The evidence that vitamin D protects against breast cancer is suggestive, but inconclusive. Many types of cancer are currently being studied under the influence of vitamin D. Colon, pancreatic, and prostate cancer are showing significant reductions of incidence with the use of vitamin D supplements of 1000 IU or more per day. One study suggests significant reductions in total cancer. Animal studies offer a surplus of evidence that vitamin D protects against cancer.
Vitamin D decreases cell proliferation and is a powerful anti-inflammatory. There is preliminary evidence that it may lower the risk of multiple sclerosis, rheumatoid arthritis, lupus, heart disease, autism, and autoimmune thyroid disease. Vitamin D also plays a role in depression. Most studies are too new to show any long term effects. Firm conclusions will be based on further research.
For now, shoot for 700 to 1,000 IU of vitamin D per day. Most multi-vitamins supply 400 IU. Expect only 40 - 150 IU in most foods fortified with vit. D. Toxicity levels have not yet been established, though studies using as much as 3000 IU/day have shown no ill effects. Farmed salmon has about 1/4 the vit. D of wild salmon. Only a few foods (like fatty fish) have more that 220 IU of vit. D per serving. Ultra violet rays from the sun can prompt your skin, liver, and kidneys to make vit. D, but UV rays are too weak in the winter (unless you live as far south as Los Angeles or Atlanta), or use sunscreen. That's why it may be simpler to get vit. D from a supplement.