CLA (Conjugated Linoleic Acid): Benefits, Side Effects, Dosage, and What the Research Says

Nutrienting Team

January 9, 202613 min read

Conjugated Linoleic Acid (CLA) is a naturally occurring fatty acid found in certain foods, often marketed as a dietary supplement for various health benefits, particularly weight management. This article explores what CLA is, its proposed uses, potential side effects, recommended dosages, and what current scientific research suggests about its effectiveness. Understanding CLA involves looking beyond marketing claims to the evidence, considering both its potential advantages and limitations.

What is Conjugated Linoleic Acid (CLA)?

CLA refers to a group of isomers of linoleic acid, an omega-6 fatty acid. These isomers differ in the arrangement of their double bonds. The most common forms found in supplements and research are c9, t11-CLA (cis-9, trans-11) and t10, c12-CLA (trans-10, cis-12).

Naturally, CLA is primarily found in the meat and dairy products of ruminant animals like cows, sheep, and goats. These animals produce CLA through microbial fermentation in their digestive systems. Grass-fed animals tend to have higher levels of CLA in their products compared to grain-fed animals.

As a supplement, CLA is typically synthesized from linoleic acid in vegetable oils, such as sunflower or safflower oil. This process often yields a mixture of CLA isomers, with varying ratios of c9, t11 and t10, c12. The specific isomer blend can influence its biological effects, a point often overlooked in general discussions about CLA.

The initial interest in CLA stemmed from animal studies in the late 1980s, which suggested potential anti-cancer properties. Subsequent research expanded to explore its role in body composition, immune function, and other areas.

CLA and Body Composition: A Closer Look

One of the most heavily promoted uses of CLA is its purported ability to aid in weight loss and improve body composition, specifically by reducing body fat and increasing lean muscle mass. The mechanisms proposed for these effects include:

Decreased fat deposition: CLA is thought to inhibit lipoprotein lipase, an enzyme involved in fat storage, and promote lipolysis (fat breakdown).
Increased energy expenditure: Some theories suggest CLA might boost metabolic rate.
Enhanced fat oxidation: CLA could potentially increase the body's ability to burn fat for energy.
Reduced adipocyte (fat cell) size and number: Certain CLA isomers may influence the development and size of fat cells.

However, the scientific evidence supporting these claims in humans is mixed, and often less dramatic than animal studies or marketing might suggest.

Research Findings on CLA and Weight Management

Numerous human clinical trials have investigated CLA's impact on body composition. Here's a summary of common observations:

Modest Fat Loss: Some studies have reported a small, but statistically significant, reduction in body fat mass, particularly with long-term supplementation (6 months to 2 years). The average fat loss observed is often in the range of 1-3 kg (2-6 lbs) over several months, which is modest compared to lifestyle interventions.
No Significant Weight Loss: Many studies show no significant overall weight loss, even when fat mass decreases. This is often attributed to a concurrent increase in lean body mass, which can offset changes on the scale.
Effect on Lean Mass: A few studies suggest CLA may help preserve or slightly increase lean body mass, especially in individuals engaging in resistance training. This could be beneficial for maintaining metabolism during weight loss.
Isomer Specificity: The t10, c12 isomer is generally thought to be more active in fat reduction, while the c9, t11 isomer may have other effects, such as anti-inflammatory properties. Most supplements contain a blend, and the exact ratio can influence outcomes.
Inconsistent Results: The variability in study designs, populations, dosages, CLA isomer ratios, and duration makes it challenging to draw definitive conclusions. Some trials show positive effects, while others report none.

For example, a meta-analysis published in the American Journal of Clinical Nutrition reviewed several studies and concluded that CLA produces a modest reduction in body fat in humans. However, the authors also noted the small effect size and the need for more long-term research.

It's important to differentiate between studies on overweight or obese individuals and those on athletes or lean individuals. The effects might vary depending on the starting body composition and activity level.

Beyond Weight Loss: Other Potential CLA Benefits

While weight management is a primary focus, research has explored other potential benefits of CLA:

Immune System Modulation

Some animal and in vitro studies suggest CLA may influence immune function, potentially enhancing certain immune responses or reducing inflammation. However, human research in this area is limited and inconclusive. More studies are needed to understand if these effects translate to practical immune benefits in people.

