Thus far, the study has shown that omega-3 fatty acids appear to increase weight loss, fat loss and abdominal fat when consumed in fish (salmon) or as capsules. This had already been reported. The next finding is the most remarkable of the study, in that the researchers discovered that the group eating a lean fish (cod), which contained only one-tenth as much omega-3 fatty acids as the salmon diet, also exhibited significantly greater weight loss than the control group, nearly as much as the salmon and fish oil capsule groups. Further, the cod group appeared to hold on to lean mass a little better than the salmon group, though the difference was not statistically significant.3

This study is the first to report the positive weight-loss effects of lean fish. In attempting to explain the finding, the researchers discussed the potential role of the amino acid taurine. Taurine is an ingredient in certain energy drinks, including Red Bull. Taurine has been reported to induce weight loss in humans when supplemented in moderate amounts (3 grams per day).10 A recent animal study has demonstrated that taurine supplementation prevented obesity, in part by raising energy expenditure.11 The authors of the animal study speculated that taurine supplementation may lead to an increase in a co-activator called PCG-1a. PGC-1a has been shown to be a key regulator of energy metabolism, and its activation is an element of beta-adrenergic stimulated fat loss (ephedrine/caffeine-like drugs).12

As taurine is limited in the typical Western diet, and concentrated in fish, it is likely that this is a major contributor to the fat-loss effect seen in the lean fish group. Given that salmon is high in both omega-3 fatty acids and taurine, one would expect it to have a much greater effect than either cod or the fish oil capsules. However, this was not the case.13 It is likely that rather than having a synergistic effect of combining the fat loss benefits of two different pathways, omega-3 fatty acids and taurine present in combination in salmon appear to have no more than a slight additive effect, suggesting they might support the same fat-loss pathway.
It was also interesting to note that adding fish to the diet did not increase weight loss or fat loss in women. It is unclear why this is the case. However, as the subjects in this study were between the ages of 20 and 40, all the women would be premenopausal, and thus have relatively high estrogen concentrations. Estrogens increase the activity of a class of receptor in fat cells called the a-2 adrenergic receptor, which reduces the fat loss effect of beta-adrenergic stimulation.14 If omega-3 fatty acids and taurine act through the beta-adrenergic pathway to induce weight loss, it is understandable how this route would be less effective for women. Further, research published in 2002 demonstrated that women have a higher capacity for synthesizing DHA (one of the two main fatty acids in fish oil) from alpha-linolenic acid (an essential fatty acid present in the diet).15,16 Thus, women would have higher levels of DHA than men and benefit less from fish oil supplementation.