Conjugated linoleic acid, or CLA, is a popular supplement purported to improve health in multiple ways.

Many people believe it helps you lose fat and build muscle, decreases your risk of poor metabolic and cardiovascular health, and increases testosterone production, all without any adverse side effects.

Many others, however, are leery of these claims. They believe the benefits of CLA are based on controvertible evidence and that taking CLA comes with significant risk.

Who should you believe?

Is CLA all pros and no cons?

Is it really safe and side-effect free?

Get an evidence-based answer in this article.

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What Is CLA?

CLA stands for conjugated linoleic acid and refers to a group of naturally occurring fatty acids.

CLA is found primarily in the meat and dairy of ruminants such as cows, goats, sheep, and buffalo, though the amount can vary depending on the animal’s diet, altitude, breed, and lactation age.

For example, some evidence shows that the dairy produced by cows free to pasture contains up to 500% more CLA than cows fed a typical dairy cow diet.

Scientists can also synthesize CLA using vegetable oils such as sunflower and safflower oil.

There are 28 possible forms of CLA. 

All of these forms share the same chemical formula, but the atoms in each are arranged differently, giving them slightly different properties.

Most of the CLA in the human diet (75-to-80%) comes from a form known as cis-9, trans-11-CLA, or c9,t11.

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What Is a CLA Supplement?

CLA supplements are dietary supplements that typically contain equal amounts of c9,t11 and another form of CLA called trans-10, cis-12-CLA (t10,c12).

People take CLA supplements because they believe that CLA confers several health benefits, including decreased risk of diabetes and improved cardiovascular health.

In fitness circles, the most common reason people supplement with CLA is to boost fat burning, though some also believe it enhances muscle growth, testosterone production, and athletic performance.

How Does CLA Work?

The most common reason people take CLA is to boost weight loss. While we don’t fully understand how CLA affects fat burning, some scientists believe it’s connected to how CLA interacts with Peroxisome Proliferator activated Receptors (PPAR).

Research on rats shows that CLA (particularly c9,t11 and t10,c12) binds to PPARa, which according to some researchers, may increase fat burning.

There’s also evidence from a human study that t10,c12 inhibits PPARy, a receptor found in fat cells that increases fat gain. That said, other research on human cells shows that c9,t11 activates PPARy, and thus has the opposite effect.

Research on human cells also suggests that CLA suppresses or inhibits enzymes that contribute to fat gain, such as lipoprotein lipase and acetyl-CoA carboxylase. Likewise, animal studies show that CLA increases levels of enzymes that boost energy expenditure, such as carnitine palmitoyltransferase-1 and acyl-CoA oxidase.

Furthermore, studies show that CLA interacts with the enzyme fatty acid synthase (FAS), though it’s not yet clear what effect this has on body fat. For instance, some studies show it increases fat burning, others that it has no effect, and still others that it increases fat gain. 

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What Does CLA Do?

Supplement sellers claim that CLA benefits your health in multiple ways, including aiding weight loss, boosting muscle growth, increasing insulin sensitivity, enhancing athletic performance, and improving cardiovascular health.

Are these claims valid, though? Here’s what science says.

CLA and Weight Loss

Many people take CLA for weight loss. However, the evidence that it’s effective in this regard is weak.

Studies involving mice regularly show that CLA significantly boosts fat burning, decreases appetite, and prevents fat storage, but scientists seldom report similar results in human studies, with most showing CLA has no effects on fat loss in humans.

The results are underwhelming in the few studies showing CLA increases fat loss in humans, too.

For example, in one study published in the journal Nutrients, people who supplemented with CLA twice daily for 12 weeks lost just ~1.5 pounds of fat. In another study by scientists at the University of Barcelona, overweight people who took CLA for 12 weeks lost only ~1.3 pounds.

Perhaps the most “dramatic” weight loss seen in a CLA study comes from research conducted by scientists at the Max Rubner Institut. In this study, researchers found that 85 obese men (~75% of whom had metabolic syndrome) who took CLA for 4 weeks lost ~2.5 pounds of fat.

Other studies show that CLA’s weight-loss effects are highly unpredictable. For instance, one study published in The American Journal of Clinical Nutrition showed that some people who took CLA for 6 months lost as much as ~6.8 pounds of fat, while others gained ~4.2 pounds.

Moreover, CLA isn’t effective at preventing weight regain after dieting, nor does it help you eat less. 

Overall, most data shows that CLA is a dud. Results from the few studies that suggest otherwise show that CLA’s effects are unreliable, inconsistent, and often inconsequential. As such, there’s little reason to add CLA to your fat-loss supplement stack.

CLA and Muscle Growth

Many people believe that CLA boosts muscle growth, but most research shows this isn’t the case

The few studies that disagree tend to be inconsistent, too.

For example, scientists at Rowett Research Institute found that young obese men who took CLA and fish oil for 12 weeks increased muscle mass by 2.4%, though young lean men and older obese and lean men saw no benefit. 

Researchers at the University of North Carolina found that obese people who took 6.4 grams of CLA (a large dose) daily for 12 weeks gained ~1.3 pounds of muscle in 12 weeks. Those who took 3.2 grams of CLA daily saw no benefit.

