A controversial nutritional test of a chemically modified fat suggests that the substance is more harmful, in at least some respects, than are the partially hydrogenated vegetable oils that it’s intended to replace.
Many food producers are phasing out partially hydrogenated oils, which contain trans fats, substances that have been linked to heart disease. For certain products, such as baker’s shortening and margarine, some companies are turning to interesterified fats.
Interesterification shuffles the fatty acids that make up each fat molecule (see “How Interesterification Works,” in this week’s Food for Thought at Science News Online). Like partial hydrogenation, interesterification produces molecules that seldom or never appear in nature.
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The new study reports worrisome changes in blood-glucose and cholesterol concentrations in 30 volunteers in Kuala Lumpur, Malaysia, who had consumed a diet containing large amounts of interesterified fat.
But scientists who weren’t involved in the study criticize it for comparing forms of fat that they say can’t be used as direct substitutes for one another.
In the study, nutritional pathologist K.C. Hayes of Brandeis University in Waltham, Mass., and two Malaysian researchers fed volunteers three diets during different 4-week periods. Each diet used a different source of fat, either a trans fat–rich, partially hydrogenated soybean oil; an interesterified soybean oil; or a natural substance, palm oil. A palm oil–industry group funded the study.
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The trans fat and interesterified-fat diets had a more negative effect on volunteers’ beneficial, or HDL, cholesterol than the palm oil diet did.
Moreover, the interesterified fat raised blood-glucose concentrations and slowed the metabolism of glucose relative to the effects of either of the other fats. Both those changes are associated with increased diabetes risk.
The study appeared Jan. 15 in the online journal Nutrition & Metabolism.
Nutrition scientist Brent Flickinger says that the study didn’t make an “apples-to-apples” comparison. His employer, Archer Daniels Midland Co. of Decatur, Ill., sells interesterified fats and other oils.
The interesterified fat used in the experiment is 59 percent saturated and “as hard as candle wax,” Flickinger says. His company’s interesterified products are more malleable.
In past studies, Flickinger says, interesterified fats have had fewer worrisome effects on cholesterol than partially hydrogenated oils have.
“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,” says Walter Willett of the Harvard School of Public Health in Boston. Furthermore, those diets—especially the interesterified diet—included unusually high amounts of saturated fat.
The experiment, he says, should be “replicated with realistic [interesterified] products at realistic intakes.”