There has been a plethora of published information on the myriad functions and benefits of Vitamin D, its normal levels, its perceived global deficiency, and the many dietary ways to supplement it.  Because our current state of knowledge is far from satisfactory, extrapolated opinions have proliferated beyond scientific evidence, and colorful myths have filled our knowledge vacuums.

         When a very specific band of Ultraviolet B Sunlight penetrates our skin, it transforms its Vitamin D precursors into Vitamin D3, which then must pass through a liver transformation step followed by a final kidney activation step before it becomes Calcitriol, which is the biologically active form of Vitamin D.  Animal sources of Vitamin D are mainly deep-sea-fatty-fish, egg yolks, and liver.

         Measuring the biologically active Vitamin D, Calcitriol, for many reasons, is not practical.  Instead, its circulating, liver-processed form, pro- Calcitriol, is used for defining deficiency and sufficiency.  Current standards, which are still debatable, state that a level less than10 ng/ml suggests deficiency, a level between 10 to 30 ng/ml suggests insufficiency, a level of 30 to 76 ng/ml suggests sufficiency, and levels above 76 ng/ml may be harmful.  Depending on which method is used for measurement, these values could vary and they will certainly change with future research.

         Historically, Francis Glisson in 1650 was the first to publish a medical treatise on the common childhood bone disease known as Rickets.  Sniadecki was the first to report in1882 the association of Rickets with the lack of sunlight.  By the 19^th century, cod liver oil had become an effective treatment for Rickets and the work of Mellanby & McCollum led to the discovery of Vitamin D as the active ingredient in cod liver oil.  This led to the fortification of milk and other foods with Vitamin D in the 20^th century causing Rickets—and the adult form of Rickets known as osteomalacia—to become uncommon in the developed world.

         Calcitriol, the only biologically active form of Vitamin D, is a true hormone that acts mainly on the first part of the small intestine, the duodenum, to enhance calcium absorption.  It also acts on the bone-making and bone-breaking cells that mobilize skeletal calcium.

         The levels of Vitamin D achieved with supplements, diets, or sun exposure vary with the individual’s race, age, body fat, and genetic factors as do the clinical manifestations of deficiency and insufficiency.  The two most important clinical conditions that result from Vitamin D deficiency are:  

         1. Nutritional Rickets, a disease of children, results from inadequate mineralization of growing bones and continues to be prevalent among those breast-fed children all over the world who have no access to preventive medical care.  It leads to leg and ribcage deformities, bone pains, delayed walking and growth, and frequent falls.

         2. Osteomalacia, results from poorly mineralized adult bones and leads to bone pains, muscle weakness and cramps, gait instability, and false fractures seen on x-rays.  The blood levels of calcium and phosphorus are usually low and the bone enzymes are high when the Vitamin D deficiency is severe.

         There are many purported benefits of Vitamin D besides improving bone and muscle health but evidence for such benefits is observational (soft) and not derived from randomized controlled studies:

         1. High quality studies have shown a reduction in falls and fractures in those elderly who take at least 800 D units/day, whereas taking ½ a million units once a year paradoxically increased the falls and fractures.

         2. An observational study in individuals older than 65 whose Vitamin D levels were over 40 ng/ml showed a 45% lesser mortality than those with levels less than 10 ng/ml, but it is not known whether the levels were the cause or the result of this lower mortality.

         3. Other observational studies hint at reduced cardiovascular mortality and reduced risks of high blood pressure, diabetes, obesity, and high triglycerides.

         4. Observational, uncontrolled studies hint at a reduced colon and breast cancer risk in those with robust Vitamin D levels.  Other common cancers do not seem to be reduced by higher Vitamin D levels.

         5. The incidence of multiple sclerosis increases in higher latitudes where less sun exposure and lower levels of Vitamin D are common.  Children with low Vitamin D levels may have more asthma, bronchitis, pneumonia, and schizophrenia.  Overweight individuals with low Vitamin D levels have more depression and those who receive Vitamin D improve.  Certain body aches and muscle cramps may also respond to Vitamin D.

         6.  Both Vitamins D2 and D3 are widely available as supplements and are equally effective.  Taking 800-2000 units daily may be wise in those who are elderly, dark skinned, or those who do not get enough sun.

         To conclude, information about Vitamin D is rapidly evolving but scientific knowledge that can guide clinical decisions is still lacking.  It is best to proportion our beliefs to the evidence.