A moderate amount of fat appears to lead to better vitamin D absorption.
Fat-soluble vitamins 101:
- There are four fat-soluble vitamins that humans need: vitamins A, D, E, and K. All the other vitamins are water-soluble.
- Fat solubility affects two primary things: how well the vitamin is absorbed with fat-containing meals, and how long it’s stored in your body.
Being fat-soluble is a double-edged sword. On the one hand, you can store those vitamins in your body fat and liver, which means you don’t necessarily need daily intake. A sunny vacation can provide you with enough stored vitamin D to last you weeks or even months.[1]
On the other hand, you can accidentally overdose on these vitamins more easily. Case studies of vitamin A overdose are actually not that uncommon. Eating the livers of some large fish, seals, or polar bears can easily cause vitamin A poisoning.[2][3][4]
But most of us don’t eat shark liver or polar bear liver, and vitamin D overdose[5] is less common than vitamin A overdose. In the context of vitamin D intake solely from dietary sources (and not supplements), our problem is less likely to be overdosing on fat-soluble vitamins than it is to be getting enough of them. Let’s take a look at how to get enough vitamin D by taking it with the right amount of fat.
Some people take a vitamin-D-containing multivitamin in the morning, either with coffee or on an empty stomach. Others take a multivitamin with three eggs and buttered coffee. Whether or not either of these is ideal is an open question.
It is commonly assumed that vitamin D is better absorbed when taken with a low to moderate amount of fat, as compared to no fat or lots of fat, based on an often-noted study. Specifically, researchers have shown that in people given a single, large dose of vitamin D, taking it with 11 grams of fat leads to higher levels of vitamin D in the bloodstream, compared to taking it with either 35 grams or 0 grams — absorption was 16% higher and 20% higher, respectively.[6] An important caveat to this work is that these apparent differences in absorption were only noted by measuring plasma vitamin D levels 12 hours after taking a single large dose of 50,000 IU (1,250 μg). When plasma vitamin D levels were assessed one month and three months after this large dose, the groups didn’t significantly differ in their levels of circulating vitamin D. One alternative interpretation of this work is therefore that vitamin D may be well-absorbed regardless of the amount of fat it is taken with, or on an empty stomach. Changes in plasma vitamin D levels 12 hours postdose with respect to dietary fat content may simply reflect altered rates of absorption into the bloodstream, rather than increased or decreased absolute absorption, as is commonly inferred from the data. Examining plasma vitamin D levels after a single dose at earlier (<12 hours) and later times (>12 hours) might have shed more light on this. Another possibility, given that participants in the above study had the same blood vitamin D levels regardless of whether they took it with some additional fat, is that the limiting factor for achieving long-term steady state vitamin D levels in the blood is more dependent on absorption of vitamin D into the tissues over time, and less on achieving maximal acute vitamin D levels in the blood after a single dose. The ability of fat to acutely enhance blood vitamin D suggests that fat may enhance intestinal absorption. However, it’s not clear that this ultimately matters in terms of achieving stable, steady-state blood vitamin D levels over a longer period of time. In other words, simply taking vitamin D may be more important than what it is taken with.
The same researchers did a later study that showed 32% higher plasma vitamin D levels when people took a large dose with a meal containing 30 grams of fat, compared to a fat-free meal. But again, they used a single large dose of 50,000 IU of vitamin D.[7] Although they tested blood vitamin D levels at additional acute time-points (10, 12, and 14 hours), testing at additional timepoints beyond 14 hours (ideally, 24-48 hours, while the vitamin D meal was still present in the intestines) would have been necessary to see whether or not the plasma levels of the people who ate the fat-free meal ever reached those of those who ate the fatty meal, as one would expect if the effect of a fatty meal is to expedite the intestinal uptake of vitamin D. Unfortunately, the study authors failed to perform any longer-term followup.
More research is therefore needed to better understand interactions between meal fat content and vitamin D absorption. Taking the evidence all together, it is safe to say that vitamin D is absorbed well enough no matter which type of meal (or no meal) it is taken with. Since vitamin D is stored in body fat, the apparent effects of meal fat content relative to changes in acute absorption may ultimately be less important than regular consumption to support optimal steady-state levels.