I've frequently mentioned that when taking vitamin D supplements, the one to take is vitamin D3, and not D2. Medscape (the medical site) has an article explaining that results of a recent study showed that vitamin D3 is twice as effective as D2 in raising blood levels of vitamin D. The vitamin D3 form is derived from animal products, while vitamin D2 is plant-based. So check any supplements you purchase because many contain the vitamin D2 form of vitamin D.
Of course, sunlight is the best because it has more benefits than vitamin D - such as also having low levels of "blue light" which energizes T cells (which are part of the immune system). From Medscape:
Vitamin D3 is significantly more effective at raising the serum biological marker of vitamin D status than vitamin D2 when given at standard doses in everyday food and drink, say UK researchers — findings that could have major implications for both current guidelines and the supplement industry.
In a randomized controlled trial of vitamin D supplements, vitamin D3, which is derived from animal products, was associated with significantly higher serum total 25-hydroxyvitamin D [25(OH)D] levels after 12 weeks than vitamin D2, which is plant-based and currently used in the vast majority of vitamin D supplements.
"The importance of vitamin D in our bodies is not to be underestimated, but living in the UK it is very difficult to get sufficient levels from its natural source, the sun, so we know it has to be supplemented through our diet," explained lead author Laura Tripkovic, PhD, department of nutritional sciences, University of Surrey, Guildford, United Kingdom, in a press release.
She added, "Our findings show that vitamin D3 is twice as effective as D2 in raising vitamin D levels in the body, which turns current thinking about the two types of vitamin D on its head." "Those who consume D3 through fish, eggs, or vitamin D3-containing supplements are twice as likely to raise their vitamin D status [compared with those] consuming vitamin D2 rich foods, such as mushrooms, vitamin D2 fortified bread, or vitamin D2 containing supplements, helping to improve their long-term health." [Original study.]