Vitamin D: Why it's essential and how to increase levels during cold, gray months

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AccuWeather
Katy Galimberti
Mar. 2, 2017

In the dark months of winter and still gray transition to spring, getting enough natural vitamin D through the sun's rays can be an almost impossible feat.

Vitamin D is produced under the skin by a reaction to the sun and is essential to regulating calcium and phosphate levels in the body. A lack of vitamin D can lead to bone softness and other complications. Low vitamin D levels can also lead to fatigue.

More... http://www.accuweather.com/en/weather-news/vitamin-d-why-its-essential-and-how-to-increase-levels-during-cold-gray-months/70000989

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Even in the summer months, the Sun must be high in the sky and you must be in direct sunlight.

Glass blocks UVB. Your cat probably favors sunbeams because sunlight makes vitamin D. But glass blocks UVB so that is in vain.