Anti-Cancer Properties

Early interest in CLA stemmed from its observed anti-cancer effects in animal models, particularly against breast, colon, and skin cancers. The mechanisms are thought to involve antioxidant activity, modulation of cell growth, and induction of apoptosis (programmed cell death). Despite promising animal data, robust human trials demonstrating CLA's efficacy in cancer prevention or treatment are largely absent. It is not currently recommended as a cancer therapy.

Bone Health

A few studies have investigated CLA's potential role in bone health, suggesting it might improve bone mineral density or reduce bone loss. This area of research is still nascent, and evidence is not strong enough to support CLA supplementation for bone health.

Cardiovascular Health

The role of CLA in cardiovascular health is complex and somewhat contradictory. Some animal studies indicated potential improvements in lipid profiles or reductions in atherosclerosis. However, human studies have yielded mixed results, with some suggesting potential negative effects on insulin sensitivity and lipid profiles, particularly with the t10, c12 isomer at high doses. This highlights the importance of considering specific isomers and dosages.

CLA Side Effects and Safety Concerns

While generally considered safe for most people when consumed in moderate amounts, CLA supplements can cause side effects, especially at higher dosages.

Common side effects often include:

Gastrointestinal issues: Nausea, upset stomach, diarrhea, and indigestion are frequently reported.
Fatigue: Some individuals experience increased tiredness.

More serious, though less common, concerns have been raised regarding CLA's impact on metabolic health:

Insulin Resistance: Some human studies, particularly those using higher doses of CLA (especially rich in the t10, c12 isomer), have shown a decrease in insulin sensitivity, potentially increasing the risk of developing type 2 diabetes in susceptible individuals. This is a significant concern for those already at risk.
Liver Function: There have been reports of elevated liver enzymes and potential for fatty liver in some animal models and a few human cases, particularly with high doses.
Lipid Profiles: While some studies show beneficial effects on cholesterol, others have reported increases in LDL ("bad") cholesterol or decreases in HDL ("good") cholesterol, again often linked to the t10, c12 isomer.

Who should be cautious?

Individuals with diabetes or insulin resistance.
People with liver conditions.
Pregnant or breastfeeding women (due to limited research).
Individuals on medications, as CLA could potentially interact with certain drugs (e.g., blood thinners, insulin).

It's crucial to discuss CLA supplementation with a healthcare provider, especially if you have pre-existing health conditions or are taking other medications.

CLA Dosage Recommendations

There is no universally agreed-upon standard dosage for CLA, as research has used a wide range of amounts, and optimal doses may vary depending on the desired effect and individual factors.

Based on available research, most studies showing modest benefits for body composition used dosages ranging from 3.2 grams to 6.4 grams per day. This amount is typically divided into 2-3 doses taken with meals.

For weight management: Many studies suggest that a daily intake of around 3.4 grams of active CLA isomers is where some effects on body fat reduction have been observed.
Duration: Effects, if any, are generally seen over long periods, often several months to a year or more. Short-term supplementation (e.g., a few weeks) is unlikely to yield significant results.

Considerations for Dosage:

Isomer Content: Pay attention to the label to see the amount of active CLA isomers (c9, t11 and t10, c12) per serving, not just the total capsule weight. Different brands will have different concentrations.
Start Low: If you choose to try CLA, starting with a lower dose and gradually increasing it can help assess tolerance and minimize gastrointestinal side effects.
Consult a Professional: Always consult with a doctor or registered dietitian before starting any new supplement, especially to determine an appropriate dosage for your specific health needs and to check for potential interactions or contraindications.

What the Research Says: A Balanced Perspective

The overall scientific consensus on CLA is one of cautious optimism tempered by inconsistent results and concerns regarding side effects at higher doses.

Here's a summary of the research landscape:

Animal Studies vs. Human Studies: Animal studies often show more pronounced and consistent benefits (e.g., significant fat loss, anti-cancer effects) compared to human trials. This difference is common in supplement research and highlights the need for human-specific data.
Modest Effects: When benefits are observed in humans, they are typically modest, particularly for weight and fat loss. CLA is not a "magic pill" for significant body transformation.
Isomer Importance: The specific CLA isomer profile in a supplement appears to be crucial. Most research implicates the t10, c12 isomer in fat reduction, but also in some of the negative metabolic effects. The c9, t11 isomer is more prevalent in natural food sources and may have different, potentially more beneficial, effects.
Individual Variability: Responses to CLA supplementation vary widely among individuals, likely due to genetic factors, diet, activity levels, and gut microbiome composition.
Long-Term Data Gaps: While some studies extend for a year or two, very long-term safety and efficacy data, particularly regarding metabolic health, are still somewhat limited.

