I first became familiar with D3 1000 by Metagenics through a chiropracter and began using the 5000 strength last spring. Up here in the Northeast USA many folks have Vitamin D deficiencies because of long winters and lack of time in the sun without being bundled up with warm clothing. I definitely noticed this passing winter season that I was not depressed like I am most winters; we had snow storm after snow storm but my ""cabin fever"" was not that bad; I actually enjoyed being in the house crocheting and organizing my photos. Recently I've switched to the 1000 formula because my vitamin D level is in the healthy range.