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Although consumers have beenbombarded with the messageto reduce or avoid dietary satu-

rated fatly acids, not all saturated fattyacids are created equal. Stearic acid,studies show, stands out from thenorm.

"Stearic acid is anomalous. It's asaturated fatty acid but doesn't act likeone as far as serum cholesterol is con-cerned," said David Kritchevsky of theWistar institute, Philadelphia, Penn-sylvania.

Saturated fatty acids with 16 orfewer carbon atoms raise serumcholesterol levels. Kritchevsky andothers, however, stress that there isscientific agreement that dietarystearic acid-an 18-carbon-atom satu-rated fat-does nOI do so.

"The question remaining is, 'Whydoesn't it?" Kritchevsky said.

He, for one, thinks it may bebecause of stearic acid's relatively

high melting point. In rat feedingstudies with various fatty acids,Kritchevsky and Patrick Tso, forinstance, found that after four hours offeeding, stearic acid was still in theintestine even though other fats hadcleared. "Stearic acid moves moreslowly," Kritchevsky said.

BackgroundObservations that stearic acid affectsserum cholesterol levels differentlythan other saturated fats first appearedin the 1960s. In separate research,O.M. Hegsted and Ancel Keys showedthat stearic acid did not significantlycontribute 10 plasma cholesterol instudies concerning dietary fatty acidsand coronary heart disease. -II

Kritchevsky and colleagues alsonoted that in rabbits fed different natu-rally occurring fats, cocoa butter wasmuch less atherogenic than one wouldexpect from its level of saturation.Subsequent human studies in 1988 byA. Bonanome and S.M. Grundy and in1991 by Grundy and Margo Denke atthe Center for Human Nutrition, Uni-versity of Texas Southwestern Medi-cal Center at Dallas, helped to estab-lish that dietary stearic acid does not

This article 011dietary stearicacid research was written byBarbara Fitch Heumann, senioreditor/writer for INFORM.

raise serum cholesterol levels inhumans.

Controlled feeding studies con-ducted at Pennsylvania State Universi-ty concerning cocoa buller and choco-late also have contributed to theunderstanding of the effects of stearicacid on blood lipids. These, too, haveproduced results showing that stearicacid does not raise plasma cholesterol.

As a result, researchers generallyclassify stearic acid as neutral con-cerning serum cholesterol, with itseffects seen as being intermediate tosaturated fatty acids that are hyper-cholesterolemic and to mono- orpolyunsaturated fatty acids, whichusually are hypocholesterolemic.

Because stearic acid was not seenas either causing or preventing heartdisease, however, "It was ignored formany years," according to Gary Nel-son, research chemist at the U.S.Department of Agriculture (USDA)Western Human Nutrition Center(WHNC), San Francisco, California.

Despite general agreement aboutstearic acid's effect 011 serum choles-terol, findings from a longer-termstudy at the WHNC, published in theMay 1995 issue of The America"

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Journal of Clinical Nutrition, seemedto raise some additional questions.

Most clinical studies with individu-al fatty acids have been conducted fortwo to four weeks. The WHNCstudy=-conducted for 40 days- ini-tially showed some suppression ofboth low-density lipoprotein (LDL)and total serum cholesterol with a diethigh in stearic acid content, but bothlater rose, although not to the originallevels. according to USDA researchersJames Iacono and Rita Dougherty,now both retired.

In the study, ten healthy male sub-jects were fed either a high stearicacid diet provided from shea butter ora semi-high palmitic acid diet. Theshea butter, which is a hard, waxy fat,was incorporated in breads. cakes, bis-cuits and dessert items fed to the sub-jects. Shea butter is a high-steariccompound used in "white chocolate"candy bars, Iacono explained.

"Our results might suggest thatperhaps the studies for stearic acidhave been too short to show its fulleffect on blood cholesterol over time,"Iacono said.

In contrast, Penny M. Kris-Ether-ton, professor of nutrition at Pennsyl-vania Stale University. and Thomas A.Pearson. now chief of Community andPreventive Medicine. University ofRochester in New York, conducted apilot study which showed stearicacid's effect on cholesterol remainedflat after the first two weeks.

