Olestra was first patented in 1971 by Procter and Gamble, which proposed to use it as a cholesterol-lowering drug. However, it was not potent enough for this purpose and the company later developed it as a calorie-free, fat-free substitute for shortening and cooking oils.

After expenditure of approximately $200 million and twenty-five years in research and development, olestra was approved in January 1996 for use as a replacement for up to 100% of the conventional fat in savory snack foods such as potato chips and crackers. Olestra may be used as a substitute for both the fat used in frying and the fat used as an ingredient in the dough. This approval comes with certain conditions. Foods containing olestra must be supplemented with vitamins A, D, E, and K. Packages must bear a label stating that olestra may cause abdominal cramping and loose stools and that it inhibits the absorption of some vitamins and nutrients. Procter and Gamble is required to monitor consumption and long-term effects.

Structurally, olestra has some similarity to ordinary fats, and this accounts for its ability to mimic the texture, flavor, and "mouth-feel" of fat. Triacylglycerols in plant and animal lipids consist of three fatty acids esterified to a glycerol molecule. In olestra, the glycerol is replaced by sucrose; and fatty acids of carbon chain length C8 to C22 are attached to between 6 and 8 of the sucrose's 8 available hydroxyl groups. The fatty acids are obtained from edible fats and oils (soybean, cottonseed, corn, and coconut). The natural triacylglycerols in these plant lipids are first esterified with methanol. Then the methyl groups are replaced with sucrose to produce a crude mixture of sucrose polyesters. Further refinement removes free fatty acids, colored compounds, flavors, and volatiles. The physical propertiesappearance, lubricity, mouth-feel, and flavor developmentof olestra preparations resemble those of corresponding triacylglycerols. By altering the chain length and degree of unsaturation of the fatty acids, olestra can be formulated for specific purposes such as baking and frying, and for specific products such as bread spreads. Although olestra has been approved only for use in snack foods, it has the potential for a wide range of uses.

Olestra, shown on right, is much bulkier than a typical triacylglycerol, shown on the left. Olestra's large size does not allow it to fit into active sites of hydrolytic enzymes and it passes through the body without being metabolized.

Like ordinary fats, olestra is stable when stored at ambient temperatures and is not degraded when heated to frying and baking temperatures. However, as with conventional fats, exposure to oxygen and temperatures above 50 °C can cause oxidation of unsaturated fatty acids, with the formation of hydroperoxides and development of rancidity. Olestra is stable in weak acids, but the sucrose polyesters can be hydrolyzed by strong acids and by exposure to moisture at high temperatures (230 °C). They are also saponified by alkali.

Despite these similarities to conventional fats, olestra cannot be digested by the enzymes in the human gastrointestinal system. Ordinary fats are hydrolyzed by pancreatic lipase to yield free fatty acids and monoacylglycerols, which are absorbed through the intestinal cell wall. However, with so many fatty acids attached to the sucrose molecules in olestra, this enzyme is unable to hydrolyze the fatty acids during their transit time in the digestive tract. Since olestra molecules are not hydrolyzed and are too large to be absorbed intact, they pass through the body unchanged and therefore provide no calories.

Lest you be concerned that olestra is so recalcitrant to biodegradation that we will be polluting our environment with discarded potato chips, certain microorganisms found in soils and sewage sludges are capable of utilizing olestra as a sole carbon and energy source. Experiments using olestra labeled with 14C in the sucrose and fatty acid moieties revealed that the ubiquitous Pseudomonas aeruginosa extensively degraded olestra during an 8-day incubation; and after 69 days, virtually all of the sucrose label and roughly three-quarters of the fatty-acid label was mineralized to CO2 (Biodegradation 1996, 7[3], 257).

If olestra looks like a fat and acts like a fat, what's wrong with it? One problem is that, like ordinary fats, it absorbs the fat-soluble vitamins A, D, E, and K and also carotenoids. In fact, one critic has charged that olestra is the first approved food additive with negative nutritional value. Because olestra passes though the digestive tract unchanged, it acts as a sponge, soaking up these vitamins and carrying them out also: hence the requirement for adding the fat-soluble vitamins to olestra-containing foods. There are two related points of contention. Supplementation with carotenoids, important fat-soluble nutrients that may aid in preventing cancer, has not been mandated. Also, there is concern that supplementation with vitamin K, an important factor in blood clotting, may be detrimental to the more than one million persons who receive the blood-thinning drug warfarin.

