Before I comment on this study, I want to say  … I noticed that Labdoor, a leading supplement testing lab, has updated their fish oil tests to include 51 supplements, up from 30. They’re still finding that all 51 of those best-selling fish oil supplements “contained measurable amounts of mercury, with 3 products recording 50% or greater of the allowable mercury content per serving.” Many contained either less or more omega-3 than was stated on the label. That’s because no agency, government or otherwise, is regularly checking that what is on a supplement’s label is what is in the bottle. But people love their fish oil, mercury or not.

On to the study … Krill and salmon oils are the next one-up from run-of-the-mill fish oils. Like fish oil, they’re not without their problems. This randomized control trial found that men who took a blend of krill and salmon oil for 8 weeks had reduced insulin sensitivity. When cells become resistant to insulin, glucose can’t get in and blood levels of glucose rise – a precursor to diabetes.

Supplementation With A Blend Of Krill And Salmon Oil Is Associated With Increased Metabolic Risk In Overweight Men, American Journal of Clinical Nutrition, July 2015

Background: Krill is an increasingly popular source of marine n–3 (ω-3) PUFA that is seen as a premium product. However, to our knowledge, the effect of krill-oil supplementation on insulin sensitivity in humans has not been reported.

Objective: We assessed whether supplementation with a blend of krill and salmon (KS) oil [which is rich in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)] affects insulin sensitivity in overweight men.

Design: The design was a randomized, double-blind, controlled crossover trial. A total of 47 men with a mean ± SD age of 46.5 ± 5.1 y, who were overweight [body mass index (in kg/m2) from 25 to 30] but otherwise healthy, received 5 1-g capsules of KS oil or a control (canola oil) for 8 wk and crossed over to another treatment after an 8-wk washout period. The primary outcome was insulin sensitivity assessed by using the Matsuda method from an oral-glucose-tolerance test. Secondary outcomes included lipid profiles, inflammatory markers, 24-h ambulatory blood pressure, and carotid artery intimamedia thickness.

Results: Unexpectedly, insulin sensitivity (per the Matsuda index) was 14% lower with the KS oil than with the control oil (P = 0.049). A mediation analysis showed that, after controlling for the likely positive effects of blood EPA and DHA (i.e., the omega-3 index), the reduction in insulin sensitivity after KS-oil supplementation was more marked [27% lower than with the control oil (P = 0.009)].

Conclusions: Supplementation with a blend of KS oil is associated with decreased insulin sensitivity. Thus, krill-oil supplementation in overweight adults could exacerbate risk of diabetes and cardiovascular disease.