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Trans fats: “natural” might not mean “healthy”

In the nutrition community, a common message has been that artificial trans fats are bad, however natural trans fats are not only okay but beneficial. This trial on blood lipids puts that to the test.

Study under review: Vaccenic acid and trans fatty acid isomers from partially hydrogenated oil both adversely affect LDL cholesterol: a doubleblind, randomized controlled trial

Introduction

Fatty acids are broadly classified as saturated, monounsaturated, or polyunsaturated based on how many double-bonds exist within the fatty acid tail (zero, one, and more than one, respectively). Unsaturated fats most commonly come in the cis- isoform, meaning that the double-bond creates a kink in the tail that causes bending. These kinks prevent the fatty acids from gathering close to one another, which lowers the fat’s melting point and explains why unsaturated fats are oils at room temperature.

In contrast, a trans- configuration causes the tail to remain straight, mimicking the structure of a saturated fat. These trans-fatty acids (TFAs) exist in nature alongside their cis- counterparts, but in far lower quantities. The most common naturally occurring TFA in the human diet is vaccenic acid, which is produced via microbial fermentation of plant matter in the stomachs of ruminant animals, and subsequently obtained in our diets through consuming the meat, fat, and milk of these animals. Additionally, vaccenic acid can be further metabolized into cis-9, trans-11 conjugated linoleic acid (c-9, t-11 CLA) within the ruminant stomach.

Although humans have been consuming ruminant TFAs throughout evolutionary history, they made up an incredibly small portion of the diet and are not the TFAs that come to mind when the term is used today. Rather, TFAs have become synonymous with partially hydrogenated oils. These industrial trans-fatty acids (iTFAs) are produced when unsaturated oil is reacted with hydrogen in the presence of a chemical catalyst (usually powdered nickel). This technique appeared within the last century, with the first marketed iTFA being a partially hydrogenated cottonseed oil called Crisco that appeared on American shelves in 1911.

The primary iTFA is elaidic acid, which is essentially oleic acid with the double-bond in a trans- rather than cis- configuration, meaning the kink in the chain is removed. Vaccenic acid (whose structure is shown in Figure 1) also has 18 carbons like oleic acid and elaidic acid, but its trans- double-bond is farther down the chain. CLA is most similar to linoleic acid as both have two double-bonds; however, CLA has its second double-bond in the trans- configuration whereas linoleic acid has two cis- double-bonds.

Figure 1: Some relevant 18-carbon fatty acids

The health effects of iTFAs are well documented, to the point that most major health authorities recommend limiting intake to as little as possible, preferably less than 1% of energy intake. The FDA has even revoked their Generally Recognized as Safe status and required that they be removed from the American food supply over the next three years. However, the FDA defines the TFA term based on chemical structure rather than metabolic or functional aspects. As such, the origin of the TFA does not matter for the FDA definition, and they must be labelled when falling under their definition of all unsaturated fatty acids that contain one or more isolated double bonds in a trans configuration.”

Based on the FDA’s definition, vaccenic acid but not CLA must be included in the label for TFAs because CLA contains conjugated double bonds (meaning that there are two double-bonds separated by one single-bond) in the trans- configuration. But the health effects of these fatty acids are far less investigated than those of partially hydrogenated oils. Therefore, the current study evaluated the effects of iTFAs and ruminant TFAs on blood lipid risk markers for cardiovascular diseases (CVD) in the context of a highly controlled dietary intervention.

The detrimental effect of partially hydrogenated oils and their trans-fatty acid component on cardiovascular disease risk is well-established. However, the effects of naturally occurring trans-fatty acids such as vaccenic acid and conjugated linoleic acid are far less investigated. The current study aimed to compare iTFA and TFAs from ruminants in a highly controlled dietary intervention study.

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