What Is Conjugated Linoleic Acid (CLA)?

Conjugated linoleic acid (CLA) is a slightly modified form of the unsaturated omega-6 fatty acid called linoleic acid—the word "conjugated" refers to the type of bond between molecules. Naturally found in dairy products and beef (it's made by microbes that live within the gut of animals), CLA can also be synthesized in the lab as a dietary supplement.

There are two major forms (isomers) of CLA: cis-9, trans-11 and trans-10, cis-12. Various physiological effects are believed to come from each type. Trans-10, cis-12 is the form most often found in supplements.

What Is Conjugated Linoleic Acid Used For?

The most widely known use for CLA supplements by far is as a weight-loss aid. It’s a common ingredient in supplements marketed for this purpose, and with claims that it can reduce fat, build muscle, and increase energy and endurance, CLA is popular among some athletes. There's a wide range of other purported benefits as well, including cancer prevention and the treatment of high cholesterol.

Weight Loss

There have been some very promising studies indicating that CLA can improve body composition and weight loss. But many of these early studies were done on animals, and when the same experiments were tried on people, the results weren't anywhere near as favorable. Researchers aren't even sure how CLA would work to boost weight loss, though it's theorized to suppress appetite as well as to block fat cells from increasing in size by affecting enzymes that contribute to fat storage.

In studies that demonstrated weight reduction with CLA in humans, the amount of weight loss was usually fairly modest. For example, a 2012 study published in the journal Nutrition found that over a 12-week period, people taking CLA lost about one pound more than those not taking CLA. That’s less than a tenth of a pound per week. The decrease in body fat percentage was very small as well. People taking a CLA supplement saw a decrease in body fat that was less than a half percentage point lower than those not taking the pill.

A 2015 review study showed mixed results. In another report from 2007, researchers evaluated the results from 18 studies where participants took the supplement for a longer period of time (six months to two years). The scientists reported that, on average, the people who supplemented with CLA lost more fat than those not taking CLA, but the amount averaged less than a quarter of a pound per week.  

Based on the current evidence at the time, a 2015 review published in Nutrition and Metabolism concluded that CLA offered no "promising or consistent health effects so as to uphold it as either a functional or medical food." And the most recent analysis, a review article from 2019 that looked at 13 studies on overweight and obese people, determined that the efficacy of CLA supplementation on body weight and body fat is "not clinically considerable."

Besides these disappointing results, other research in 2004 shows CLA may actually be harmful in some people. For example, in men with obesity and metabolic syndrome or at high risk for heart disease, CLA supplementation caused insulin resistance, a silent blood sugar problem that increases the risk for prediabetes, type 2 diabetes, and a host of other serious health problems, including heart attacks, strokes, and cancer.

What's more, trans-10, cis-12, the key component of CLA supplements, was found to have a negative impact on blood sugar and could potentially contribute to the development of insulin resistance and atherosclerosis (hardening of the arteries).

Athletic Performance

Beyond its use in weight loss, proponents of CLA supplementation believe that it can enhance athletic performance in various ways, including stimulating testosterone production in the Leydig cells of the testicles. While it's true that CLA has this effect in laboratory tests on cells, the level of stimulation doesn't appear to translate to increased energy expenditure (the total number of calories you burn each day) or muscle development.

A 2014 study from the University of Nebraska reported that athletes provided a daily, 800-milligram dose of CLA for six weeks showed no improvement in endurance (as measured by VO2 max, a measure of how much oxygen is used during exercise) compared to athletes given a placebo. Likewise, a 2015 study on 80 non-trained healthy young men who took CLA for eight weeks saw no effect on VO2 max, and no change in time to exhaustion, weight, BMI, or waist circumference compared to those who took a placebo.

CLA has also gained attention among resistance-trained athletes as a way to preserve muscles by reducing catabolism (the breakdown of muscle for fuel), as well as to reduce body fat and improve muscle mass during training. However, supplementation of 6,000 milligrams a day of CLA coupled with 3,000 milligrams a day of fatty acids for four weeks did not significantly affect changes in total body mass, fat-free mass, fat mass, body fat percentage, bone mass, strength, serum substrates, or general markers of catabolism during training in one early study from 2002.

Taken as a whole, there's little convincing evidence that CLA improves athletic performance in any significant way. It's important to note that some studies reporting benefits such as strength gains and improved body composition used CLA in combination with creatine monohydrate, a supplement that's been widely shown to increase muscle mass and strength on its own.

Other Health Benefits

Other health benefits for CLA supplementation are also largely unsupported, including its use in treating diabetes, the common cold, allergic rhinitis (hay fever), or asthma.

Heart Health

Studies show that while CLA reduces total cholesterol levels, it also reduces HDL cholesterol. HDL is what's referred to as “good” cholesterol, so a decrease in HDL is not a good thing. As for other effects on heart health, the 2015 review study found that a host of both beneficial and detrimental effects of CLA were observed during clinical studies. For instance, while participants who took 6,400 milligrams of CLA daily for 12 weeks in one study in 2007 saw slight increases in lean body mass, they also saw significant decreases in HDL and significant increases in markers like C-reactive protein, a sign of inflammation that's associated with a higher risk of heart attack.

Cancer

Test tube and animal studies have shown that CLA has antioxidant properties and may play a role in disrupting cancer cell replication to reduce the spread of cancer, including breast and colorectal cancer. Other possible mechanisms of action include modulation of intracellular signaling; when cells lose the ability to respond to signals from other cells, they may become cancer cells. It's important to note, however, that these benefits are often seen with the type of CLA found in food, not supplements. While preliminary studies in humans suggest potential anticancer effects, the Memorial Sloan Kettering Cancer Center says that clinical trials to confirm safety and effectiveness are needed.

Possible Side Effects

Some people may experience mild to moderate side effects with this supplement, including stomach upset, diarrhea, and nausea.

Conjugated linoleic acid may also slow blood clotting. Taking a CLA supplement along with an anticoagulant ("blood thinners") or nonsteroidal anti-inflammatory drug (NSAID) can further enhance this effect, leading to easy bruising and bleeding.

Possible drug interactions include:

• Advil (ibuprofen)
• Aleve (naproxen)
• Aspirin
• Advil (ibuprofen)
• Coumadin (warfarin)
• Fragmin (dalteparin)
• Heparin
• Lovenox (enoxaparin)
• Plavix (clopidogrel)
• Voltaren (diclofenac)

Dosage and Preparation

CLA supplements are typically produced as a gel cap and filled with either sunflower or safflower oil. CLA is classified by the U.S. Food and Drug Administration as GRAS ("generally regarded as safe") when taken as instructed. Since various formulations contain different amounts of CLA, it's wise to take a pure CLA supplement or make sure you're getting the right levels from combination supplements. Doses typically range from 3 to 6 grams per day.

Keep in mind that dietary supplements haven't been tested for safety, and, due to the fact that they're largely unregulated, the content of some products may differ from what's specified on the product label. Also be aware that the safety of supplements in pregnant women, nursing mothers, children, and those with medical conditions or who are taking medications hasn't always been established.

What to Look For

Due to the disappointing results on CLA supplementation and weight loss, as well as the potential adverse side effects, some researchers have suggested that eating foods that naturally contain CLA might be an alternative to losing weight and deriving other health benefits. For instance, in one study published in 2007, people who had more of the cis-9, trans-11 CLA isomer in their fat had a lower risk of diabetes. That isomer is the type found in meat (grass-fed animals may have higher levels) and dairy products. CLA is also in sunflower and safflower oil.