In fact, vitamin D is best absorbed when taken with a meal containing fats or oils, so fish oil can only help with that. Absorption of the omega-3 fatty acids in fish oil won't be affected by the vitamin D.

The only excellent dietary source of vitamin D is fish liver oil — such as cod liver oil — which contains upwards of two times the Reference Daily Intake (RDI) in a single tablespoon (15 ml).

Research shows that vitamin D and omega-3s offer protection against cancer and cardiovascular disease. Life Extension recommends doses of 5,000 IU to 8,000 IU of vitamin D and 2,400 mg of EPA/DHA from fish oil daily to maintain optimal levels. Regular blood testing is important to guide adjustments to these doses to achieve the maximum benefits.