The relationship between low levels of vitamin D and metabolic syndrome (or type 2 diabetes) are confounded by the fact that fat cells tend to capture vitamin D, keeping blood levels depressed. Lower vitamin D levels may simply be a result of obesity, which could be the real culprit behind these related conditions, and there’s not much evidence that vitamin D helps control blood sugar levels after diabetes is diagnosed.
Vitamin D is unique in that you can, and in how you can, make your own. Remarkably, exposure to sunlight turns a form of cholesterol stored in your skin into a precursor of the active form of vitamin D. However, the advertised dangers of overexposure (or any exposure) to direct sunlight keeps many people from getting sufficient exposure for adequate vitamin D production year round.
Plus, other factors, like where you live, the color of your skin, and how much body fat you store (body fat captures and holds vitamin D) make consistently adequate production of vitamin D by exposure to sunlight nearly impossible for many people.
Nevertheless, getting adequate vitamin D is crucial to bone health because vitamin D is essential for adequate absorption of calcium and may have many more benefits to your health. For the purpose of this book the following two aspects of vitamin D are most important:
Beyond its crucial role working in tandem with calcium and phosphorous for bone health, evidence that vitamin D has much broader positive impacts on your health is growing.
Vitamin D appears to help regulate your immune system and reduce inflammation responses, may work to prevent several cancers, seems to reduce the buildup of dangerous plaques in arteries and help reduce blood pressure, may work to prevent the metabolic syndrome, which is often associated with type 2 diabetes, and may even reduce the risk for both type 1 and type 2 diabetes.
Inadequate levels of vitamin D does suppress insulin production, and the potential positive effect of adequate levels of vitamin D on heart health and general inflammation would suggest that maintaining adequate levels of vitamin D could help reduce the risk for diabetes complications. Excess weight, however, works against your efforts to increase levels of vitamin D.
It’s very difficult to get adequate vitamin D from food. Because vitamin D is a fat soluble vitamin, fatty fish like salmon and mackerel tend to be the best natural sources, and animal sources contain the most active form of vitamin D, called D3.
Non-animal sources of vitamin D are a different, less active, form known as D2, but mushrooms exposed to ultraviolet light can provide significant amounts of D2. Plant-based foods supply virtually no significant vitamin D (mushrooms are a fungus).
Many foods, like milk and orange juice, are fortified with vitamin D, but you may have to drink six cups of milk each day to reach the daily recommended intake for people 1 to 70 years of age of 600 international units (an IU for vitamin D equals .025 mg, so the daily recommendation is for 15 micrograms). The daily recommendation rises to 800 IUs at age 70.
The daily recommended intake for vitamin D is targeted to achieve a minimum blood level of 20 nanograms per milliliter (ng/ml) of 25-hydroxyvitamin D (the active compound that should be measured in the lab). Ultimately, the cautions and challenges with getting adequate sun exposure, coupled with the relative difficulty of consuming an adequate dose of vitamin D from food, makes vitamin D supplementation necessary for many people to maintain an appropriate blood level.
Although overdosing on vitamin D is possible, unless you spend a lot of time outside in the sun without sunscreen, and take high doses of supplements too, it is highly unlikely. Most instances of vitamin D toxicity are related to accidental consumption of huge doses. For adults, the upper limit for recommended daily intake is of vitamin D is 4,000 IUs.