Possible mechanismsMeanwhile. others have undertakenfurther research chiefly to understandwhy stearic acid acts as it does.

''The key question is, why doesn'tit raise serum cholesterol levels? Themechanism is not completely under-stood," Kris-Ethertcn said.

One hypothesis was that stearicacid is poorly absorbed by the bodyand thus is excreted. In the 1950s.Fred Mattson showed that tristearinwas not well absorbed, but tngtyc-erides in which stearic acid represent-ed one or two of the fatty acids werewell-absorbed. In their 1991 study,Denke and Grundy found that stearicacid absorption was relatively high,Iacono and colleagues subsequentlyshowed absorption for major dietary

In humans, oleic acid absorptionis about 97-98%,

stearic acid absorption is about 90%,and that of palmitic acid

falls somewhere in between.

fatty acids, except stearic acid, was95% or greater. whereas in a highstearic acid diet, stearic acid's absorp-tion averaged approximately 90%.

In fact, studies generally haveshown that more than 90% of stearic

acid is absorbed, according to areview wriuen by Vikkie A. Mustadand Kris-Etherton published in Topicsif! Nutrition and Food Safel)" Issue 4,from Hershey Foods Corp.

Edward A. Emken, recently retiredfrom the USDA National Center forAgricultural Utilization Research andwho is now operating MidwestResearch Consultants in Princeville,Illinois, notes that in humans, oleicacid absorption is about 97-98%,stearic acid absorption is about 90%,and that of palmitic acid falls some-where in between.

A recent study completed at USDAAgricultural Research Service'sBeltsville Human Nutrition ResearchCenter (BHNRC) has measured thedigestibility of stearic acid and otherfatty acids in humans fed mixed diets.Study collaborators included David J.Baer and Joseph T. Judd of BHNRC'sDiet and Human Performance Labora-tory, Kris-Ethertcn, and Emken. Find-ings from the study are to be present-ed at the Experimental Biology '98meeting to be held April 18-22 in SanFrancisco.

'The Beltsville data are fairly con-sistent with Grundy's work with liquiddiets, and our studies with stable iso-tope-labeled fats. These studies allshow that if you add stearic acid tonormal diets, it is well-absorbed bypeople. These results are in contrast toresults generated over the years con-cerning absorption of stearic acid inanimal models," Emken said.

In fact. he noted. "Animal data

show stearic acid is not well-absorbed,and studies with people show it iswell-absorbed. My guess is that boththe human and rat data are accurate.This is one example where we havesome metabolic evidence to show that

men really aren't rats-contrary towhat some wives might think abouttheir husbands."

Another hypothesis has been thatstearic acid is converted rapidly intooleic acid in the body. Rabbit studiesby William E. Connor of OregonHealth Sciences University, Portland,Oregon. have shown that a substantialamount of stearic acid converts tooleic acid. However. Emken's datafrom stable-isotope tracer studies sug-gest this is not true in humans.

"Our data, and that of others, showthat only 10 ± 5% of the labeledstearic acid was converted to oleicacid, which is not a huge amount;'Emken said.

Other metabolic aspects are beingexplored.

John M. Dietschy of the Depart-ment of Internal Medicine, Universityof Texas Southwestern Medical Centerat Dallas, for instance. has conductedanimal studies using synthetic triacyl-glycerols that contain only one fattyacid to study the effects of dietary fattyacids on regulating LDL receptoractivity and the resulting plasma LDLcholesterol concentrations.

The liver is the site of very lowdensity lipoprotein (VLDL) choles-terol biosynthesis, Dietschy noted. Aportion of the VLDL cholesterol cir-culating in plasma is converted toLDL cholesterol, and this lipoprotein.in tum, is cleared from the plasma byLDL receptors. located primarily inthe liver. Dietary cholesterol will sup-

(contil/lled 011 page 205)

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[continued from page 203)

press LDL receptor activity andincrease the amount of VLDL beingproduced, thus increasing the amountof LOL circulating in the plasma.Although fatty acids modify this regu-latory process, they have little effectin the absence of dietary cholesterol.

