Evidence suggests that less than 5% to 10% of the US population have vitamin D levels above 100nmol/L (40ng/mL) which is the level cited as optimal by numerous sources. In an Australian study of 11,247 individuals, it was found that 73% had suboptimal vitamin D levels consistent with vitamin D insufficiency (<75nmol/L).
There are many plausible reasons to explain why the majority of these populations are afflicted by suboptimal vitamin D status including:
- Greater amount of time spent indoors at work and home
- Increased adherence to sun protection (SPF 15+ sunscreen blocks 99% of vitamin D synthesis in the skin)
- Higher proportion of people adhering to plant-based diets (vitamin D levels are higher in animal products overall)
- Lower levels of vitamin D in conventional diets (mushrooms for example are typically grown in dark growing rooms precluding exposure to sunlight which is essential for vitamin D synthesis)
- A growing elderly and institutionalised population with lower exposure to sunlight
Rising rates of obesity are also likely to be partially responsible for suboptimal vitamin D status. Given that vitamin D is soluble in fat, people tend to store vitamin D in adipose tissue. Historically, this vitamin D would be released over winter as our hunter-gatherer ancestors were starved of high density caloric diets over the winter period.
Nowadays, people tend to maintain their belly fat over winter, and instead tend to spend more time indoors, with less exposure to sunlight and a lower UVB index over winter contributing further to vitamin D insufficiency.