How much should you worry about trans fats?

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William Saletan points to an old but still relevant Gina Kolata article on the newest nutritional scare trans fats. A little background first. A fatty acid is basically a long string of carbons attached to a carboxyl (COOH) group, like this:
  H H H H H O-H
  | | | | | |	
H-C-C-C-C-C-C=O
  | | | | |
  H H H H H

That = sign is a double bond. You don't need to know the quantum theory here, but the rule of thumb here is that each carbon (C) can form a total of four bonds, each oxygen (O) can form a total of two, and each hydrogen (H) 1. So, in this structure, the right-hand C and the singleton O attached to it need to be double-bonded in order to make the bond count come out right. If we remove one H each from a pair of Cs, we can form a double bond between a pair of carbons too, like so:

  H H     H O-H
  | |     | |	
H-C-C-C=C-C-C=O
  | | | | |
  H H H H H

Fats with all single bonds (every available position taken up with a hydrogen) are called saturated. Fats with double bonds are called unsaturated and fats with more than one double bond are called, you guessed it, polyunsaturated. Saturated fats tend to be more solid and unsaturated fats tend to be more liquid. Also, they tend to spoil faster. For both these reasons, food manufacturers like to hydrogenate them: add hydrogens to some of the C-C double bonds (that's what e.g., "partially hydrogenated soybean oil" means. The "partially means that it's still partially unsaturated).

Now, the other thing you need to know here is that single bonds rotate freely. So, for instance, the carboxyl group (the COOH, remember) rotates on the bond to the next carbon (call it C2). But double bonds don't rotate freely, so the group attached to C3 (the one next to C2) is fixed with respect to the group attached to C4. These pictures don't capture the geometry well, but think of the bonds around C3 and C4 being arranged in a planar triangular geometry:

  H     H
   \   /
    C=C 
   /   \ 
  X     Y

Where X and Y represent the non-hydrogen groups.

Because the double bond is flat, that means that there are two orientations around it, the one where the hydrogens are on the same side, as shown above (called cis) and the one where they're on the opposite side, (called trans), shown below:

  X     H
   \   /
    C=C 
   /   \ 
  H     Y

Now, despite the fact that saturated fats taste good and are good for cooking with, there's also a fair amount of evidence that they're bad for you. See the 2002 Institute of Medicine Dietary Reference Intakes report for a survey of the current literature. In particular, there's a strong observed correlation between consumption of saturated fat and elevated low-density lipoprotein (LDL, "bad cholesterol")/high-density lipoprotein (HDL, "good cholesterol") ratio as well as to coronary heart disease (CHD).

This brings us to the topic of trans fats, which have also been shown to be associated with high LDL/HDL ratio and with CHD. In fact, trans fatty acids appear to have a higher impact on LDL/HDL ratio than do saturated fats, as seen below. However, it's unclear whether this difference is in fact clinically significant.


Figure from Ascherio et al. (1999) by way of the IOM report.

Why this is all relevant now is that there's been a lot of news about how bad trans fats are for you. In particular, groups like the Center for Science in the Public Interest are pressuring manufacturers to remove them from their products--with some success. For instance, many of this year's girl scout cookies are now trans fat free. Now, there's not necessarily anything wrong with removing trans fats from food, but these self-same cookies (thin mints in this case) have 6g of saturated fats (75% of the total fat load) per 170 calorie cookie.

Interestingly, girl scout cookies are made by two licensed bakers, ABC/Interbake and Little Brown Bakers. ABC's version is trans-fat free. Little Brown Bakers is not. Here's the relevant nutrititional info for the respective thin mints:

-ABC/InterbakeLittle Brown Bakers
Calories160140
Total fat (g)87
Saturated fat (g)64
Trans fat (g)01

Two things to notice here. First, the saturated + trans load is lower on the non-trans free cookie. Even if trans fats are significantly worse than saturated fats, it's not clear that 6g of saturated fats is better than 4g of saturated fats + 1g of trans fats. Of course, it's certainly possible that you could reformulate the LBB cookie to be trans fat free while retaining the 5g total load, but one wonders why ABC didn't do that if it were easy. Note also that the ABC cookies have a higher carbohydrate load, so it could just be a subtly different recipe. Nevertheless, I think it makes the point that you can't just look at trans fats.

The second thing to notice is that the total saturated fat load is much higher than the trans fat load, even in the non-trans-fat free cookie. This suggests that it's probably not a good idea to focus on trans fat reduction to the exclusion of saturated fats, since saturated fats comprise a much higher portion of the average person's diet, and so a rather higher fraction of your total risk.

So, why all the interest in trans fat? Here's Paul Rozin's explanation:

It is also an unsurprising one, said Paul Rozin, a psychology professor at the University of Pennsylvania who studies people's psychological relationships with the food they eat. While trans fat occurs naturally in foods like milk and meats, most trans fat in processed foods was created by chemically altering oils like corn oil, turning them into fats that add texture and stability. That means most trans fat is artificial, which causes many people to recoil.

"Food is one of the areas where people think to leave it alone is better," Professor Rozin says.

I guess not everyone believes in better living through chemistry.

Anyway, based on the data so far, it seems like it's worth being cautious about trans fats, but not treating it like plutonium. Given the data about LDL/HDL ratio, it's probably pretty to treat 1g of trans fat as somewhere between 1 and 1.5g of saturated fat.

2 Comments

Great little primer, ekr. Would that more teachers could lay out scientific info as straightforwardly and entertainingly as that.

Actually, the recent interest in trans-fats seems to have arisen not out of concern for their effect on lipid profiles (which, as you point out, is bad but not fatal) as much as the fact that they seem to cause systemic cellular inflamation, which leads to a greater incidence of cardiac disease, even after adjusting for the lipid effects.

The apparent link between trans-fats and cardiovascular health apart from lipid impact was identified by a Harvard School of Public Health study by Dr. Lopez-Garcia et al.

Their findings are republished at http://jn.nutrition.org/cgi/content/full/135/3/562

The final conclusion of the report is, "[O]ur findings provide strong evidence that trans fatty acids adversely affect endothelial function, which might explain in part the association of these fatty acids with the risk of cardiovascular disease. These data lend further support for the recommendation to minimize the content of trans fat in the diet."

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