Dietschy and colleagues haveshown that lauric, myristic. andpalmitic acids, in the presence ofdietary cholesterol, decrease LDL-receptor activity and thus increase cir-culating plasma LOL-cholesterol con-centrations. Oleic and linoleic fattyacids increase receptor activity, result-ing in lower plasma LOL-cholesterolconcentrations. By comparison,caproic. caprylic. capric, stearic, and/ral/s fatty acids do not alter relativereceptor activity or LDL-cholesterolproduction. and thus have a neutraleffect on circulating LDL-cholesterol.

"The liver becomes enriched with18:0 and yet it does not shut off recep-

tors. but we really don't know why:'Dierschy said, noting his studies havefocused on genetically altered mice.

Human data thus far are confusing.Dietschy said, adding. "A major techni-cal problem is that it is difficult to feedstearic acid alone since it has a veryhigh melting point. As a result. it is usu-ally necessary to feed this fatty acidwith other long-chain compounds, andthis makes it difficult to elucidate thespecific effect of the stearic acid itself."

Also. he noted. comparisons inhuman studies are usually canied outby substituting one fatty acid foranother. rather than by substituting anactive triacylgtycerol for a fat that isconsidered to be biologically neutral.Also, most human studies only lastabout four to eight weeks. which maybe too short for definitive results.

Meanwhile. Yu-Van Yeh, professorof nutrition in the Department ofNutrition at Pennsylvania State Uni-versity and doctoral student Tonkun

Pai have studied fatty acid effects oncholesterol levels in rat-liver cells.

In their studies. they treated cul-tured rat hepatocytes with variousfany acids-palmitic. myristic, oleic.linoleic. and stearic. Cells treated withstearic acid produced much lower lev-els of triacylglycerol than did thosetreated with the other fatty acids. This.in turn. stymied the cells' ability toproduce and secrete VLDL, the pre-cursor of LDL.

"Inhibiting VLDL formation inliver cel1s means the cells wouldrelease less of it from the liver into theblood. We speculate that this couldlead to a decrease in LDL production.This suggests that this could be one ofthe mechanisms for stearic acid to notraise plasma LDL cholesterol." Yehsaid. adding. however, that he believesthe effects of stearic acid are not dueto a single action but rather to concert-ed mechanisms.

[continued 011 page 207)

Facts about stearic acidStearic acid, an l8-carbon fatty acid (CtSHJ(j02), is a sat-urated fatty acid. It is a primary fatty acid in lard, beeftallow, and cocoa butter. The predominant food sourcesare dairy products, meat, and processed grain products. Itis also found in shea butter, produced from the nuts of anAfrican plant, and babassu oil, a kernel oilfrom the Central and South Americanbabassu palm.

BUller fat is composedroughly of 13% stearic acid.53% other saturates, 31 %rnonounsaturatcs and 4%polyunsaturares. Beef fat andcocoa butter are composed of 22% and35% stearic acid, respectively.

Stearic acid contributes approximately 3% of totalenergy and 27% of total saturated fatty acid in the diet.according to a review by Vikkie A. Mustad and Penny M.Kris-Etherton published in Topics ill Nutrition and FoodSafer),. Issue 4 from Hershey Foods Corp.

a major dietary concern."Not too many people are wonied about it:' according

to David Kritchevsky of the Wistar Institute.Gary Nelson of the U.S. Department of Agriculture's

Western Human Nutrition Center agreed. "Certainty themechanism of stearic acid's effect onf".__ serum cholesterol is an open

question. From an academicpoint of view, I'd like to knowmore, but from a health point of

view, it isn't a major concern."Margo Denke, associate pro-

fessor of internal medicine at the Univer-sity of Texas Southwestern Center for Human

Nutrition, sees no practical reason for expanding stearicacid's use in the diet.

"Stearic acid will be limited in the food supply becausefats rich in stearic acid have a mouthfeel like candle wax,"Denke said. Experiments attempting to incorporate it in anartificial fat have not been promising. "It doesn't taste goodand won't succeed as an artificial fat," she said.

The important point of research findings, she noted, isthat it means beef fat is no more cholesterolemic thanchicken fat-both contain approximately 25-30% of sat-urated fat as stearic acid (Table I)-and that cocoa butlercan be incorporated into the diet, according to Denke.

