Rancidity is a legit concern with fish oils and other omega-3 products. If these supplements aren't sourced, produced, distributed, stored or taken properly, they become more vulnerable to oxidation—which might make them stink, taste terrible, lose efficacy or even be harmful. Knowing consumers worry about this, our Secret Shopper surveyed how one retailer explained the issue and offered assurance.

Natural Foods Merchandiser: How can I be certain to avoid rancid omega-3 supplements?

Retailer: Well, it starts with choosing a high-quality fish oil or other omega-3. Fish goes bad quickly, and you know how terrible that can smell! So always check the "best buy" date on the bottle to make sure it's still good. We keep a close eye on that, so you should never find expired omega-3s in in our store, but it never hurts to check. Other than that, you're asking about capsules, right? Or liquid?

Natural Foods Merchandiser: Both. But I usually buy capsules.

Retailer: OK, liquids should always be kept cold, like they are here. But capsules too should be stored in the fridge after you open them to prevent contact with the air—that's what turns omega-3s rancid.

Natural Foods Merchandiser: Alright, so if I use an omega-3 before its "best buy" date and keep it refrigerated, I should be good? Can anything else cause rancidity?

Retailer: Hmmm … not that I can think of, although I guess anything is possible. But if you ever do come across an omega-3s that smells bad, definitely toss it!

How did this retailer do?

Our expert educator: Elana Natker, R.D., director of consumer and health care practitioner communications at the Global Organization for EPA and DHA Omega-3s (GOED)

This retailer seems to take good care of their omega-3 products, which is great! I love that they prioritize high-quality supplement brands and monitor the "best by" date, since that's the manufacturer's quality guarantee.

I disagree with the retailer's recommendation to store capsules (or softgels) in the refrigerator, as there really is no benefit unless you prefer swallowing cold pills. But they are right that liquids should be stored in the fridge. I'll also point out that you should never store omega-3s in the freezer since the gel coating can become brittle.

Oxidation happens when a supplement is exposed to light and air. A properly closed bottle helps with limiting air exposure, and it doesn't matter if that closed bottle is in a refrigerator, cabinet or drawer. And to limit light exposure, most omega-3 supplements come in opaque or dark bottles that act as sun blocks.

As for using your nose as a guide, know that some fish oils do have a fish smell. But it should smell "fish-fresh," not unpleasant. So if a product smells bad, yes, you should probably toss it.

The hardest part for consumers is knowing which omega-3 supplements are high quality. Price isn't an indication of quality, and the most expensive supplement isn't necessarily the best. I recommend buying from GOED member companies, which have agreed to uphold strict standards of quality and ethics.