What Conjugated Linoleic Acid (CLA) Does for Body Composition

Conjugated linoleic acid (CLA) has been studied for possible roles in weight loss, exercise performance, cancer, and heart protection, but evidence in humans is limited. It carries a risk of side effects that include gastrointestinal issues, inflammation, and liver problems.

This polyunsaturated fatty acid is found naturally in dairy products and beef but also can be synthesized from sunflower and other oils.

What Foods Have CLA?

In general, people in the U.S. consume between 150 and 212 milligrams of CLA daily through the food they eat.

Foods rich in CLA include meats and dairy products like the following:

• Beef
• Lamb
• Turkey
• Butter
• Whole milk

What Is CLA 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.

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, including cancer prevention and the treatment of high cholesterol, but evidence of this is lacking.

Weight Loss

Researchers theorize that CLA affects enzymes that contribute to fat storage, thereby suppressing appetite and blocking fat cells from increasing in size.

Some studies indicated that CLA improved body composition and weight loss. However, many of these early studies were performed on animals. When the same experiments were applied to people, the results weren't nearly as favorable.

Overall, trials of CLA in humans have met with mixed results. There's not enough evidence to recommend it for weight loss, according to several extensive reviews, including:

• An analysis of 32 clinical trials concluded CLA did not affect waist circumference or blood sugar levels.
• A review of 20 clinical trials showed that although CLA plus exercise reduced body fat, it didn't decrease total body weight.
• A systematic review of 13 studies on overweight and obese people determined the efficacy of CLA on body weight and body fat was not clinically significant.

Even in studies that demonstrated weight reduction with CLA in humans, the weight loss was reasonably modest. 

For example, one trial found that in 12 weeks, people taking CLA lost about one pound more than those not taking CLA. That's less than one-tenth of a pound per week, and the decrease in body fat percentage was also slight.

People taking a CLA supplement experienced a decrease in body fat that was less than half a percentage point lower than those not taking it.

Athletic Performance

Beyond its use for weight loss, proponents of CLA supplementation believe it can enhance athletic performance in various ways, including by:

• Stimulating testosterone production
• Increasing strength
• Increasing muscle mass

However, human studies haven't lived up to these claims.

A small study of 10 male athletes suggested that CLA may increase muscle mass. However, more extensive studies are necessary to know this for sure.

Other studies have shown no benefit of CLA on exercise performance. One clinical trial reported that athletes who took CLA for six weeks showed no improvement in endurance compared to athletes given a placebo.

Likewise, a study of 80 non-trained healthy young men who took CLA for eight weeks saw no effect on VO2 max (a measure of how much oxygen is necessary during exercise) or time to exhaustion compared to those who took a placebo.

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 combined with creatine monohydrate, a supplement widely shown to increase muscle mass and strength.

Heart Health

Studies of CLA's effects on heart protection show conflicting results.

Some recent research determined that CLA does not affect cholesterol levels or blood pressure in humans and that its role in heart health is uncertain.

On the other hand, a systematic review showed that CLA decreased low-density lipoprotein cholesterol (LDL, or "bad") in healthy adults.

Of note, CLA from the diet and CLA from supplements seem to affect cholesterol profiles differently.

CLA obtained through the diet was more beneficial. CLA from supplements slightly increased total cholesterol and decreased high-density lipoprotein cholesterol (HDL, or "good" cholesterol").

However, CLA from foods did the opposite, causing a decrease in total cholesterol and an increase in HDL cholesterol.

Safety

Avoid CLA if you're allergic to it or its components.

Seek immediate medical attention if you have a severe allergic reaction (itching, hives, shortness of breath).

Compared to vitamins, a study showed that the following types of supplements increased the risk of severe medical events threefold:

• Muscle building supplements
• Energy supplements
• Weight loss supplements

The use of these types of supplements is not advisable.

Side Effects

Any supplement you take may cause side effects. These side effects may be mild or severe.

Generally speaking, CLA is well tolerated. It may cause mild gastrointestinal side effects such as the following:

• Stomach pain
• Constipation
• Diarrhea
• Nausea
• Vomiting
• Heartburn

Some serious side effects are also possible with CLA supplementation, including the following:

• Inflammation
• Insulin resistance
• Fatty liver disease
• Increased liver enzymes
• Colon cancer
• Asthma exacerbations

Drug Interactions

CLA may interact with the following medications:

• Insulin or other diabetes medications. CLA can cause insulin resistance. So, higher doses of insulin or medicines that treat high blood sugar may be necessary if taken with CLA.
• Retinol (vitamin A). CLA can increase retinol levels and, therefore, may intensify side effects if taken together.
• Cholesterol medications. Some research has shown that CLA can decrease LDL cholesterol. Because of this, CLA may enhance the effects of medicines like statins, used for treating high cholesterol.

It is essential to carefully read a supplement's ingredients list and nutrition facts panel to know which ingredients and how much of each ingredient is included. Please review the supplement label with your healthcare provider to discuss potential interactions with foods, other supplements, and medications.

Precautions

Do keep the following precautions in mind when using CLA:

• Pregnancy: CLA supplementation is not recommended during pregnancy. If you are pregnant, discuss supplementation with your healthcare provider before adding any supplements to your regimen.
• Breastfeeding: There isn't enough safety data to recommend CLA supplements during breastfeeding. CLA may affect your milk supply or its fat content. Some research recommends getting fatty acids like CLA through the diet rather than in supplement form during this period. If you are breastfeeding, discuss it with your healthcare provider before adding any supplements to your regimen for safety.
• Children: CLA has been studied in a clinical trial of children ages 6–10 who were overweight or obese. While the supplement did decrease body fat in those who took it, researchers note that more data is necessary to determine safety in children. CLA has also been studied in children ages 8–10 who were obese. Always discuss any supplement you're considering for your child with your pediatrician for optimal results.

Dosage

CLA supplements are typically produced as a gel cap and filled with safflower oil. CLA is classified by the U.S. Food and Drug Administration as GRAS (generally recognized as safe) when taken as instructed.

Doses typically range from 3–6 grams per day.

Research suggests daily doses of up to 6 grams were safe for up to a year, and daily doses of up to 3.4 grams were safe for up to two years.

Summary

CLA is a fatty acid obtained through meats and dairy products or taken as a dietary supplement.

It's often marketed for weight loss and exercise performance, though there isn't much data to support these claims. Clinical trials have had conflicting results, so more research is necessary to determine CLA's effects in humans.

Side effects typically involve the gastrointestinal tract but can also include inflammation and liver problems. CLA is not recommended during pregnancy or breastfeeding.