CLA Supplements: Body Fat Evidence, Doses Studied and Side Effects

Conjugated linoleic acid (CLA) is a family of fatty acids found naturally in dairy products and beef. It is sold as a softgel supplement for fat loss. The honest summary of the human trials is that a CLA supplement trims body fat by a small amount at best. A 2012 systematic review and meta-analysis in the European Journal of Nutrition found about 1.3 kg (about 2.9 lb) more fat loss than placebo over six months to a year. Its authors judged that effect small and of uncertain clinical relevance.

There are trade-offs. Digestive complaints are the most common side effect, and some studies found unfavorable changes in cholesterol. One of the two main CLA isomers also worsened insulin sensitivity in men with metabolic syndrome. So two questions matter most: how big a change could you realistically expect, and does your health history make the risks more relevant to you?

What CLA is

Linoleic acid is a common omega-6 fat. “Conjugated” describes a particular arrangement of its chemical bonds, and that arrangement can take several shapes, called isomers. According to the NIH Office of Dietary Supplements (ODS) fact sheet on weight-loss supplements, two isomers matter for supplements: c9,t11-CLA and t10,c12-CLA. Most products contain a mix of the two, supplied either as a triglyceride (the usual form of fat in food) or as free fatty acids.

Researchers have suggested, but not shown, that CLA might work in four ways: increasing fat breakdown, raising how much fat muscles use for energy, reducing fat storage and prompting fat cells to die off. ODS notes that CLA appears to reduce body fat in animals. Results in people suggest the effects are small and of questionable clinical relevance.

What the trials found

The five studies below give a fair picture of the human evidence, from the largest pooled analyses to single trials that pointed in different directions. BMI is body mass index. A crossover trial gives each person both CLA and a comparison oil, one after the other.

Study	Who	Daily dose	Length	Result
Meta-analysis of 18 trials (2007)	Adults in placebo-controlled trials	Median 3.2 g	Up to 2 years	About 0.09 kg more fat lost per week than placebo
Meta-analysis of 7 long-term trials (2012)	Adults with overweight or obesity	2.4 to 6 g	6 to 12 months	0.70 kg more weight and 1.33 kg more fat lost than placebo
One-year trial (2004)	180 adults with overweight	3.4 to 3.6 g	1 year	Fat mass fell 6.9% to 8.7% more than placebo
Crossover trial	35 postmenopausal women with obesity and type 2 diabetes	6.4 g	16 weeks	Lower BMI and body fat than with safflower oil
Trial with exercise	28 young women with obesity	3.2 g	8 weeks	No reduction in body fat versus placebo

A 2007 meta-analysis in the American Journal of Clinical Nutrition pooled 18 randomized, double-blind trials that measured body composition with validated methods. At the median dose of 3.2 g a day, people taking CLA lost about 0.09 kg (roughly 0.2 lb) of fat a week more than those on placebo. The effect grew steadily for about six months. Then it leveled off and flattened out toward two years.

The 2012 meta-analysis took a stricter view and included only trials that lasted at least six months. Of 15 candidate trials, seven qualified, and four of those had serious flaws in how their methods were reported. Pooled, CLA led to 0.70 kg (about 1.5 lb) more weight loss and 1.33 kg (about 3 lb) more fat loss than placebo. The authors concluded that the evidence does not convincingly show clinically relevant long-term effects on body composition.

The largest single trial in the ODS summary was a one-year trial published in the American Journal of Clinical Nutrition in 2004. It followed 180 adults with overweight and compared CLA as free fatty acids, CLA as triglycerides and placebo. Both CLA forms reduced fat mass, by 6.9 and 8.7 percent on average, and the free fatty acid form also increased lean mass. Other trials were less positive. In 28 young women with obesity, 3.2 g a day combined with aerobic exercise for eight weeks did not reduce body fat compared with placebo.

Putting the numbers in context

A difference of roughly 1 to 2 kg (about 2 to 4 lb) of fat over six months to a year is real in statistical terms. Next to most people’s goals, it is modest. It is also an average, so individual results vary.

Several trials, including the one-year study, let participants eat as they normally would. CLA was not tested as a substitute for changes in eating or activity, and the results do not suggest it can be one.

