This study, like many others on inexpensive nutrients, seems designed to fail with it's extremely low dosage of 400IU.

If you look into vitamin D, you'll find that the body requires something like 10s of thousands of IU per day. Vitamin D experts have long been saying that daily supplementation should be more like 4000 IU rather than 400.

Also, our receptors can become dormant or muted when our vitamin D tank is low.

Any study of the effects of vitamin D on health should include a group that has been tested and is vitamin D replete.

There are currently NCI studies underway studying higher doses of vitamin D on various cancers---both as preventives and treatments.