Can fish oil cause weight gain? Unpacking the omega-3 myth

October 4, 2025 •

5 min read

While a single teaspoon of fish oil contains around 45 calories, experts suggest it's highly unlikely that the supplement will cause weight gain when consumed within the recommended daily dosage. The question, 'Can fish oil cause weight gain?', often stems from misconceptions about fats and calories, but the science points to a more nuanced answer.

Quick Summary

Fish oil provides a small number of calories, but typically does not lead to weight gain at standard doses. Research indicates omega-3s can aid weight management by boosting metabolism, controlling appetite, and improving body composition by increasing lean mass and reducing body fat.

Key Points

Low Caloric Impact: A standard fish oil softgel contains very few calories, making it unlikely to contribute to weight gain when taken at recommended dosages.
Metabolism Boost: The omega-3s (EPA/DHA) in fish oil may increase your resting metabolic rate, leading to more calories burned throughout the day.
Improved Body Composition: Fish oil can help increase lean muscle mass and reduce body fat, potentially resulting in no net change on the scale but a healthier physique.
Fat-Burning Effects: Some studies suggest that fish oil can amplify the number of calories and amount of fat burned during exercise.
Appetite Regulation: Research on fish oil's effect on appetite is mixed; some studies report increased satiety, while others show no significant change.
Quality is Key: Poor-quality or rancid fish oil can cause inflammation, which may negatively impact weight management. Always choose a high-quality, pure product.

Fish oil and calories: A closer look

Fish oil, like all dietary fats, contains calories. Each gram of fat provides approximately 9 calories, meaning a standard 1,000 mg fish oil softgel contains around 10 calories. In the grand scheme of an average daily caloric intake, this is a negligible amount. The focus on fish oil's potential to cause weight gain often overlooks the broader picture of total daily energy balance.

The importance of omega-3s

Fish oil's primary health benefits come from its omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These are essential fats that the body needs for various functions, including supporting heart and brain health, reducing inflammation, and regulating metabolism. Most health professionals recommend getting 200–300 mg of combined EPA and DHA daily, an amount easily obtained without significant caloric impact.

The complex relationship with metabolism

Far from causing weight gain, omega-3s may have a positive influence on metabolism. Several studies have explored how fish oil affects the body's metabolic rate, which is the speed at which it burns calories.

Boosting resting metabolic rate (RMR): Research has shown that omega-3 supplementation can increase a person's RMR, particularly in individuals who are overweight or obese. A higher RMR means more calories are burned at rest, which can support weight management efforts.
Enhancing fat burning during exercise: Some studies suggest that fish oil may encourage the body to use fat, rather than carbohydrates, as a primary fuel source during exercise. This metabolic shift could potentially enhance the fat-burning effects of physical activity.
Improving insulin sensitivity: Better insulin sensitivity allows the body's cells to utilize glucose more effectively, reducing the likelihood of storing excess sugar as fat. Omega-3s have been shown to improve insulin sensitivity, creating a more favorable environment for fat burning.

Impact on appetite and body composition

While some research on appetite and fish oil has yielded mixed results, several studies point to its potential benefits for weight control. In a study of individuals on a calorie-restricted diet, those with higher fish oil intake reported feeling significantly fuller for up to two hours after a meal. However, other studies found no significant effect on overall appetite scores. These inconsistent findings suggest that the effect on appetite may vary among individuals depending on factors like health status and diet.

It is also important to consider body composition. Weight is not the only metric for success. Some studies that found no change in overall body weight still reported a reduction in body fat and an increase in lean muscle mass among participants taking fish oil. This change in body composition is often more desirable and beneficial for long-term health than a simple change in weight on the scale. Omega-3s may specifically target visceral fat, the dangerous fat stored around organs, which is linked to metabolic issues.

The crucial role of supplement quality

The quality of a fish oil supplement can also impact its effects. Poor quality or rancid fish oil can undergo oxidative damage, which generates free radicals and can lead to inflammation. Chronic inflammation can hinder weight management efforts and contribute to weight gain. Choosing a high-quality product that has been independently tested for purity and freshness ensures you get the intended health benefits without unwanted side effects. Factors such as the source of the fish, processing methods, and freshness can all influence the supplement's effectiveness and purity.

