakes is safe during pregnancy.”4

I cannot offer advice any better than that offered in the ADA position paper “Use of Nutritive and Nonnutritive Sweeteners”: “Dietetics professionals can lead the dialogue to help consumers and others in addressing the following issues of concern:

• recognize that sweeteners can add to the pleasure of eating and that these sweeteners can assist consumers in improving the quality of the diet if selected in appropriate quantities and in forms that are high in micronutrients;

• assist consumers in reading food and beverage labels to determine appropriate personal choices about consumption of nutritive and nonnutritive sweeteners; and

• facilitate the incorporation of sweeteners within the context of the total diet instead of simply examining the health benefits or risks of individual foods or beverages.”

Humans are naturally drawn to sweet tastes. Many of our clients start their day with a bit of sugar in their coffee, consume sodas and sports drinks, and celebrate holidays with sweets. Dean Bingham, professional truffle maker (www.deanssweets.com), has witnessed this strong attraction to sweets. He thoroughly enjoys watching treat eaters smile and raise their eyebrows. “That is the finest reward—to see the change in a person’s countenance,” Bingham says. “I’ve had chocolate lovers ask me to marry them, and more.”

— Carol M. Meerschaert, RD, is a freelance writer and a consultant to food and beverage companies.

References
1. Oldways Scientific consensus Statement. Available at: http://www.oldwayspt.org/index.php?area=consensus_reports.

2. DuBois GE. Francis FJ, Ed. “Sweeteners: Nonnutritive.” Encyclopedia of Food Science & Technology. New York: John Wiley & Sons, Inc., 2000.

3. “Experts Look to the Future of Sweetness” lecture at Oldways Preservation and Exchange Trust “Managing Sweetness” conference. October 22, 2004.

4. Sicard PJ, Le Bot Y. Manufacturing Opportunities With Non-Sugar Sweeteners — 1994 Royal Society of Chemistry.

5. Position of the American Dietetic Association: Use of nutritive and nonnutritive sweeteners. J Am Diet Assoc. 2004;104:255-275.