LDL Cholesterol: You Aren't Always What You Eat, or Roundabout Ways to Improve Your Diet

Sometimes you read these papers and shake your head.

"Lowering Dietary Saturated Fat and Total Fat Reduces the Oxidative Susceptibility of LDL in Healthy Men and Women"

The troll who pointed me to this was trying to argue that this invalidated the claim that omega-6 polyunsaturated (n-6) fats are important in cardiovascular disease (I gather), since lowering saturated fat (see the title) lowered oxidation.

Oddly enough, he was almost right, but not quite.

Fat components

The intervention was to have fixed protein (15%), but a variable rate of saturated fat (SFA), to be replaced by carbohydrate (CHO), and fixed rates of monounsaturated and polyunsaturated fats (MUFA and PUFA, respectively). And they did a good job of detailing what the diet included, right down to the individual fats.

However you have to go to a different study to find all the details ("Effects of Reducing Dietary Saturated Fatty Acids on Plasma Lipids and Lipoproteins in Healthy Subjects")

To sum the variation, SFAs were 16%, 9%, and 5%; and CHO was therefore 48%, 55%, and 59%. MUFA was 14%, and PUFA was 7%.

The diet part of it seems to have been very well done, indeed. If you want some idea from where the food may have come, may I introduce you to their sponsors?

"The DELTA Investigators express thanks to the following contributors: AARHUS, Bertolli, USA., Best Foods, Campbell Soup Company, Del Monte Foods, General Mills, Hershey Foods Corp., Institute of Edible Oils and Shortenings, Kraft General Foods, Land O'Lakes, McCormick Incorporated, Nabisco Foods Group, Neomonde Baking Company, Palm Oil Research Institute, Park Corporation, Procter & Gamble, Quaker Oats, Ross Products Division/Abbott Laboratories, Swift-Armour and Eckrick, Van Den Bergh Foods, Cholestech, Lifelines Technology Incorporated."

Wow.

The troll was almost right because LDL oxidation did indeed go down in this study. But it went down for a reason that I wouldn't have expected, and which leaves me somewhat perplexed to explain mechanistically. 

The paper goes on ad nauseum along these lines:

"Convincing evidence suggests that oxidative modification of LDL plays an important role in the pathophysiology of atherogenesis (Steinberg 1997). In recent years, numerous molecular mechanisms have been proposed to explain the different oxidation pathways that lead to modification of LDL (Steinberg 1997). 

"One of the earliest steps in the generation of oxidatively modified LDL is the peroxidation of its polyunsaturated fatty acids (PUFA).3 The oxidative breakdown products of these fatty acids, such as malondialdehyde [MDA] and 4-hydroxynonenal [HNE], form covalent bonds with apolipoprotein B (apo B)..."

Yeah, yeah. Old news (the paper is from 2000), but here's where it gets odd:

"The results of the present study also show that LDL composition (LDL quality) affects susceptibility to oxidation. [It] was inversely correlated with the quantity of LDL oleic acid (r = −0.29, P < 0.01), and positively correlated with the quantity of LDL linoleic acid (r = 0.23, P = 0.04) and the 18:2-to-18:1 ratio (r = 0.52, P < 0.001). The oxidation rate was positively correlated with the 18:2-to-18:1 ratio (r = 0.24, P = 0.03)."

To lower n-6, cut SFA?

What?

So lowering SFA indeed made the LDL more resistant to oxidation. But it did so by lowering n-6 and raising MUFA in the LDL! SFA was basically unchanged.

A little Lipidology 101, MDA and HNE, mentioned above, aren't made from SFA, they're made from PUFA, and HNE is made exclusively from n-6. MDA and HNE pretty much are oxidized LDL. So you're not getting oxidized LDL from SFA in the body. It just won't happen.

Of course the authors don't mention that little fact. More below.

For some reason I am unable to explain, lowering SFA (16:0, palmitate in the chart) in the diet lowered n-6 (18:2, linoleate) in the LDL, and increased MUFA (18:2, oleate), protecting LDL from oxidation. SFA stayed  the same, basically.

My best guess is that it was the increased CHO in the diet (is this what's behind the Japanese diet, which is low in SFA and high in CHO?) but it is just a guess.

"In the present study, the ratios of 18:2 to 18:1 and PUFA to MUFA in the LDL from subjects when they consumed the Step-1 and Low-Sat diets were significantly lower than they were in the LDL from subjects when they consumed the AAD. Linoleic acid (n-6) in LDL from subjects when they consumed the Low-Sat diet also was significantly lower compared with those from subjects when they consumed the AAD. "

The first citation for this paper is:

"Oxidation of low-density lipoproteins: intraindividual variability and the effect of dietary linoleate supplementation"

"LDL oxidized faster after the linoleate diet than after the oleate diet... and produced more conjugated diene [that's bad] in proportion to the increase in LDL linoleate."

Here's where we get to the shaking my head part.

Why in blazes wouldn't you reduce the thing that is actually capable of causing the harm, as your first cite shows, and as your own study shows, is the source of the problem? Why keep n-6 flat and reduce something else, which through some round-about effect, lowers what you need to lower, the n-6?

Just lower the n-6 and you reduce the susceptibility for oxidation! They cite other papers that have done exactly that!

This study was done in 2000, did I mention that?

Think of the progress we would have made if they'd actually done what any engineer reading this paper would have told them to do. 

"N-6 converts to these toxins? You're proved that? Why not reduce the n-6?"

Let's not get too into conspiracies here, but on the face of it this study is clearly little more than a misleading advertisement for the sponsors listed above, who are forced to follow the United States Dietary Guidelines and replace saturated fats with the n-6 fat that their own research claims is harmful.

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This post started with a twitter troll who cited the first-mentioned study:

I unblocked you and looked at your study. Declining oxidation in LDL tracks w/ declining n-6 LA content in LDL, while SFA flat and MUFA went up.

Interactions between FA is a fascinating topic, & may be due to CHO, but reinforces n-6 claim for LDL.

Figure things out, you said? pic.twitter.com/qSCu2qDkSF

— Tucker Goodrich (@TuckerGoodrich) April 26, 2018