(continued 011 page 207)

Dietary significanceA typical U.S. diet containing 80 grams of total fat perday includes about 11 grams of stearic acid, apprcxi-mately 14% of total fat in a typical diet.

Some researchers, however. do not see stearic acid as

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throughout the testing period. The testmeals consisted of liquid Ensure con-taining a labeled fatty acid.Researchers then measured the sub-jects' breath for He-labeled expiredCO2, Fatty acids tested were lauric.palmitic. stearic, myristic, oleic.elaidic, linoleic, and linolenic. withsubjects fed two fatty acids a week.

Findings revealed oxidation for thesaturated fats was inversely related tochain length, with lauric and myristicacids the most oxidized. These werefollowed by linolenic, elaidic, oleic.linoleic. palmitic, and finally stearicacid, Ihe least oxidized.

"Over nine hours after eatingstearic acid, the subjects showed thatvery little had been used for energy, sowe surmise it must have been used forstorage," DeLany said.

The palmitic acid connectionConnor, meanwhile, points out adilemma concerning stearic acid: inthe diet, stearic acid is not a purifiedsubstance but is always accompaniedby palmitic acid.

(COil/billed/rom page 205)In addition, Yeh and Pai in one

study observed that VLDL particlessecreted by cells treated with stearicacid were almost twice as large as par-ticles from cells treated with palmiticor myristic acid. The larger VLDLparticles are removed more rapidlyfrom circulating blood than smallerVLDL particles, Yeh said, explaining,"If you remove VLDL faster from cir-culating plasma. there is less VLDLthat will be converted to LDL"

Yeh next plans to undertake animalfeeding studies, perhaps with rats,hamsters, or guinea pigs.

In other work. Jim Delany, associ-ate professor at the PenningtonBiomedical Research Center/Louisiana State University in BatonRouge, has conducted research on theoxidation of individual fatly acidsusing stable isotopes. To do so. he fedDC-labeled fatty acids to 12 sub-jects-four on a high-fat diet andeight on a low-fat diet. The subjectswere fed, under controlled conditions,either the high-fat or the low-fat diet

"The problem with stearic acid isthe company it keeps," Connor said,noting that, from a practical point ofview, the presence of palmitic acidoffsets any neutral effect stearic acidmay have on serum cholesterol. "Ourhuman studies using cocoa butter haveshown plasma levels going up com-pared to unsaturated fatty acidsbecause of the palmitic acid content,'Connor said, adding, "The only time itwouldn't be present with palmitic acidis in a vegetable oil that is completelyhydrogenated, and this would beunpalatable, solid, and impractical touse in food:'

Pearson noted that palatable andtasty products can be designed to bal-ance the amount of stearic andpalmitic acid content (Table I). Forinstance, although milk chocolate con-tains substantial amounts of palmiticacid, dark chocolate contains mostlystearic, as opposed to palmitic, acid.

ThrombosisAnother unresolved question concern-ing stearic acid centers on thrombosis.

[continuedfrom page 205)

Labeling questionCocoa butter interests have been pursuing a possiblechange in nutrition labeling to reflect the differencebetween stearic acid and other saturated fatty acids.

Currently, the saturated fat value on nutrition labelsincludes stearic acid grouped with other saturated fats.In April 1996, the American Cocoa Research Insti-tute-the research arm of the Chocolate ManufacturersAssociation based in McLean, Virginia-filed a citi-zens' petition with the U.S. Food and Drug Adminis-trutton (FDA) asking the agency to exclude stearic acidfrom the saturated fat declaration on nutritional label-ing because of scientific evidence that stearic acid doesnot affect blood cholesterol levels.

"We have no formal response yet, but we're stillinterested in pursuing this, and hopefully there will bea successful conclusion;' Carol Knight, vice presidentfor scientific and regulatory affairs at the ChocolateManufacturers Association and National ConfectionersAssociation, said.

FDA spokeswoman Judy Foulke confirmed thatFDA has not yet responded to the petition. and said itis premature to say what the agency's response will beon the issue.

The USDA Food and Safety Inspection Service,however. does permit the voluntary declaration of

stearic acid content in the nutrition facts panel on meatand poultry products.

