Every once in awhile I check in with the trans fats research, to see if they are living up to their bad reputation.
Fats contain long chains of carbon molecules. Each carbon molecule has positions (or bonds) at which a hydrogen can be attached. If all the potential hydrogen bonds on all the carbons are filled, the fat is saturated. Monounsaturated fats have one open bond, while polyunsaturated fats have two or more. The more open bonds, the less stable (and more liquid) a fat is.
Hydrogenation is the process of adding hydrogen molecules to an unsaturated fat’s open bonds. If you take a batch of unsaturated fats and add hydrogen molecules to much of it, it will make the batch more stable and improve some of the cooking qualities.
If you completely hydrogenate a batch of oil you end up with saturated fat. But if you only partially hydrogenate the batch, you will produce some saturated fats as well as some unsaturated fats with an unusual shape, called trans.
An unsaturated fat normally has a curved, or cis structure (unlike a saturated fat which is straight). But a trans fat is an unsaturated with a structure that is more straight.
When reading food labels, if an ingredient is called “hydrogenated oil,” then it is fully hydrogenated and will contain practically no trans fats. If an oil is listed as “partially hydrogenated” then it will contain trans fats.
The research is clear that trans fats increase LDL cholesterol and lower HDL. They also increase inflammation and harm the walls of blood vessels. The American Heart Association recommends that trans fats be less than 1% of energy intake.
In 2006 a meta-analysis by Mozaffarian et al. (1) was published in the New England Journal of Medicine. It included four prospective studies that calculated the increased risk for coronary heart disease based on substituting 2% of carbohydrate calories with trans fats:
A. Nurse’s Health Study (2005) – 33%
B. Health Professionals Follow-up Study (2005) – 26%
C. Alpha-Tocopherol Beta-Carotene Cancer Prevention Study (1997) – 14%
D. Zutphen Elderly Study (2001) – 28%
A & D were statistically significant; C & D were close to statistical significance. The combined results showed an increased risk of 23% (1.23, 1.11 – 1.37).
They also looked at three retrospective studies in which those in the top quarter of trans fat levels in their fat tissues were compared to those in the bottom quarter to measure an increased risk of heart disease:
E. EURAMIC (1995) – minus 3%
F. Costa Rica (2003) – 194%
G. Australia (2004) – 150%
Study E was not even close to statistically significant, while F and G were highly significant.
The pooled risk in all seven studies showed a 29% increase in risk (1.29, 1.11 – 1.49).
Thus, the findings have been pretty consistent (especially for nutrition studies on fatty acids). I looked for other, more recent studies such as these and did not find any.
Interestingly, the only place where trans fats are found naturally are animal products, where they are produced by bacteria in the gut of ruminant animals. In fact, a trans fats in dairy products, conjugated linoleic acid, is touted for its health benefits and sometimes pointed to by people arguing against veganism as necessary for optimal health. Here is what Mozaffarian et al. say about trans fats from ruminant animals:
[D]ietary trials indicate that consumption of conjugated linolenic* acid reduces insulin sensitivity, increases lipid peroxidation, and has mixed effects on markers of inflammation and immune function. Of four prospective studies evaluating the relation between the intake of trans fatty acids from ruminants and the risk of CHD, none identified a significant positive association, whereas three identified nonsignificant trends toward an inverse association….[T]he sum of the current evidence suggests that the public health implications of consuming trans fats from ruminant products are relatively limited.
*This appears to be a typing error and should say “linoleic” (not “linolenic”). The studies they cite are done on conjugated linoleic acid.
In the USA, a food label is allowed to list the trans fat level as zero if it contains less than .5 g per serving, and apparently many foods listed as zero have close to .5 g. So, it’s best to rely on the ingredients list and avoid foods with “partially hydrogenated oil.”
1. Mozaffarian D, Katan MB, Ascherio A, Stampfer MJ, Willett WC. N Engl J Med. 2006 Apr 13;354(15):1601-13.
Moloney F, Yeow TP, Mullen A, Nolan JJ, Roche HM. Conjugated linoleic acid supplementation, insulin sensitivity, and lipoprotein metabolism in patients with type 2 diabetes mellitus. Am J Clin Nutr. 2004 Oct;80(4):887-95.
Remig V, Franklin B, Margolis S, Kostas G, Nece T, Street JC. Trans fats in America: a review of their use, consumption, health implications, and regulation. J Am Diet Assoc. 2010 Apr;110(4):585-92.
Risérus U, Smedman A, Basu S, Vessby B. Metabolic effects of conjugated linoleic acid in humans: the Swedish experience.Am J Clin Nutr. 2004 Jun;79(6 Suppl):1146S-1148S.