by Randy Hartnell and Craig Weatherby
We received this perceptive inquiry from a reader perplexed by an aspect of last week’s article on vitamin D and bone health:
So, do we know how salmon become vitamin D rich? I was surprised to hear the skinless fish was so rich in vitamin D, since it is stored in fat. They probably don’t process sunshine. Is their liver making vitamin D?
Sockeye and vitamin D: case solved
Peggy’s question was a good one, and we weren't certain of the answer, so we sought a scientific explanation for the unexpectedly high vitamin D levels in our skinless sockeye fillets. Vitamin D is fat-soluble, so it concentrates in the fat under the skin of fish, and in their fat-rich livers.
Yet, as the chart in our last issue shows, lab tests show that our sockeye salmon contains more vitamin D than fattier Vital Choice fish such as sardines, sablefish, and king salmon (To view it, click here).
Why would our skinless sockeye fillets test higher in vitamin D than fillets of fish with fattier flesh?
I had a hunch that answer might lie in the zooplankton that constitute so much of their diet. Zooplankton is a term that covers a variety of tiny marine animals, including larval-stage crustaceans.
Indeed, a quick search of the scientific literature confirmed that phytoplankton (single-cell marine organisms) and zooplankton are the key sources of vitamin D in the marine food chain. Both types of minuscule sea life create large amounts of vitamin D from sunlight, and zooplankton feed on phytoplankton, thus driving their vitamin D content to even higher levels.
Unlike most other fish and salmon species (except chum), sockeye feed largely on zooplankton through all stages of life, so it makes sense they would accumulate unusually high levels of vitamin D in the fat deposited throughout their flesh. Note: There are some seasonal and geographic variations in sockeye diets, so vitamin D levels will vary.
It seems that my hunch was right, and that the mystery of sockeye and vitamin D is solved. Thanks, Peggy… your question prompted us to dig a little deeper!
- Bjorn LO, Wang T. Vitamin D in an ecological context. Int J Circumpolar Health. 2000 Jan;59(1):26-32.
- Holick MF. Evolutionary biology and pathology of vitamin D. J Nutr Sci Vitaminol (Tokyo). 1992;Spec No:79-83. Review.
- Holick MF. Evolution and function of vitamin D. Recent Results Cancer Res. 2003;164:3-28. Review.
- Davis ND, Armstrong JL, Meyeres KW. Bering Sea salmon food habits: Diet overlap in fall and potential for interactions among salmon. Accessed online November 4, 2005 at http://www.fish.washington.edu/research/publications/pdfs/0311.pdf