Vitamin D is known as the “sunshine vitamin”, because skin exposed to the sun can produce vitamin D for you. If you live nearer to the equator, it is more likely that you have OK blood vitamin D levels, because the sun is at the right angle most of the year to produce vitamin D via skin exposure.
But in the U.S., if you live above the 42nd parallel (roughly a line from Boston to Chicago to Portland, Oregon), the sun is at the correct angle only from mid-March thru mid-October. This means that for over half of the year, people above this line cannot produce their own vitamin D, even if you were outside in the sun all day. (More quick facts about Vitamin D here.)
It was not common to be tested for Vitamin D levels until fairly recently. And if you were found to have a low vitamin D blood level, your provider often recommended supplementation with vitamin D, such as 1000 to 2000 units of vitamin D daily, or if the levels were very low, a prescription for 50,000 units vitamin D weekly.
(By the way, it is not very wise to take extra vitamin D “just because”, as it can build up in the body to toxic levels. To be safe, it’s best to ask for a blood test to see if you even need extra vitamin D.)
Researchers at the Cleveland Clinic Foundation Bone Clinic noticed that some of their patients did not achieve optimal vitamin D levels despite taking large doses of vitamin D for months. They noted that many patients took the supplement either on an empty stomach or with a light morning meal.
Because vitamin D is fat soluble, meaning that it is better absorbed into the body with a fat-containing meal, they asked the study participants to take the vitamin D with the biggest meal of the day, thinking it may help improve absorption.
Sure enough, after two to three months, average blood levels of vitamin D in the participants jumped 50 percent, from about 30.5 nanograms per milliliter, to 47.2 ng/ml.
So the lesson to be learned: If you need to take vitamin D to correct a deficiency, you are likely to get the most benefit from vitamin D if you take it with your biggest meal of the day (and be sure the meal contains some fat).