Doses studied

Human trials have used roughly 2.4 to 6.4 g of CLA a day, most often around 3 to 4 g. Most used a near-equal mix of the two main isomers.

On a label, look for the amount of CLA itself rather than the total weight of oil in each softgel. Products differ in purity, so the oil weight and the CLA content are not always the same number.

Food provides CLA too, mainly from dairy fat and beef. A 2015 review in the Journal of the International Society of Sports Nutrition puts beef fat at roughly 1.7 to 10.8 mg of CLA per gram of fat. To match a 3 g (3,000 mg) trial dose from beef alone, you would need somewhere between about 280 g (about 10 oz) and well over a kilogram of beef fat a day. So diet and supplements are not comparable sources. That gap is not a reason to eat more fat, and it is not a reason to start a supplement.

Some studies asked whether CLA helps keep weight off. The 2015 review describes a year-long trial in 101 adults who had already lost more than 8 percent of their body weight. They took 3.4 g of CLA a day alongside a modestly reduced-calorie diet. Compared with an olive oil placebo, CLA did not reduce regain of weight or fat. White blood cell counts also rose in the CLA group.

If you decide to try it anyway

This is not a recommendation, but if you want to see for yourself, a few steps keep the experiment honest and safe.

1. Check first. Talk to your doctor or pharmacist before starting if you have diabetes, prediabetes, high cholesterol or liver disease, or if you are pregnant or breastfeeding.
2. Read the label. Choose a product that lists the CLA content per softgel. Keep your daily total within the 3 to 6 g range used in trials, not above it.
3. Measure before and after. Record something objective before you start, such as your waist measurement or your body weight at the same time of day. Measure again after three to six months. The trials suggest any change will be small, so a single weigh-in will not tell you much.
4. Know when to stop. Stop if stomach symptoms persist. Stop and seek advice if you notice yellowing of the skin or eyes, or a change in the color of your urine or stool. MedlinePlus lists these among the symptoms of liver disease.

Side effects

ODS describes CLA as generally well tolerated, with most reported effects minor. The ones worth knowing about fall into five groups.

• Digestive upset: the most common. This includes stomach discomfort and pain, constipation, diarrhea, loose stools, nausea, vomiting and indigestion.
• Blood fats: results are inconsistent, but some research shows certain isomers may lower HDL (the protective cholesterol) and raise lipoprotein(a), another blood fat.
• Blood sugar: the t10,c12 isomer has been reported to increase insulin resistance (the body responding less well to insulin) and blood glucose in men with obesity and metabolic syndrome.
• Liver: three case reports have linked CLA products to hepatitis (liver inflammation). The products were not tested for contaminants, so the cause was never established.
• Other signals: CLA might raise some markers of oxidative stress (a type of wear on cells) and lower the fat content of breast milk. Both need confirmation.

A note on weight-loss supplements generally

The FDA does not approve dietary supplements for safety or effectiveness before they go on sale. Weight-loss products deserve extra caution. ODS reports that from 2004 to 2012, weight-loss supplements made up 27 percent of the dietary supplements subject to the FDA’s most serious class of recall. Most of those recalls happened because the products contained undeclared drug ingredients.

A product that lists only CLA is not in that category. Multi-ingredient weight-loss blends that pair CLA with stimulants are a different matter.

If your real interest is body composition, two other guides may be more useful: our psyllium husk guide covers the evidence on fiber, and our creatine basics guide covers muscle. Our review of detox cleanses looks at another category sold with weight-loss promises.

Who should be careful

• Type 2 diabetes, prediabetes or metabolic syndrome: the t10,c12 isomer has been linked to worse insulin resistance and higher blood sugar. Ask your doctor before starting.
• Managing cholesterol or heart risk: some studies reported lower HDL and higher lipoprotein(a). Ask your doctor first.
• Liver disease: three hepatitis case reports involved CLA products, so check with your doctor first.
• Pregnancy and breastfeeding: CLA may reduce the fat in breast milk, and the trials offer no guidance for pregnancy.
• Sensitive digestion: stomach complaints are the most common side effect.
• Multi-ingredient products: these may contain stimulants or undeclared ingredients beyond CLA. Ask your pharmacist to look over the label before you take one.