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 week or even months.
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 liver of some large fish, seals, or polar bears can easily cause vitamin A poisoning.
But most of us don’t eat shark liver or polar bear liver, and vitamin D overdose is less common than vitamin A overdose. Our problem is less likely overdosing on fat-soluble vitamins than 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.
Vitamin D is one of four fat-soluble vitamins. We don’t need daily intake, due to it being stored in body fat. But with many people being low in vitamin D, knowing how much fat you need can be very helpful.
Some people take a vitamin-D containing multivitamin in the morning, either with coffee or on an empty stomach. That might not be a great idea. Others take a multivitamin with three eggs and buttered coffee. That might also not be a great idea, even though vitamin D is fat soluble and that meal provides a ton of fat.
It turns out that vitamin D is best absorbed with a low-to-moderate amount of fat, compared to no fat or lots of fat. Specifically, researchers have showed that 11 grams of fat leads to higher absorption than either 35 grams or 0 grams, at 16% higher and 20% higher respectively.
Nobody knows quite why large amounts of fat hinder vitamin D absorption. It could be due to vitamin D becoming almost too soluble and staying in the fat globules, making some fat globules in the intestine too large to efficiently cross through the intestinal lining, or something entirely different.
A major limitation of this study was that they used 50,000 IU of vitamin D once a month, whereas most people dose vitamin D daily. And possibly influenced by this, when they measured vitamin D levels one month and three months after dosing, the groups didn’t significantly differ in their vitamin D levels. So basically, we don’t have any great studies of daily vitamin D dosing and different fat levels.
These same researchers did a later study that showed 32% higher vitamin D levels when people ate a meal with 30 grams of fat, compared to a fat-free meal. But yet again, they used 50,000 IU of vitamin D all at once, rather than spread out over days. However, they did find something new: that eating polyunsaturated fat (which is high in corn oil) versus monounsaturated fat (high in olive oil) didn’t affect absorption.
Even if you don’t include much fat when you take your vitamin D, you’ll absorb some of it. It’ll just be somewhere around 15-30% less than if you eat some fat with your meal.
A “high-fat” meal may lead to lower vitamin D absorption than a low-moderate fat meal. In terms of previous experiments, a high-fat meal would be around 35 grams of fat, which equates to roughly three eggs fried in butter, plus coffee with cream. But eating a high-fat meal should also provide higher vitamin D absorption than a no-fat meal.
When you synthesize vitamin D from sun exposure, it skips the fat absorption issue altogether. This could be important for people with intestinal issues that affect fat absorption. Vitamin D can especially be a problem in elderly people, who are both more likely to have intestinal absorption issues and reduced vitamin D synthesis from sun exposure. So optimizing fat intake can provide an important edge in these people. If your older family members like food and nutrition, talk to them about this interesting tidbit!
Want to learn more about vitamin D supplementation? Check out our page on vitamin D.