Low vitamin D linked to longer lifespan
A study focused on the families of people who had at least two siblings who were 89 years or older. Middle-aged children of the long-lived siblings and the life partners of the middle-aged children were included in the Leiden Longevity Study. The vitamin D levels were found to be lower in the middle-aged children than for life partners. The life partners were used as the control group in comparison to the middle-aged children based on the idea that they would have shared similar diets and lifestyles. Vitamin D levels but not hormone D levels were mentioned.  The numerical lab values were not available.The middle-aged children of the families with at least two 89-91 or older siblings were found to have less cardiometabolic disease indicators such as myocardial infarction, diabetes and hypertension compared to their life partners. The group of middle-aged children of 90 year olds also had lower values for blood sugar, insulin, triglycerides and improved insulin sensitivity. 
Low levels of vitamin D have been associated with many diseases, however hormone D levels were not measured in most of the studies. The vitamin can be converted to the active hormone by an enzyme that is typically well controlled by the kidneys but is also made by some types of cancer cells, white blood cells and in some autoimmune conditions. Elevated hormone D levels can cause excessive loss of calcium from bone tissue and calcification of soft tissue.
Supplements of vitamin D frequently need to be taken in amounts far greater than the standard recommendation in order to raise vitamin D levels. An association with disease does not prove whether the depressed vitamin D levels are a cause or a side effect of chronic disease. Measuring both the vitamin 25(OH)D and the hormone 1, 25(2OH)D levels would be more likely to reveal whether excessive enzyme activity was causing a decrease in vitamin D levels and an increase in hormone D levels rather than there being an actual deficit of vitamin D. Chronically elevated hormone D is associated with osteoporosis and other health issues.
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Canadian Medical Association Journal (2012, November 5). Low vitamin D levels linked to longevity, surprising study shows. ScienceDaily. Retrieved November 11, 2012, from [http://www.sciencedaily.com /releases/2012/11/121105130355.htm]
van Heemst D., Leiden Longevity Study, Longevity Research Background, in Dutch, [http://www.langleven.net/index.cfm?p=1B8B2369-3048-9110-624DBCCA13F65FE9]
/Disclosure - the author of this post had two grandparents who lived beyond 90 years old; and has had low to normal values for vitamin D and normal to elevated lab values for hormone D for more than five years of testing.
*January 2021 addition: Too much vitamin D seems to be problem for me - genetic over activity of the vitamin D receptor may be part of it. Transport proteins are also needed for holding a supply of vitamin D in the inactive 25-D form. Without the protein transport the body can't keep the 25 D from converting to the 1, 25-D form which is the hormone form that can activate the Vitamin D Receptor. /
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