Some people who eat olestra-containing foods report abdominal cramping and diarrhea. The percentage of the population that experiences these symptoms is controversial. Procter and Gamble reported that in a 5-month trial with 3,357 subjects, some of whom suffered with gastrointestinal disturbances such as ulcerative colitis, 2% reported some kind of gastrointestinal disturbance both with and without olestra. Critics contend that such data are insufficient. They are also concerned that more widespread use of olestra in a broader range of foods in the future will increase the occurrence of such symptoms.

In early trials, some subjects experienced anal leakage of olestra. The undigested olestra passed through the digestive tract and arrived at the anal sphincter as a liquid mass. The sphincter muscle could not contain this mass and some leakage occurred. By increasing the proportion of longer-chain and more highly saturated fatty acids, olestra was reformulated to remain stiffer at body temperatures, and this problem appears to have been corrected.

Long-term studies with laboratory rats and mice have not indicated any increase in cancer or other adverse health effects when diets were supplemented with olestra at levels up to 100 times (on a weight % basis) the amount ingested by people who eat olestra snacks (see, for example, Food Chem. Toxicol. 1994, 32, 789).

Another important question remains: Will olestra be effective in aiding weight loss or preventing obesity? One possibility is that the human body will detect the decreased caloric intake on an olestra-supplemented diet and stimulate compensatation by overeating other high-calorie foods at later meals. In one set of experiments testing this scenario, subjects were fed a control meal containing 73.2 g of fat (53 wt %) or a similar olestra-supplemented meal containing 24 g of fat (27 wt %) (Br. J. Nutr. 1996, 75, 545). This represented a decrease in fat-energy content from 43% in the regular meal to 28% in the olestra-supplemented meal and was realistic in terms of common Western diets. During the remainder of the first day and the next day, subjects did not appear to compensate for this dietary change. However, when diets were restricted more severely, from a fat-energy content of 32% to one of 20%, subjects did rate themselves as being more hungry and they compensated for 74% of the energy (fat) deficit while eating during the second day (Am. J. Clin. Nutr. 1996, 63, 891). These short-term studies suggest that olestra may be useful in controlling dietary fat intake only if the caloric restriction is not too great.

Olestra is but one of a large number of fat substitutes that are in various stages of development, testing, and marketing. These products have a broad range of functional and physiologic properties, potential applications, advantages, and problems. In many cases the most satisfactory result is achieved with a mixture of two or more components. Applications include meat products, bakery products, and frozen desserts. Some of the many types of fat substitutes are listed in the table. This information was derived from Chem. Ind 1996, 13, 494, which with the references it includes, provides a rich source of backgroung material.

Olestra may help to combat the pervasive problem of overweight in the U.S.A., but there is still much to be learned about its long-term effects on people, particularly in the youngest and oldest age groups. The FDA plans to formally review and evaluate all data on consumption of olestra and reported adverse effects within 30 months of its approval, to determine whether there continues to be reasonable certainty regarding its decision on the olestra's safety.

Good general articles on olestra include J. Chem. Educ. 1991, 68, 476; Chem. Ind. 1996, 13, 494; Newsweek Jan. 8, 1996, p 60; The Wall Street Journal July 31, 1996, p B1; and Frito-Lay's Web site (found at URL: http://fritolay.com/olestra.html). The FDA's role in the olestra story has been covered extensively. See, for example, C&EN Nov. 20, 1995, p 11, and May 13, 1996, p 25; and an opinion piece denouncing the FDA (New Engl. J. Med. 1996, 334, 984), which was followed by a group of informative letters in response to the opinion piece (New Engl. J. Med. 1996, 335, 668). Britain's Institute of Food Science and Technology (home page URL: http://www.easynet.co.uk/ifst/) has an informative report on olestra (URL: http://www.easynet.co.uk/ifst/hottop13.htm) that was available at press time.