I do not absorb my Vitamins very well; which have resulted in a Vitamin D deficiency. This is not uncommon especially as we age and take such precaution with being in the SUN. I live in Texas; SUNNY year round weather and still have a deficiency. I experienced extreme Fatique, Headaches, Muscle Pain like Fibromyalgia. I've had Fibromyalgia for 20 years. After starting on Rx. Vitamin D and a 5,000 IU supplement 3 x's a week, I feel so much better!!! My endocrinologist, routinely checks my Vit. D levels since it is involved with your Thyroid function. (I'm ypothyroid). Learn something new every day!! D.White