Dear Mark: CLA Supplements

Today’s question comes from Ola and regards CLA, or conjugated linoleic acid. What is CLA? CLA is the “good” trans-fat that occurs naturally in meat and dairy, especially from grass-fed animals. In the stomach of ruminants like cows, sheep, or goats, millions upon millions of bacteria help the animal digest its food. They also help convert dietary linoleic fatty acids into saturated fatty acids. Well, that conversion takes several steps, and one of the steps is the creation of CLA, some of which never gets fully saturated and instead shows up in the animal’s body and milk fat. 28 different CLA isomers, or structural arrangements of the molecules, appear in CLA-rich animal fat. It’s very complex and quite different from trans-fat created by partially hydrogenating vegetable oils. Those lab-created trans-fats have definite negative metabolic and health effects, while the panoply of various CLA isomers from grass-fed dairy and meat seem to be beneficial. With that said, let’s get to the question.


Thank you for this great and informative website. I have been primal for about a year now. I still don’t seem to get rid of those last 5 lbs so I read a bit about CLA supplements. What is your take on them? You have commented briefly on them and promised to write a more detailed piece about supplements.

If you don’t recommend CLA, what do you recommend for me? I lift “heavy things”/ push ups etc.. but I don’t sprint as much as I probably should! Thank you!


Conjugated linoleic acid production is a booming industry with many players. You’ve got the new guys creating the stuff on a massive scale, getting their hands dirty in the lab, converting linoleic acid derived from safflower or sunflower oil into various isomers of CLA. Then there are the stalwarts, those ruminant stomachs filled with microscopic bacterial sweatshops toiling away as they convert unsaturated fats to saturated fats and make various CLA isomers in the process. An isomer called cis-9, trans-11 (or c9, t11) isomer is the primary one. CLA with a trans-10, cis-12 isomer is also evident, but in far scanter quantities. Same type of molecules – different arrangement. In fact, c9, t11 CLA  accounts for between 80-95% of the CLA in ruminant and dairy fat, with t10, c12 showing up in trace amounts. Supplement makers have the luxury of focusing on other isomers, of course, so they typically produce CLA supplements containing equal amounts c9, t11 and t10, c12.

Why would they try to improve on an impossibly complex and delicately balanced natural system millions of years in the making by messing with the ratios?

Heh. Do I really have to answer that?

It turns out that the t10, c12 isomer has performed well in some studies. T10, c12 can inhibit the growth of human colon cancer cells in vitro (with c9, t11 having no effect). In another in vitro study, this time connective tissues isolated from human body fat, t10, c12 inhibited lipogenesis, or (something analogous to) body fat creation, while c9, t11 did not. It also showed promise as a promoter of lean mass versus fat mass in humans.

In a totally unsurprising twist, however, results change when you start feeding the stuff to live organisms and paying attention to the full effects (beyond just “does it result in 2% more fat loss?”).  Let’s take a look at a few examples.

Healthy humans taking trans-10, cis-12 CLA supplements had increased triglycerides, LDL-HDL ratios, and total cholesterol-HDL ratios when compared to patients taking supplements based on cis-9, trans-11. In both wild-type and lab mice, the t10, c12 isomer stimulated mammary tumor growth, while c9, t11 isomers had a neutral effect.

As Stephan Guyenet points out in a blog post, CLA loses a head-to-head match with safflower oil (!). The safflower oil group saw improved insulin sensitivity, higher HDL, and lower inflammation. The CLA was 50% trans-10, cis-12 and 50% cis-9, trans-11. In other words, it wasn’t CLA as you’d get from grass-fed butter or pastured lamb shoulder chops. Stephan also cites two other studies using t10, c12 and c9, t11 at a 50:50 ratio that had similarly negative results – here (t10, c12 supplements worsened metabolic syndrome in men) and here (increased c-reactive protein and insulin resistance). If you can’t beat safflower oil, you should probably just throw in the towel.

Another study found that while t10, c12 supplementation decreased fat mass, it also raised LDL, lowered HDL, and overall worsened the cholesterol profile, as well as increased insulin resistance, blood glucose levels, and insulin. C9, t11, on the other hand, improved lipid metabolism overall.

In post menopausal women, high t10, c12 CLA supplementation increased inflammatory markers and lipid peroxidation when compared to CLA “supplementation” with milk (containing, remember, mostly c9, t11).

Mice fed t10, c12-enhanced diets experienced reductions in liver fatty acid oxidation and liver detoxification enzymes. In short, t10, c12 CLA gave mice fatty liver and reduced the liver’s ability to do its job. It had similar effects on hamster livers.

T10, c12 led to dysregulated glucose and lipid metabolism.

Are you noticing a pattern? Again and again, individual CLA isomers appear to be protective or beneficial in isolated studies, usually in vitro, but when you actually feed an animal or a human a CLA supplement with the same isomer ratios, the benefits either disappear or get counterbalanced by a negative effect. You might burn some body fat, but you’ll also become insulin resistant. You may keep off the baby weight, but your breast milk will contain less fat as a consequence. I’m a big supporter of supplementation, but in my opinion, CLA supplementation simply isn’t worth it.

The right CLA supplement employing the right isomers in grass-fed ruminant-fat proportions could be helpful, but after taking an admittedly brief look at the top CLA supplements results on Amazon, I couldn’t point you toward any that fit that description. They may exist. Heck, they probably do exist, but it’s not obvious. I think you’d be better served simply eating grass-fed animal products: butter, cheese, and meat with fat intact.

And as for what you should do to lose the last five pounds read this, this, this, and see this. Grok on!

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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