I am currently recommending almost all of my patients to be on Vitamin D from now until early summer. Vitamin D is an essential nutrient to bolster the immune system. People on Vitamin D are less likely to get colds and flu. For adults, I recommend 5,000 IU per day. For children, I recommend 2-4,000 IU/day. In honor of the flu that has been going around, this February Vitamin D will be 20% off. Read more on Vitamin D here.