Effects of supplemental fish oil on resting metabolic rate, body composition, and salivary cortisol in healthy adults
Journal Title (Medline/Pubmed accepted abbreviation): J Int Soc Sports Nutr
Year: 2010
Volume: 7
Number: 31
Page numbers:
doi: 10.1186/1550-2783-7-31

Summary of Background and Research Design

Background:Several rodent studies have shown that diets rich in omega 3 fatty acids, specifically those found in the oil from cold-water fish, lead to significantly lower total body fat stores versus diets rich in other fatty acids. Despite these data in animal models, very little is known about the effects of omega 3 fatty acids on body composition and metabolic rate in humans. Preliminary findings suggested a negative regulatory effect of cortisol on body fat distribution, and there is limited evidence that fish oil supplementation can reduce cortisol levels. However, it is unknown whether treatments that lower cortisol levels can positively affect body composition.

Hypothesis/purpose of study: To determine the effects of supplemental fish oil on body composition and resting metabolic rate, determine the effects on morning salivary cortisol concentrations, and determine whether there is a relationship between changes in salivary cortisol concentration and body composition changes

Subjects: Fourteen men and 30 woman (mean ± standard deviation: age 34 ± 13 years, weight 71 ± 15 kg, body fat 29% ± 9%) participated in the study. Subjects were healthy, active and maintained their current diet and exercise practices throughout the study. Individuals who ate fatty fish at least 3 times a month, or were supplementing their diet with omega 3 fatty acids, or had a known metabolic or endocrine disorder were excluded.

Experimental design: Randomized, double-blind study

Treatments and protocol:Subjects reported to the laboratory in the morning following a 10- to 12-hour overnight fast for body composition assessment using air displacement via the Bod Pod® (n = 44) and resting metabolic rate (RMR) determination using open-circuit indirect calorimetry (n = 24). Following these tests, a saliva sample was collected via passive drool and later analyzed for cortisol content. Subjects were randomly assigned to 1 of 2 groups: 4 g/day of Safflower oil (SO), or 4 g/day of Fish oil (FO) supplying 1,600 mg/day eicosapentaenoic acid (EPA) and 800 mg/day docosahexaenoic acid (DHA). All tests were repeated following 6 weeks of treatment.

Summary of research findings:
  • No significant differences were observed for body mass between the treatments (SO = 0.2 ± 0.8 kg; FO = 0.0 ± 0.9 kg; P = .52)
  • There were significant differences for fat free mass (SO = -0.1 ± 1.2 kg; FO = +0.5 ± 0.5 kg; P = .03) and for fat mass (SO = 0.2 ± 1.2 kg; FO = -0.5 ± 1.3 kg; P = .04)
  • Percentage of body fat also tended to change differently over time between the treatments (SO = 0.3% ± 1.5%; FO = -0.4% ± 1.3%; P = .08)
  • No significant differences between groups were observed for resting metabolic rate (SO = -62 ± 184 kcal, FO = 17 ± 260 kcal; P = .40) or for the respiratory exchange ratio (SO = 0.023 ± 0.54; FO = -0.019 ± 0.85, P = .16)
  • There was a tendency for salivary cortisol concentrations to change differently over time between the 2 treatments (SO = 0.016 ± 0.272 µg/dL; FO = -0.072 ± 0.142 µg/dL; P = .11)
  • The change in salivary cortisol concentration in the FO group was significantly correlated with the change in percentage of body fat (r = 0.638, P = .001), the change in fat-free mass (r = -0.504, P = .02) as well as the change in fat mass (r = 0.661, P = .001)
  • No significant correlations were observed in the SO group between the change in salivary cortisol concentration and the change in percentage of body fat ( r = -0.321; P = .17), change in fat-free mass (r = 0.007; P = .98), or the change in fat mass (r = -0.309; P = .19)

Interpretation of findings/Key practice applications:

Similar to previous studies, the current trial showed that 6 weeks of supplemental fish oil significantly increased lean mass, and significantly reduced fat mass in healthy adults. The change in fat mass following fish oil treatment significantly correlated with the change in salivary cortisol (r = 0.661, P = .001). Given the short duration of this study, it is unclear how these changes would impact long-term body composition changes and more research is needed to determine the impact of chronic fish oil supplementation on long-term body composition. Also, more research is needed regarding the impact of alterations in cortisol levels that occur within the normal physiological range on body composition. on body composition.
One potential strength of the study was the use of enteric coated fish oil capsules, which could prevent the “fish burps” that might allow subjects to ascertain which treatment they were receiving. However, the authors did not report data regarding the subjects' ability to correctly guess which treatment they were receiving. A major limitation of the study is that the authors did not analyze dietary intake of the subjects. Thus, it is not known if the subjects might have altered their food intake more so in the FO vs SO group, especially if they were able to correctly guess which treatment they were receiving. In addition, the subjects in this study were not engaged in an exercise program. Given that the changes in body composition in this study were modest, albeit statistically significant, it would have been interesting to see whether the fish oil supplement would have produced more pronounced results when used in conjunction with exercise.
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