Multivitamins and fish oil are two of the most popular supplements on the market, and many people who take one also take the other. In fact, some multivitamin supplements also contain fish oil. In most cases, it's fine to take a multivitamin and fish oil together.
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Be aware, however, that some types of fish oil are high in vitamins A and D which, combined with your multivitamin, could increase your risk of overdoing it on those nutrients.
You can take most types of fish oil with a multivitamin, but use caution when taking cod liver oil.
Multivitamin and Fish Oil
There's no best time to take fish oil. Taking your fish oil supplement and your multivitamin together may affect absorption of certain nutrients, but it's not clear by how much. The absorption of the fat-soluble vitamins A, D, E and K may be enhanced when taken with a meal including fat. Taking those nutrients in combination with fish oil could have a similar effect.
In a small study published in February 2015 in the Journal of the Academy of Nutrition and Dietetics, participants who took a fish oil supplement with a meal containing 30 percent of calories from fat had 32-percent higher plasma blood levels of vitamin D3 12 hours after administration compared to participants who ate a low-fat meal.
But, even though it's essentially fat, there's not a significant amount of fat in one serving of a fish oil supplement — around 2 grams and 20 calories. That's likely not enough to make a large impact on absorption. Otherwise, there isn't any other reason why taking the two pills at the same time would be recommended or contraindicated. More importantly, it's the amounts of those fat-soluble vitamins in both supplements, since excess amounts of fat-soluble nutrients can be dangerous.
Read more: The 10 Best Supplements
Fish Oil Versus Liver Oil
The majority of fish-based, omega-3 supplements you'll see in stores and online are fish oils, obtained from pressing the whole body of the fish. Fatty fish including salmon, herring, mackerel, anchovies and sardines are the most common sources. Liver oils are obtained by pressing only the liver of the fish, which is usually cod.
What's the difference? In terms of omega-3 fatty acid contents, not much. Omega-3 content varies widely among different fish oil and cod liver oil supplements, so you can find both types of supplements with lower doses of omega-3s and both types with higher doses of omega-3s. It just depends on the brand and the dose you want to take.
The major difference between the two oils is the amount of naturally occurring vitamins A and D that they contain. Fish oils typically do not provide these vitamins; however, cod liver oil is a very rich source.
For example, one cod liver oil supplement provides 750 micrograms RAE (or retinol activity equivalent, used to measure retinol) of vitamin A in one teaspoon, which is 107 percent of the recommended dietary allowance (RDA) of vitamin A for women and 83 percent of the RDA for men, according to the National Institutes of Health (NIH). If you are taking cod liver oil and multivitamins together, you may need to rethink the combination.
The same supplement also provides 6.35 micrograms of vitamin D, which is a little less than half of the RDA for both women and men, reports the NIH. That's likely not a problem, unless you are taking multivitamins with a large amount of vitamin D, a single vitamin D supplement, and/or you are taking more than one dose of cod liver oil. But it's still a concern when you're taking cod liver oil, specifically, and not just fish oil.
Problems with Excess A and D
Unlike water-soluble vitamins, excess amounts of which are excreted in the urine, fat-soluble vitamins are stored in the fat cells for long periods of time. Getting too much of these nutrients can cause them to build up to dangerous levels in the body, leading to side effects and more serious health consequences.
Hypervitaminosis A is an especially concerning condition, in which chronic high intakes can cause liver damage, according to the NIH. Other side effects may also include headache, diarrhea, nausea, dizziness and pain in the joints and bones. The tolerable upper intake level, called UL, for adults for vitamin A is 3,000 micrograms or 10,000 international units. Exceeding this amount on a regular basis increases your risk of liver toxicity and other negative side effects.
If you're taking a single serving of a cod liver oil supplement with 750 micrograms and you eat an otherwise normal diet, you're not at great risk of overdoing it with vitamin A. However, if you take more than a single dose and you take a multivitamin with high amounts of vitamin A — or a separate vitamin A supplement — you could be at risk.
While many multivitamins don't contain abnormally large doses of vitamin A, some do. For example, one commercially available multivitamin provides 25,000 international units of vitamin A, which is more than double the UL.
As for vitamin D, the risk of toxicity isn't as great. With 6.25 micrograms of vitamin D per serving, you're not likely going to go over the UL for vitamin D, which is 100 micrograms daily. However, if you are taking a multivitamin with a lot of vitamin D, you increase your risk.
Some people take a multivitamin with vitamin D in addition to a separate, single-nutrient vitamin D supplement. Together, these may provide as much as the UL in one serving. If you also take a cod liver oil supplement, your risk increases.
Should You Take Both?
In conservative doses, there's no reason not to take a multivitamin and fish oil, especially if your diet is not as healthy as it could be and/or you don't eat seafood. If you take fish oil, and not cod liver oil, you don't have to worry.
However, you should be sure to read the labels of your chosen fish oil and multivitamin products to make sure that, combined, you are not exceeding safe intake levels. You should also double check the labels of other supplements you take, because they may also contain additional nutrients such as vitamins A and D.
If you eat a healthy diet, including fresh fish twice weekly, chances are that you don't need a fish oil supplement, according to the Harvard T.H. Chan School of Public Health. The same is true for multivitamins.
In certain cases, specific nutrient supplements may be necessary to repair a deficiency. Your doctor may also recommend that you take a certain nutrient supplement if you are predisposed to a particular deficiency, or if you have a health condition that requires it. In the absence of that, before you worry about taking a fish oil and multivitamin supplement together, perhaps it's best to check in with your doctor and get her advice on the subject.
- ConsumerLab.com: "Vitamin D Supplements Maintain Top Spot in Popularity, as Probiotics Surpass Multivitamins to #4 Behind Fish Oil and CoQ10 in Latest ConsumerLab.com Survey "
- Journal of the Academy of Nutrition and Dietetics: "Dietary Fat Increases Vitamin D-3 Absorption"
- ConsumerLab.com: "Fish Oil vs. Cod Liver Oil"
- NIH: "Vitamin A"
- NIH: "Vitamin D"
- University of Colorado: "Fat-Soluble Vitamins: A, D, E, and K"
- Harvard T.H. Chan School of Public Health: "Fish: Friend or Foe?"