By Ben Harder

Web edition: February 8, 2007

A controversial nutritional test of a chemically modified fat suggests that the substance may be more harmful, at least in its potential to cause diabetes, than is a trans fat–rich, partially hydrogenated vegetable oil.

Many food producers are phasing out partially hydrogenated oils, which contain harmful trans fats. For certain products such as baker's shortening and margarine, some companies are turning to fats in the class called interesterified fats.

Interesterification shuffles the fatty acids that make up each fat molecule (see "How Interesterification Works"). Like partial hydrogenation, which generates unnatural trans fats, interesterification produces some molecules that are rare or nonexistent in nature.

The new study reports worrisome changes in blood glucose and cholesterol concentrations in 30 volunteers in Kuala Lumpur, Malaysia, who had consumed an interesterified-fat–heavy diet.

"In one month, the blood glucose was raised from a normal level to prediabetic," says study leader K.C. Hayes, a nutritional pathologist at Brandeis University in Waltham, Mass.

But scientists who weren't involved in the study criticize it for comparing forms of fat that they say aren't interchangeable.

In the study, Hayes and two Malaysian colleagues fed volunteers three separate diets during different 4-week periods.

The diets varied in their source of fat. One used a trans fat–rich, partially hydrogenated soybean oil, and another contained a saturated fat–rich, interesterified soybean oil. The third used palm oil. All three researchers have financial ties to the Malaysian palm oil–industry group that funded the study, and the volunteers were employees of the organization.

The interesterified-fat and trans fat diets reduced volunteers' beneficial, or HDL, cholesterol relative to the effect of the palm oil diet. Trans fat, but not interesterified fat, significantly raised harmful, LDL cholesterol.

"The interesterified fat behaved somewhat like trans in its effect on LDL and HDL," Hayes says.

Moreover, the interesterified fat raised blood-glucose concentrations more than either of the other fats did, and it slowed the metabolism of glucose. Both those changes are associated with increased risk of diabetes.

The study appeared on Jan. 15 in the online journal Nutrition & Metabolism.

Interesterified fats "are being marketed big-time around the world as a replacement [for trans fats]," Hayes says. "We made a big mistake with trans 40 or 50 years ago. Let's not make the same mistake now."

Nutrition scientist Brent Flickinger says that the study didn't compare "apples to apples." His employer, food giant Archer Daniels Midland Co. of Decatur, Ill., sells interesterified fats and other oils.

Flickinger points out that the interesterified fat used in the experiment is 59 percent saturated and "as hard as candle wax." His company's interesterified products are more malleable because a smaller proportion of their fatty acid content is saturated. That consistency makes them more appropriate substitutes for most partially hydrogenated oils, he notes.

Michael Jacobson of the Center for Science in the Public Interest in Washington, D.C., says the study's interesterified fat had a "bizarrely high" proportion of saturated fatty acids. "The kind of interesterified oil used in this study is not what would replace partially hydrogenated oil," he says. "It looks to me like the palm oil industry was looking for a way to put a potential competitor in a bad light [by using] unrealistic conditions."

In past studies, Flickinger says, interesterified fats have generally had more healthful effects on cholesterol than have partially hydrogenated oils.

The Malaysian study's partially hydrogenated oil was relatively low in trans fatty acids—10 percent—and was a "creamy, pourable oil" that couldn't be used in most recipes requiring a solid fat, Flickinger says. The palm oil was also a liquid, he notes. In terms of baking and spreading properties, therefore, the three fats "are just not the same," he says.

According to epidemiologist Walter Willett of the Harvard School of Public Health in Boston, "The fatty acid composition of the three diets was very different, so [Hayes and his colleagues] could not really distinguish the effects of interesterification from the effects of the fatty acids. In other studies, when interesterified fats were used in realistic amounts, they did not adversely affect blood lipids."

People wouldn't realistically consume the quantity of interesterified fat or the large quantity of saturated fatty acids that the trial's volunteers ingested, Willett says. The experiment, he suggests, should be "replicated with realistic [interesterified] products at realistic intakes."

Hayes agrees that the effects of interesterified fats need further investigation. "The intent was to examine the issue in the extreme," he says. "If this [study] showed no effects, then the issue could be laid to rest. Now it seems more work needs to be conducted."

"There's no general rule that [interesterifying an oil] is going to make it better or make it worse," says David Klurfeld of the U.S. Department of Agriculture in Beltsville, Md. In the 1980s, he collaborated on experiments in pigs that suggested that some interesterified oils are less healthful than their unmodified counterparts, and some are more healthful.

Nutrition researcher Mary Enig of the Weston A. Price Foundation in Washington, D.C., says she isn't "terribly worried" about the metabolic effects of "tearing triglycerides apart and putting them back together." However, she cautions, some interesterification methods use potentially hazardous chemical solvents that could end up in the final product. The new study didn't investigate that possibility.

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How Interesterification Works

A fat molecule, or triglyceride, resembles a capital 'E,' in which three fatty acids run roughly perpendicular to the molecule's glycerol backbone.

Food chemists interesterify fats by chemically or enzymatically removing fatty acids from fat molecules and transferring them to other fat molecules. Because this process recombines fatty acids randomly, chemical interesterification is sometimes called randomization.

To make a fat with new and useful properties, manufacturers typically interesterify blends of different kinds of fats. These blends often consist of a natural vegetable oil and a solid fat such as the fully hydrogenated soybean oil used in the Malaysian study. Full hydrogenation forms saturated fats rather than trans fats, which are products of partial hydrogenation.

K.C. Hayes of Brandeis University suggests that randomizing the positions of saturated fatty acids could potentially have adverse health effects. In nature, those fatty acids tend to form on one end of a fat molecule or the other. But reshuffling can put them in the middle.

"The position of the fatty acids in the triglyceride molecule might have an effect separate from the composition of the fatty acids," says David Klurfeld of the U.S. Department of Agriculture. Research suggests the health effects of interesterified fats are unpredictable, he cautions. "You're creating a new molecule, so you basically have to test it every time."