Q: Is it necessary to have some fat in a meal for the body to absorb fat-soluble nutrients such as vitamin D?
A: Probably just a small amount of fat, though the research is not clear and many variables are involved.
The key difference between fat-soluble vitamins (A, D, E, K) and water-soluble ones (C and the Bs) is that the former are stored in the liver and fatty tissues and are eliminated much more slowly than the latter, which need to be consumed more regularly.
A small recent Brazilian study did find that high-dose vitamin D supplements were better absorbed with a very-high-fat meal than a very-low-fat meal. However, some other studies have not found a difference in the vitamin’s absorption.
The evidence is somewhat clearer for beta carotene, a precursor to vitamin A. Studies have found, for instance, that beta carotene in a salad is much better absorbed when regular dressing is used instead of nonfat dressing.
Keep in mind, many factors affect the bioavailability of nutrients, notably other nutrients and compounds in the food or meal, what you ate earlier, whether the vitamin is in a food or pill and your body’s needs.
If you have a varied, nutritious diet, none of this should be a concern, unless you have a medical condition that affects nutrient absorption. As for supplements, we generally don’t recommend them (with the exception of calcium and vitamin D for many people). Supplements are usually overkill anyway—supplying 100 percent or more of the RDA of nutrients—so it hardly matters if absorption is reduced.