“None of the studies reported a statistically signiﬁcant effect of ﬁsh oil treatment relative to placebo. Further analysis showed no relationship between body weight change with intervention duration, or n-3 dosage, nor any funnel effect with study sample size. This analysis does not support the hypothesis that daily n-3 oil supplementation reduces body weight and BMI in the overweight and obese.”
First, drawing such a definitive conclusion based on data from only 4 studies, that are heterogeneous, is misleading. Second, the studies didn’t provide enough information to allow the meta-analysis to adjust for factors that might abrogate the fat loss effect of the long-chain omega-3 fatty acids EPA and DHA, found in fatty fish and fish oil.
Factors abrogating the anti-obesity effect of omega-3 fatty acids
The macronutrient composition of the diet affects the effects of fish oil [6, 7]. For example, the amount and type of carbohydrates, the levels of omegs-6 fats, linoleic acid in particular, in the background diet might influence the anti-obesity effect of n-3 PUFAs . Also, medications have been shown to interfere with the effects of omega-3 fatty acids [8, 9].
However, none of the studies included in the meta-analysis provided detailed data on diet composition. The following was the information given:
Krebs 2006 ;
Low-fat high-carb energy restricted diet – 50% carbs, 35% fat, 15% protein.
Hill 2007 ;
All subjects were instructed to maintain their normal diet during the study. If not asked to exercise as part of the intervention, subjects were instructed to maintain their normal level of physical activity. Data on diet composition not provided. Kabir 2007 ;43% carbs, 37% fat, 20% protein.
The subjects (diabetics) were asked to keep their initial caloric intake and nutrient proportions constant throughout the study. Patients were asked to complete a 7-d food diary just before the start of the treatment period. They were recommended to keep their initial caloric intake and nutrient proportions constant throughout the study. To determine compliance with the dietary recommendations, the patients were asked to keep another food diary to be completed the last 7 d of each treatment period. Even if this method (7-d food diary) of measuring food intake might slightly underestimate true calorie intake, the same method was used before and after treatments, and hence the results can be compared.
Kratz 2009 ; Non-energy restricted ad libitum diet.
The omega-3 diet contained 1.4% of energy in the form of marine omega-3s (EPA + DPA + DHA), and 2.2% of energy in the form of α-linolenic acid (18:3n-3) from plant oils. The control diet provided 0% of energy in the form of marine omega-3s and 0.5% of energy as alpha-linolenic acid.
As discussed in a review published in 2010, the lack of consensus on the fat loss effect of fish oil in human intervention studies is mostly related to study design . This review concluded that human intervention trials indicate potential benefits of long-chain omega-3 supplementation, especially when combined with energy-restricted diets or exercise, but more well-controlled and long-term trials are needed to confirm these effects and identify mechanisms of action .
Many factors have to be taken into consideration when evaluating the effect of fish oil on fat loss; dose (both total omega-3 and individual omega-3 fatty acids), ratio of EPA to DHA in the supplement, background diet, calorie restriction versus non-calorie restriction, duration etc. As of this writing, there aren’t enough studies on which to draw a definitive conclusion, as was done in the meta-analysis. If the meta-analysis had added as an inclusion criterion “studies that report data on confounding factors” there would be no meta-analysis.
1. Harden, C.J., et al., Preliminary meta-analysis of the effect of fish oil on body weight and body mass index in overweight and obese subjects does not support a link. Proc Nutr Soc, 2013. 72: p. E283.
2. Kratz, M., et al., Dietary n-3-polyunsaturated fatty acids and energy balance in overweight or moderately obese men and women: a randomized controlled trial. Nutr Metab (Lond), 2009. 6: p. 24.
3. Hill, A.M., et al., Combining fish-oil supplements with regular aerobic exercise improves body composition and cardiovascular disease risk factors. Am J Clin Nutr, 2007. 85(5): p. 1267-74.
4. Krebs, J.D., et al., Additive benefits of long-chain n-3 polyunsaturated fatty acids and weight-loss in the management of cardiovascular disease risk in overweight hyperinsulinaemic women. Int J Obes (Lond), 2006. 30(10): p. 1535-44.
5. Kabir, M., et al., Treatment for 2 mo with n 3 polyunsaturated fatty acids reduces adiposity and some atherogenic factors but does not improve insulin sensitivity in women with type 2 diabetes: a randomized controlled study. Am J Clin Nutr, 2007. 86(6): p. 1670-9.
6. Hao, Q., et al., High-glycemic index carbohydrates abrogate the antiobesity effect of fish oil in mice. Am J Physiol Endocrinol Metab, 2012. 302(9): p. E1097-112.
7. Madsen, L. and K. Kristiansen, Of mice and men: Factors abrogating the antiobesity effect of omega-3 fatty acids. Adipocyte, 2012. 1(3): p. 173-176.
8. de Lorgeril, M., et al., Recent findings on the health effects of omega-3 fatty acids and statins, and their interactions: do statins inhibit omega-3? BMC Med, 2013. 11: p. 5.
9. Eussen, S.R., et al., Effects of n-3 fatty acids on major cardiovascular events in statin users and non-users with a history of myocardial infarction. Eur Heart J, 2012. 33(13): p. 1582-8.
10. Buckley, J.D. and P.R. Howe, Long-chain omega-3 polyunsaturated fatty acids may be beneficial for reducing obesity-a review. Nutrients, 2010. 2(12): p. 1212-30.