In winter our thoughts turn to vitamin D

True facts:

Winter = dark

Sunshine = vitamin D

Therefore Winter ≠ vitamin D.

Strictly speaking, “vitamin” D is not a vitamin at all.  It can be manufactured by your body when specific UVB rays (290 to 315 nanometers) of sunlight hit your skin.  Those rays cause a biochemical reaction that transforms a molecule in skin cells called 7-dehydrocholesterol to vitamin D3 (cholecalciferol) which enters the blood.  The UVB rays are strongest in summer and in equatorial locations year round.  So getting your skin to produce vitamin D in the dead of winter at higher latitudes can be nearly impossible, especially given that it’s winter.  People living in cold northerly locations are covered in clothes in winter.  So vitamin D must be obtained some other way.

For decades, vitamin D has been added to milk at a level that corresponds to a recommended daily intake of 400 IU (now called 100 micrograms) per quart of milk.  The assumption was that people drink a quart of milk a day, or at least children do.  Prior to fortification, vitamin D deficiency, or rickets, had been a widely recognized public health problem in children.  This deficiency leads to bone growth failure and deformities.  Rickets was largely eliminated.  That was thought to be the end of the story about vitamin D.

But a few researchers maintained an interest in this vitamin over the years, and raised the alarm about the impact of widespread insufficiency on health for all age groups.  Those concerns were widely publicized and now vitamin D is one of the most popular supplements.  Meanwhile, there’s been plenty of research on vitamin D and diseases, but strangely no consensus about what constitutes an “optimal” vitamin D level, either for intake or for blood levels.  The recommended daily intake for people who have “adequate” blood levels remains stuck at 100 micrograms/day (the old unit of 400 IU).  For older adults, it was recently increased to 150 mcg (600 IU) because even with adequate sun exposure, older skin is less able to produce vitamin D.

As I note in my book “Food Wisdom for Women“, recommended blood levels are all over the map.  For example:

  • The National Institutes of Health says deficient is less than 12 ng/ml; adequate is 20+ up to 50 ng/ml, above which is the potential for adverse effects
  • The Vitamin D Council and Endocrine Society says deficient is 10-30 mg/ml; adequate is 30-50 and greater than 100 ng/ml is potentially harmful.

If you get tested, the lab may have its own standards, and may use another unit of measurement: nanomoles per liter.  Nanomoles per liter and nanograms per milliliter are very different numbers.  Then if you’re deficient according to some standard, what dose of supplement should you take?  Again, no one knows.  Some doctors just recommend initially large doses to bring your levels up in 6-8 weeks, and then you are kept on a lower maintenance dose.  The key in this process is to get re-tested so you know if that dose was adequate.  Many supplements have more than the recommended daily intakes, sometimes a lot more.

Because vitamin D is so popular, it’s become common for people to just buy supplements without knowing if they even need them.  Which is too bad, because vitamin D is one nutrient that is now commonly measured during routine lab tests.  If your level is reasonable, you don’t need any more supplementation.  Whatever you’re doing is apparently fine for you.  That could be some combination of sun exposure, intake from fortified foods or intake from supplements like multiples or calcium pills or vitamin D supplements.  In which case you don’t need more.  More is not necessarily better and can be harmful.  Vitamin D is fat soluble, meaning excess is not flushed out of your system.  It hangs around, being slowly used.  But excess has to go somewhere, and can accumulate in the liver, causing damage.  So the key message is: don’t over do it!

Does vitamin D cure diseases?

Answer: No.  Unfortunately, because of its reputation, it’s being studied as a cure or preventative for all kinds of things: diabetes, heart disease, cancer, Alzheimer’s, depression.  You name it.  A week doesn’t go by without some headline about vitamin D:

  • supplements (of varying doses) don’t prevent heart disease… but…
  • people with low levels are more at risk for heart disease
  • it doesn’t prevent diabetes … but ….
  • people with low levels are more at risk for Type 2 diabetes
  • it doesn’t prevent cancer
  • it doesn’t cure osteoporosis

and on and on.

What to make of all this seemingly conflicting information?  First, vitamin D is a vitamin, not a drug.  It has many metabolic roles, and as such contributes to overall health.  With adequate vitamin D, all those metabolic systems that depend on it can function properly.  When a study finds that people with low levels are at risk for a disease, the most likely conclusion is not that vitamin D would prevent the disease.  Rather, that adequate vitamin D is part of a healthy lifestyle that lowers risk for diseases.  It’s good to have a sufficient blood level.  Other than that, don’t expect miracles.

What’s sufficient?

A blanket recommendation is not even feasible.  Based on what I’ve read, I’d say for my own purposes, a blood level between 30 and the high 40’s ng/ml is sufficient.  Higher than that doesn’t appear to provide any additional benefits.  So that’s my own personal plan.  If you’ve been tested, your doctor may have other ideas.  If your level is low/inadequate, you might need short term supplementation, or you might benefit from long tern lower dose supplements.  My main point: don’t just take supplements without knowing you actually need them.

Who might need them?  Some circumstances can lead to increased concern about vitamin D status:

  • vegan diet
  • avoidance of dairy foods (especially children)
  • pregnancy
  • osteoporosis
  • malabsorption syndromes

Again, in these circumstances, getting tested before supplementing is the best idea.

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