Can omega-3 cause weight gain? A closer look at the scientific evidence

November 24, 2025 •

4 min read

According to a 2021 systematic review, while some studies found that omega-3 supplementation had no effect on body weight, others noted a potential reduction in body fat mass. The question, can omega-3 cause weight gain, is therefore met with strong scientific evidence suggesting it does not, and in many cases, may even offer weight management benefits.

Quick Summary

Omega-3 fatty acids do not typically cause weight gain when consumed responsibly. Instead, they may aid in fat loss by boosting metabolism and enhancing exercise effectiveness. Factors like overall calorie intake, product quality, and the omega-6 to omega-3 ratio play a more significant role in weight fluctuations.

Key Points

Not a Cause for Concern: Omega-3s generally do not cause weight gain when taken within recommended dosages as part of a healthy diet.
Metabolism Booster: Some studies suggest that omega-3s can increase your metabolic rate, which helps your body burn more calories.
Fat Loss, Not Weight Loss: Omega-3s may help reduce body fat and waist circumference, even if the number on the scale doesn't change due to a simultaneous increase in muscle mass.
Quality Matters: Contaminated or rancid supplements can be harmful and potentially interfere with weight regulation, so choose high-quality products.
Balance is Key: Maintaining a healthy balance between omega-3 and omega-6 fatty acids is more important for weight management than worrying about omega-3s alone.
Calorie Awareness: Omega-3 supplements contain calories, but the amount is typically too small to cause weight gain unless you are taking excessively high doses and not accounting for them in your overall diet.

For many, the idea that consuming fat can help manage weight seems counterintuitive. However, the science behind omega-3 fatty acids paints a different picture. Far from being a cause of unwanted weight gain, these essential fats are more commonly associated with fat loss and improved body composition. Understanding the mechanisms and potential misconceptions is key to leveraging their health benefits.

The Role of Omega-3s in Metabolism and Weight

Omega-3s, particularly EPA and DHA found in fish oil, influence several metabolic processes that support weight management. They are crucial for cellular function and hormone regulation, and have been shown to positively affect metabolism and energy expenditure.

Potential Mechanisms for Weight Management

Several studies have explored how omega-3s may influence body weight and fat stores:

Increased Metabolic Rate: Some research has found that omega-3 supplementation can increase a person's metabolic rate, which is the number of calories burned at rest. A higher metabolic rate means more calories are burned throughout the day, which can aid in weight loss or maintenance. For example, a study showed that healthy adults taking 3 grams of fish oil daily for 12 weeks saw an increase in their metabolic rate by an average of 5.3%.
Enhanced Exercise Effectiveness: Fish oil has been shown to potentially amplify the calories and fat burned during physical activity. The theory is that omega-3s help the body switch from using carbohydrates to using fat as a fuel source during exercise. This could make workouts more effective for fat loss, as demonstrated in a study where women who took fish oil burned more fat during exercise compared to those who didn't.
Reduced Appetite: While results vary, some studies suggest that fish oil can increase feelings of fullness, particularly for individuals on a calorie-restricted diet. This can help curb cravings and reduce overall food intake, making it easier to stick to a diet plan.
Reduced Fat Storage: At a cellular level, omega-3s influence lipid metabolism. They can suppress the enzymes involved in fat storage and promote the breakdown of fat. This may lead to a reduction in body fat, even if total body weight remains unchanged due to a simultaneous increase in muscle mass.

Omega-3 vs. Omega-6: Striking a Balance

A critical factor in the discussion of omega-3s and weight is the balance with omega-6 fatty acids. The typical Western diet is often high in omega-6s and low in omega-3s, leading to an imbalance that promotes inflammation and is linked to weight gain. A balanced ratio is crucial for health and weight management.


Feature	Omega-3 Fatty Acids	Omega-6 Fatty Acids
Effect on Weight	Generally associated with fat loss and metabolic benefits.	High intake and ratio linked to increased risk of weight gain.
Role in Metabolism	Promotes fat oxidation and regulates adipokines (hormones related to fat).	High intake can contribute to inflammation and fat accumulation.
Dietary Sources	Fatty fish (salmon, mackerel), flaxseed, chia seeds, walnuts.	Vegetable oils (soybean, corn), poultry, and many processed foods.
Inflammatory Effect	Primarily anti-inflammatory.	Primarily pro-inflammatory when consumed in excess.

Gaining Muscle, Not Fat

When people see their weight stay the same or even slightly increase while taking omega-3s, it may be due to an increase in lean muscle mass and a simultaneous decrease in body fat. Since muscle is denser than fat, the scale can be misleading. Precise measurements of body composition often reveal a positive shift, even without a significant change in overall weight.

When Omega-3 Intake Could Be Problematic

While rare, there are scenarios where omega-3 intake could be linked to an increase in weight. It is crucial to distinguish these from the typical effects.

High-Calorie Supplements: Like any fat, fish oil contains calories (around 9 calories per gram). If you consume very high doses without adjusting your overall caloric intake, it could contribute to a calorie surplus, leading to weight gain. A standard dose of fish oil is unlikely to impact total calorie intake significantly.
Low-Quality Supplements: Rancid or contaminated omega-3 supplements can be detrimental to health and may interfere with hormonal processes that regulate weight. Choosing a high-quality, third-party tested product is vital to ensure purity and effectiveness.
Specific Medical Conditions: In some patients with conditions like cancer cachexia, high-dose omega-3 supplementation is used therapeutically to help slow involuntary weight loss. This is a targeted medical intervention, not a general side effect.

Conclusion: The Verdict on Omega-3 and Weight Gain

For the vast majority of people, omega-3s do not cause unwanted weight gain. When used in recommended dosages as part of a balanced diet, they are more likely to support weight management and improve overall body composition by enhancing metabolism and fat burning. Any perceived weight increase is more likely to be an increase in lean muscle mass, not fat. It is the imbalance of fatty acids, particularly a high omega-6 to omega-3 ratio, or poor quality products that pose a risk. As always, consulting a healthcare professional before starting any new supplement regimen is a wise step.

NIH Office of Dietary Supplements fact sheet on Omega-3s

Frequently Asked Questions

No, omega-3 supplements are not known to cause weight gain. In fact, many studies link them to benefits for weight management, such as a boosted metabolism and reduced body fat.

A standard fish oil capsule contains a very small number of calories. The additional calories from a typical fish oil supplement are unlikely to be a cause for weight gain when consumed at recommended levels.

Yes, some research suggests that omega-3 fatty acids may help reduce the accumulation of fat, particularly around the waist, and can reduce waist circumference.

Unwanted weight gain from omega-3s is extremely unlikely. Consuming excessive doses could contribute to a calorie surplus, but the amount would be substantially higher than standard recommendations. Overall dietary habits are a much larger factor.

A high ratio of omega-6 to omega-3, common in the Western diet, is linked to inflammation and weight gain. Increasing your omega-3 intake helps balance this ratio and may aid in weight management.

Fish oil may indirectly help with weight loss by increasing your metabolic rate, reducing appetite, and enhancing the effectiveness of exercise. However, it is not a magic bullet and works best when combined with a balanced diet and regular physical activity.

If your weight increases while taking omega-3s, it's more likely due to an increase in lean muscle mass, especially if you are also exercising. Omega-3s can aid in muscle building, and since muscle is denser than fat, your weight on the scale might not change even as your body fat decreases.

While ALA (from plants), EPA, and DHA (from marine sources) are all omega-3s, the long-chain marine-based EPA and DHA have been the focus of most research related to metabolism and fat loss.