Those arguing for deleting stearic acid from the def-inition note that some foods, like chocolate products,contain a high proportion of stearic acid (Table I).Thus, they argue, the saturated fat listed on food labelsoveremphasizes its potential to raise blood cholesterol."Separate consideration of stearic acid on the labelwould more accurately reflect the potential of a food toaffect blood cholesterol. It also would provide moreflexibility for planning diets to reduce the risk for heartdisease," reads an informational pamphlet, "DietarySaturated Fat: Stearic Acid Is Unique," published byHershey Foods Corporation in September 1995.

A monograph covering what currently is known aboutstearic acid was edited by Thomas A. Pearson and pub-lished as a supplement to the December 1994 issue ofThe American Journal of Clinical Nutrition. EntitledStearic Acid: A Unique Saturated Fatty Acid, it summa-rizes the proceedings of a symposium on "MetabolicConsequences of Stearic Acid Relative to Other Long-Chain Fatty Acids" held in November 1993 and a sympo-sium on "Chocolate in Perspective: Cocoa Butter, AUnique Saturated Fat" held during February 1994. Forfurther information about the monograph, contact TheAmerican Society for Clinical Nutrition lnc.. 9650Rockville Pike, Bethesda. MD 208814-3998.

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Table 1Saturated fat (SF) and stearic acid (SA) in selected foods

Food Amount Total SF (g) SA (g) SAas%oftoter SF

Butter I pat 2.5 0.5 20American cheese I oz 4.4 0.8 ISHamburger (lean, raw) 40' 9.4 2.8 30Chicken breast (raw) 112 breast 0.4 0.1 25Milk chocolate bar 1.550z 8.7 4.4 50Semi-sweet chocolate bar 1.450z 8.3 4.8 58Peanut butter cup candy bar 1.6oz 5.5 2.5 45

Source: "[);ewy Salurnlffi Fat: S~aric Acid Is Unique," Topic~ in Nutrition and Food StJf~ry. H.....hey Foods Corp.. He""'-y. Pellft'lyl,;mia. SqM..lnber I99S.

Some research findings suggest thatstearic acid adversely affects plateletfunction and hemostasis (blood clot-ting) and thus is potentially thrombo-genic.

Connor is among those researcherswho believe dietary stearic acid resultsin postprandial particles that have athrombogenic effect. His studies. how-ever, have centered on injecting stearicacid into animals.

Kris-Etherton pointed out thatother findings linking stearic acid toblood clot promotion are based onexperiments in which researchers col-lected blood samples, placed them in apetri dish, and then added stearic orother fatty acid.

"However, some researchers dis-miss this as nonphysiological evi-dence, and question what it means,"she said.

"Our studies along with those of

Thomas Pearson have shown thatwhen humans are fed high levels ofstearic acid, there are no adverseaffects on platelet function. Yet, whensubjects are fed butter, there is anincrease in the excretion of metabo-lites of eicosanoids that promoteplatelet aggregation. thus indicatingincreased thrombogenesis," she said.

"It is important that evidence fromcontrolled feeding studies doesn't sup-port the findings from studies inwhich animals were intravenouslyinjected with stearic acid;' Kris-Ether-ton added.

More recently, investigators inEurope have shown that high levels ofstearic acid had no effects on hemo-static factors (Factor VIr) involved inblood clot formation.

In conjunction with the study byIacono mentioned earlier, NorbertaSchoene of USDA's BHNRC analyzed

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the effects of dietary stearic andpalmitic acids on platelet activity sta-tus in whole-blood samples. Shefound that palmitic and stearic acidsaffect a platelet response parameterdifferently, with stearic acid resultingin inactivated disc-shaped platelets,and palmitic acid resulting in moreactive spherical-shaped platelets.Schoene presented these findings atthe Third lruemanonul Congress onEssential Fatty Acids andEicosanoids, held in 1992 in Adelaide,Australia, and summarized in a pro-ceedings published by the AmericanOil Chemists' Society in 1993.

Kris-Etherton and Pearson current-ly are examining data from existingstudies to develop predictive equationsto determine the effects of stearic andother fatty acids on hemostatic factors.They expect to have results from thisstudy available in a year. •

,

IalystJr 'I.;

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