I had muscle cramps, fatigue, dry skin .... the calcium helps the Vitamin D or D3 absorb and most have calcium diphosphate included I think.
Definitely speak with your doctor ... Vitamin D can be toxic if taken too much and what's too much for one is different than another person depending on your personal levels.
Here's super info :
http://www.medhelp.org/health_pages/Weight-Loss/Dietary-Supplement-Fact-Sheet-Vitamin-D--Page-1-of-3/show/1005?cid=129
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Without treatment, vitamin D deficiency can have significant health effects including:
Bone and muscle pain
Rickets (soft, weakened bones) in children
Osteopenia (weak, fragile bones – also known as osteomalacia or osteoporosis) in older adults
Muscle weakness
Chronic fatigue
Cold/Influenza
Allergies
Asthma
Tuberculosis
Depression
Seasonal Affective Disorder
Gum disease/tooth decay
High blood pressure
Coronary heart disease
Cancer
Type 1 diabetes mellitus
Multiple sclerosis
Rheumatoid arthritis
Treatment for a vitamin D deficiency is vitamin D3 supplements. You can find this supplement at pharmacies and health food stores. The Recommended Daily Amount for vitamin D is 400IU - 600IU (many health professionals state this is far too low) and the Upper Tolerable Limit is 4000IU daily. Higher amounts of vitamin D can be taken temporarily to correct a deficiency state. I wouldn't recommend over 10 000IU of vitamin D3 without medical supervision.
Magnesium is the most important co factor for vitamin D absorption. Most magnesium is stored in the tissues, so leg cramps, foot pain or muscle twitches are usually the first signs of a magnesium deficiency. To add, both vitamin D and magnesium are needed for calcium absorption.