Abstract
The omega-3 index, which is the red blood cell (RBC) level of two omega-3 (n-3) fatty acids [FA; eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)], is inversely related to risk of cardiovascular disease in humans. In the US, the average human omega-3 index is about 5% (i.e., EPA+DHA constitutes 5% of the FA in the RBC membrane). The RBC FA composition is a valid surrogate for that of other tissues, most importantly, the heart. Little is known about range of physiologically-possible levels for the omega-3 index in mammals, or which other RBC membrane FA levels fall as the omega-3 index rises. Although an examination of the FA composition of RBCs in piscivorous mammals would be of interest in this regard, there are, to our knowledge, no published data available. Therefore, the purpose of this study was to compare the RBC FA composition of a common marine mammal, the bottlenose dolphin (Tursiops truncatus), with that of humans. RBCs were isolated from routine blood samples collected from healthy dolphins at two SeaWorld facilities (San Diego, CA and Orlando, FL). We used pooled human RBC FA composition data from our laboratory (n = 11,329) as a comparator. FAs (n = 25) were analyzed after direct methylation by gas chromatography and expressed as a percent of total FAs. There were 26 female and 9 male dolphins included whose average (SD) age was 15 (11) years, with a range of 1 to 45 years. The omega-3 index did not vary by sex or age. The mean omega-3 index of the dolphins was 19.9% (1.4%) percent compared with 5.6% (2.4%) for humans. The primary driver of the omega-3 index was EPA 15.3% (1.6%) vs 1.0% (1.1%); DHA levels were virtually identical [dolphin 4.6% (0.9%) vs 4.6% (1.6%)]. Other FA differences included the n-6 essential FAs linoleic acid and arachidonic acid. The former was markedly lower in dolphins vs humans [0.5% (0.1%) vs 12.2% (2.5%)] whereas the latter was similar [12.3% (2.2%) vs 14.4% (1.9%)]. Dolphins, which only consume fish (providing > 100 g of EPA+DHA/day), have an omega-3 index that is 3.6-fold higher than that found in US adults (who consume only 150 mg of EPA+DHA per day on average). The difference in the omega-3 index is due entirely to a 15-fold higher level of EPA in dolphin RBC membranes; DHA levels are virtually identical in humans and dolphins. It is surprising that only EPA is elevated when dolphins are consuming similar amounts of EPA and DHA. The high levels of the "essential" fatty acid, linoleic acid, in human RBC membranes come from the pervasive amounts of vegetable oils in the US diet; dolphins consume very little linoleic acid as reflected in their membranes. Arachidonic acid, which is derived from the diet and from conversion from linoleic acid, is at similar levels in human and dolphin RBC. These differences in RBC FA composition reflect major differences in diet but also in metabolic control of RBC membrane composition. The physiologic implications of the high EPA levels in dolphins remain to be determined.
Acknowledgements
The authors would like to thank the veterinary and animal training staff at SeaWorld San Diego and SeaWorld Orlando for their help with sample collection.
*Presenting author