Scientists at the Scandinavian Clinical Research AS found that overweight people who took CLA for one year increased their muscle mass by an average of 1.8%. Importantly, this result wasn’t consistent among everyone, with some losing as much as 2.5% of their muscle mass.

The only other study worth mentioning was published in the International Journal of Obesity and Related Metabolic Disorders and found that people who took CLA for 13 weeks while regaining weight after following a very low-calorie diet regained more weight as muscle than those who took a placebo (12-to-13.7% vs. 8.6-to-9.1%, respectively). Taking CLA didn’t help these people maintain a lower body weight over time, though.

Some people also believe that CLA can enhance muscle growth by boosting testosterone levels. 

While research on human cells found CLA may be able to increase “T” production, the results weren’t replicated when scientists repeated the study in living humans. 

In one animal study, scientists found that injecting mice with a mushroom extract containing high c9,t11 levels may prevent the enzyme aromatase from converting testosterone to estrogen. That said, the mushroom extract contained other compounds that could have been responsible for the result, so it’s impossible to say whether the c9,t11 contributed. It’s also impossible to know whether we’d see similar effects in humans.

Evidence that CLA positively influences muscle growth is lacking, and any evidence in support is inconsistent. Thus, it’s reasonable to conclude that CLA is ineffective at boosting muscle growth.

CLA and Insulin Sensitivity

Insulin sensitivity refers to how sensitive your body is to the effects of insulin

People who are sensitive to insulin require less insulin to “shuttle” glucose (blood sugar) from their blood to their cells and remove glucose from their blood quicker. The more resistant to insulin you become, the worse your metabolic health and the higher your risk of type 2 diabetes.

While some animal studies show that c9,t11 increases insulin sensitivity, other research shows that t10,c12 causes inflammation that prevents glucose and fatty acids from entering cells, increasing insulin resistance.

Human studies on how CLA affects insulin sensitivity are inconsistent.

For instance, in one study conducted by scientists at the University of Guelph, 10 men took 3.2 grams of CLA daily. Six experienced an increase in insulin sensitivity, 2 experienced a decrease, and the remaining 2 experienced no change.

In another study published in Applied Physiology, Nutrition, and Metabolism, researchers found that out of 9 people who took CLA daily, 3 increased insulin sensitivity by 9-to-13% and 6 decreased insulin sensitivity by 9-to-79%. 

Most other studies show that CLA has little effect on insulin sensitivity.

Given the inconsistent results regarding CLA’s effect on insulin sensitivity and its potential to increase insulin resistance, it’s sensible to avoid supplementing with CLA until more human research shows it’s safe to do so.

CLA and Athletic Performance

Studies looking at CLA’s effect on athletic performance are inconsistent and unpredictable. Some show that CLA has no effect on endurance, power, and strength, and others report that CLA may boost athletic performance to a small but significant degree.

Furthermore, one study shows that CLA increases bench press strength but not leg press strength in men but doesn’t increase women’s performance on either exercise.

The current evidence is too inconsistent to draw any firm conclusion about whether CLA improves athletic performance. Until more human trials illustrate a benefit, it’s probably not worth investing in a CLA supplement to boost athletic performance.

CLA and Cardiovascular Health

Studies investigating CLA’s effect on cardiovascular health are so diverse that they’re difficult to analyze.

For instance, some animal studies show that CLA may protect against cardiovascular disease (CVD) by boosting arterial health, improving cholesterol levels, and reducing oxidative stress.

On the other hand, human studies looking at how CLA affects cholesterol levels are conflicting, with some showing CLA improves cholesterol, others showing CLA doesn’t alter cholesterol levels, and still others suggesting CLA has a detrimental effect on cholesterol.

Furthermore, research suggests CLA increases oxidative stress and levels of blood markers such as c-reactive protein, both of which are associated with higher CVD risk. Despite this, research shows that taking CLA doesn’t increase your chance of developing CVD.

With such discordant results, it’s impossible to draw firm conclusions about how CLA affects cardiovascular health. Given that some research suggests CLA may be detrimental to cardiovascular health, it’s probably sensible to avoid CLA supplements until we have more evidence they’re safe.

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CLA: Side Effects

CLA doesn’t appear to be toxic to humans, even when people take it for extended periods.

However, as we’ve already seen, CLA may decrease insulin sensitivity and impair cardiovascular health. Moreover, some animal research shows that taking large doses of CLA can cause increased fat accumulation in the liver.

Thus, we need more human research before we can be sure CLA is safe.

FAQ #1: What is CLA good for?

Emptying your wallet?

Jokes aside, there’s little evidence that CLA aids weight loss, boosts muscle growth, enhances athletic performance, or improves cardiovascular health.

FAQ #2: How long does it take for CLA to work?

It depends.

Some studies show that people see benefits (fat loss, for example) within weeks, others have to wait months or years, and still others never experience any benefit from taking CLA.

FAQ #3: What are CLA’s benefits and dangers?

While many people believe taking CLA will aid weight loss, boost muscle growth, enhance athletic performance, and improve cardiovascular health, few people experience these benefits. 

Unfortunately, everyone who takes CLA is susceptible to experiencing adverse side effects, most notably increased insulin resistance and CVD risk.

+ Scientific References