Is vitamin D hiding in fat cells

This is a common scenario: an obese person with low vitamin D is told to take a supplement. The person takes the supplement faithfully. Vitamin D levels do not go up. A dose that works fine for other people seems to have no effect. What’s happening?

The vitamin D molecule is lipophilic. Meaning, it’s fat soluble. So vitamin D is attracted to other fat-like molecules. That’s why you’re told to take vitamin D supplements with a meal that includes some fat, to stimulate absorption. Unfortunately, being lipophilic is suspected of causing a problem: vitamin D ends up trapped in fat cells, stored in the tiny droplets of fat that make up the cells.

Researchers have known about this effect for sometime. In studies of vitamin D supplementation, blood levels were inversely related to amount of body fat. In other words, give a supplement to people with different amounts of body fat, and the people with more fat will end up with lower vitamin D in blood. Fat tissue acts like a sink, draining vitamin D out and trapping it in fat cells.

This isn’t just a problem for officially “obese” people. It would be a problem for anyone with a high percent of body fat. In other words, a normal weight person who is out of shape and has little muscle mass may have a high percent of body fat. This person might have more trouble maintaining adequate vitamin D than a heavier person who has lots of muscle mass, such as a highly trained athlete.

I had been thinking about this issue recently for some reason, and then found this very comprehensive report on a planned study of the effect of exercise on vitamin D levels. The theory is that exercise mobilizes fat metabolism. As fat is released from fat cells, some of the stored vitamin D goes along with it. The study plans to evaluate the effect of 10 weeks of exercise training, without vitamin D supplementation, while also controlling for dietary intake and keeping weight stable, to see if exercise alone can raise blood vitamin D.

It’s an interesting theory. Certainly exercise is critical for health, but why limit the focus to exercise? I’m guessing weight loss would also release vitamin D into the blood, as fat stores are depleted. This might be especially pronounced in people losing lots of weight quickly, such as from bariatric surgery, or the weight loss associated with medical conditions like cancer chemotherapy. In fact, you have to wonder what other lipophilic substances are hiding out in fat cells? Vitamin E? Hormones? Environmental chemicals like pesticides?

This study will run into late 2021, so we won’t have any results for awhile. Meanwhile, if you’ve been taking vitamin D with poor results, and you have a high percent body fat, perhaps simple weight loss and/or exercise might help you achieve a sufficient level.

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