Comparison of CLA Research Outcomes

Research Area	General Findings (Human Studies)	Strength of Evidence	Notes
Fat Loss	Modest reduction in body fat mass (1-3 kg over 6-12 months). Often no significant change in overall body weight.	Moderate	Inconsistent results across studies; effects are small and require long-term use. Isomer ratio seems to play a role.
Lean Muscle Mass	Some studies suggest a slight preservation or increase in lean mass, especially with resistance training.	Weak to Moderate	Effects are not dramatic; more research needed to confirm.
Immune Function	Limited and inconsistent human data. Some animal studies show immune modulation.	Weak	Not enough evidence to recommend for immune support in humans.
Anti-Cancer	Promising in animal models (in vitro and in vivo). Very limited and insufficient human data to support claims.	Weak	No clinical recommendation for cancer prevention or treatment.
Cardiovascular	Mixed results. Some studies show potential for adverse effects on insulin sensitivity and lipid profiles (e.g., increased LDL, decreased HDL), particularly with t10, c12 isomer at high doses. Other studies show no effect.	Weak to Moderate	Potential negative metabolic impacts are a concern, especially for individuals with pre-existing risks.
Bone Health	Very limited and preliminary research.	Weak	Insufficient evidence.

CLA as a Functional Food

Beyond supplements, CLA occurs naturally in foods. The primary dietary sources are:

Beef: Especially from grass-fed cattle, which can have significantly higher CLA content than grain-fed beef.
Dairy products: Milk, cheese, and yogurt from grass-fed animals.
Lamb and Veal: Also good sources, particularly from pasture-raised animals.

The CLA found in these natural food sources is predominantly the c9, t11 isomer, which is associated with fewer potential negative metabolic effects compared to the t10, c12 isomer often found in higher concentrations in supplements derived from vegetable oils.

Consuming CLA through whole foods as part of a balanced diet is generally considered safe and may offer broader nutritional benefits than isolated supplements. However, the amount of CLA obtained through diet alone is typically much lower than what is used in supplementation studies.

Conclusion

Conjugated Linoleic Acid (CLA) is a fatty acid that has garnered attention for its potential role in body composition, immune function, and other health areas. While animal studies have shown promising results, human research, particularly concerning weight loss, indicates only modest effects, if any, and often requires long-term supplementation. Some studies have raised concerns about potential negative metabolic impacts, such as reduced insulin sensitivity and altered lipid profiles, especially at higher doses and with specific isomer blends.

For curious readers seeking clear, trustworthy information, the takeaway is that CLA is not a miracle supplement. Its benefits for fat loss are small and inconsistent, and it comes with potential side effects that warrant caution. If considering a CLA supplement, it's essential to consult with a healthcare professional, understand the specific isomer content, and be aware that dietary sources of CLA, primarily from grass-fed animal products, may offer a different, potentially safer, profile of benefits. Prioritizing a balanced diet and regular physical activity remains the most effective and evidence-based approach to managing weight and improving overall health.

FAQ

What does CLA conjugated linoleic acid do?

CLA is believed to influence fat metabolism by potentially reducing fat storage, increasing fat burning, and affecting the size and number of fat cells. It has also been explored for its effects on the immune system, bone health, and its anti-cancer properties, though human research in these areas is less conclusive.

Does CLA really work for belly fat?

Some studies suggest CLA may lead to a modest reduction in overall body fat, which could include abdominal fat. However, the effects are generally small, inconsistent, and not specifically targeted to "belly fat" in a significant way compared to comprehensive diet and exercise interventions. Many studies show no significant reduction in overall weight or body circumference.

What are the cons of taking CLA?

The main cons of taking CLA supplements include potential side effects such as gastrointestinal upset (nausea, diarrhea, indigestion), fatigue, and, more seriously, potential negative impacts on metabolic health. Some research links higher doses of certain CLA isomers (particularly t10, c12) to decreased insulin sensitivity (increasing risk for type 2 diabetes) and adverse changes in cholesterol levels (e.g., increased LDL, decreased HDL) in some individuals. It's also not recommended for pregnant or breastfeeding women due to insufficient research.