Comparison of fish oil and other fats

To understand why fish oil is beneficial for health despite being a fat, it's helpful to compare it to other common dietary fats. The key lies in the type of fatty acids present and their physiological effects.


Feature	Fish Oil (Omega-3 Rich)	Saturated Fat (e.g., Butter)	Omega-6 Rich Fat (e.g., Corn Oil)
Fatty Acid Profile	High in EPA and DHA polyunsaturated fatty acids.	Predominantly saturated fatty acids.	High in linoleic acid polyunsaturated fatty acids.
Effect on Metabolism	May increase metabolic rate and fat oxidation.	Can be stored as fat if consumed in excess.	High intake linked to weight gain and obesity.
Impact on Inflammation	Anti-inflammatory effects.	Neutral or pro-inflammatory in excess.	Pro-inflammatory when the omega-6 to omega-3 ratio is high.
Heart Health	Improves triglycerides, blood pressure, and vascular function.	Excess intake can increase LDL cholesterol levels.	High intake and ratio linked to increased risk of cardiovascular disease.

Conclusion: Can fish oil cause weight gain?

So, can fish oil cause weight gain? In short, it is highly unlikely when consumed in moderate, recommended doses. While it does contain calories, the amount is minimal and typically overshadowed by its metabolism-boosting and fat-burning properties. The key to weight management is a holistic approach involving a balanced diet, regular exercise, and an overall caloric balance. High-quality fish oil supplements can be a beneficial part of this strategy, supporting your body’s metabolic function, improving body composition, and aiding in appetite control. The fear of gaining weight from fish oil should not deter anyone from leveraging its numerous health advantages.

For more information on the health benefits of omega-3s, you can refer to authoritative sources like the National Institutes of Health.

Choosing the right fish oil

When selecting a fish oil supplement, prioritize purity, freshness, and the concentration of EPA and DHA. Look for products that provide a certificate of analysis or are third-party tested to ensure they are free of contaminants like mercury and PCBs. High-quality fish oil provides the benefits of omega-3s with minimal risk.

Final summary

Key takeaways: Fish oil, when taken in recommended doses, is not likely to cause weight gain due to its minimal caloric content. Instead, its omega-3 fatty acids may support weight management by boosting metabolism, enhancing fat burning during exercise, and improving body composition. The quality of the supplement is important, with high-quality, non-rancid options being the most beneficial. A balanced approach combining fish oil with a healthy diet and regular exercise is the most effective strategy for managing weight and overall health. The potential benefits far outweigh the minimal risk of weight gain associated with its calorie content.

Disclaimer: Always consult with a healthcare provider before adding any new supplement to your diet, especially if you have pre-existing medical conditions or are taking other medications.

Frequently Asked Questions

A standard 1,000 mg fish oil softgel typically contains around 10 calories. This is a very small number and is unlikely to cause weight gain when taken in recommended doses.

While not a magic weight loss pill, fish oil may support weight management. It can boost your metabolism, increase fat burning during exercise, and help reduce body fat, especially around the waist.

Studies on fish oil's effect on appetite are inconsistent. Some have found that it can increase feelings of fullness, especially for those on a diet, while others have reported no significant change or even a slight increase in appetite, depending on the individual.

For weight management, it is recommended to choose a high-quality fish oil supplement that is rich in the active omega-3 fatty acids EPA and DHA, as these are the components linked to metabolic benefits.

Even at a high dose, the risk of gaining weight is low, but excessive calorie intake from any source can cause weight gain. The key is to maintain overall caloric balance in your diet. A typical recommended daily dosage is unlikely to impact your weight significantly.

There is some evidence suggesting that omega-3s in fish oil can help reduce abdominal or visceral fat, especially when combined with a healthy diet and regular exercise. These effects are often tied to its anti-inflammatory benefits.

Fish oil can positively affect body composition by helping to reduce body fat while potentially preserving or increasing lean muscle mass. This can lead to a healthier body shape even if the number on the scale stays the same.