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Your Health: Vitamin D Is Good for You, But Too Much Can Cause Problems

By Eva Briggs, M.D.

Vitamin D — what is it and what does it do?

The scientific name is calciferol. It comes in two forms. Vitamin D2 (ergocalciferol) is found in plants. One natural source is mushrooms. Commercially manufactured D2 is made by exposing yeast to ultraviolet light. Vitamin D3 (cholecalciferol) derives from animal sources. It’s found in fatty fish such as salmon and tuna, fish liver oil, beef liver, egg yolks and cheese. The major commercial source of D3 is sheep’s wool. The small intestine easily absorbs both D2 and D3. Dietary fat improves absorption of vitamin D. Both seem equally effective, although some purists prefer D3 since that is the form naturally found in humans.

After absorption into the body, the liver first metabolizes it to a form called calcidiol. Next the kidney further metabolizes it to into the active form calcitriol. Calcidiol has a longer half life in the body, therefore that’s the measure used to monitor a person’s vitamin D status. Levels less than 12 ng/mL indicates deficiency. 12-20 ng/mL is borderline. 20-50 ng/mL are adequate. Levels more than 50 ng/mL should be avoided and more than 60 ng/mL are toxic.

Vitamin D promotes absorption of dietary calcium. It helps maintain calcium levels appropriate for bone health, anti-inflammatory processes, cell growth regulation and glucose metabolism.

Vitamin D deficiency in children can cause rickets. This disease leads to soft bones with skeletal abnormalities such as bowed legs. Because breast milk is low in vitamin D, breast fed infants should receive vitamin D supplements. Vitamin D deficiency in adults can produce weak bones and dental problems. If severe it can cause seizures and cardiac
rhythm abnormalities.

What about sunlight? Ultraviolet B light enables the skin to metabolize cholesterol into vitamin D. Adequate vitamin D synthesis requires 30 minutes per day of sun exposure three times per week on the face, arms, hands and legs without sunscreen between 10 a.m. and 4 p.m. Ultraviolet B light does not penetrate window glass or sunscreen. The necessary exposure can be difficult to obtain in northern climates, in dark skinned individuals and in people who use sunscreen to reduce the risk of sunburn, skin cancer and wrinkles.

Should your vitamin D levels be checked? The US Preventive Task Force found insufficient evidence to recommend universal screening. Studies have shown conflicting evidence as to whether taking calcium plus vitamin D reduces bone fracture risk. But the studies have some drawbacks, so the recommendation is that adults consume 600 international units (IU) of vitamin D and one gram of calcium per day from food or supplements. Pregnant and lactating women and people older than 70, need more.

Other potential benefits of adequate vitamin D levels include decreased cancer risk and mortality, lower cardiovascular disease risk and benefits for Type 2 diabetes. But the data at this point is not conclusive as to whether vitamin supplementation actually provides all these benefits.

Too much vitamin D can raise calcium levels and cause gastrointestinal upset, muscle weakness, neuropsychiatric changes, kidney stones, renal failure, cardiac arrhythmias and death. Toxic levels of vitamin D increase the risk of falls and fractures.