Dear Mark: Rancid Fat, Store-Bought Mayo and Rice Bran Oil

It’s been a while since I published a Dear Mark post and it’s been a fairly anemic news week, so I thought I’d address a trio of (related) reader questions.

If you’ve ever wondered what it means for a fat to be rancid (both for the fat and your body if you consume it), whether eating just a little store-bought every once in a while is all that bad, or where rice bran oil falls in the spectrum of Primal fats read on.

The first comes from reader Timre –

I am a newcomer to the blog and I have been reading up on oils and fats in here and there is a lot of great information but I am having trouble understanding what fats going rancid and oxidizing under heat means to my health. Can you elaborate on this for me?


Fantastic question. Before I answer, let’s make sure we understand exactly what we mean by “going rancid” or “oxidizing under heat.” Rancidity is popularly defined as the development of obnoxious tastes and odors; a rancid fat or oil is also generally unhealthy. If a fat or oil has gone rancid, it’s usually because it has been oxidized due to light, heat, or oxygen exposure. What’s oxidation? Oxidation occurs when an oxygen molecule fills an opening on a fatty acid. Unsaturated fatty acids are most vulnerable to oxidation, owing to the unsaturated, open double bonds that react with free oxygen molecules. The greater the number of open double bonds, the faster oxidation occurs, because there are simply more openings for the oxygen molecules to target. PUFAs are the most vulnerable, followed faintly by MUFAs, while saturated fats are highly resistant.

The general idea is that oxidized dietary fats and oils negatively impacts health in a number of ways. In pigs (which are omnivores, not unlike ourselves), consuming thermally oxidized oils disrupted thyroid hormone status and decreased plasma cholesterol; perhaps the body was upregulating cholesterol clearance due to the sudden appearance of oxidized lipids. In rats, oxidized frying oil was a potent peroxisome proliferator in the liver; peroxisomes are required to process lipids in the liver, so the fact that oxidized fats stimulated their proliferation might mean that processing the unwanted oxidized fat was a first priority for the body. So, no, this isn’t direct evidence that they cause us harm, but I find it very interesting that in animal studies the bodies’ lipid clearance mechanisms are upregulated whenever oxidized fats come on the scene.

It’s also worth noting that rancid oils taste just awful. I think that’s by design, don’t you? My tastebuds’ immediate knee jerk rejection of rancid fats tells me that I probably shouldn’t be eating rancid fats. Well, that and the aforementioned circumstantial clinical evidence against their inclusion in our diets. Also, check out the answer to the second question. Too much PUFA in general is a problem, and it might be compounded by oxidizing that PUFA beforehand.

I was wondering how bad is vegetable oil in small dozes? I know it’s bad and don’t use it for any kind of cooking, but when it’s a little soy oil or something like that in canned food like Mackerel with tomato sauce (norwegian product) is it ok to eat it? What about store bought mayonnaise? Ok once in a while?


Short answer: it’s real bad. Avoid it always.

Longer answer: Obviously, you want to look for canned seafood in its own oil, water, or maybe packed in olive oil. They exist, but you have to look around. The tomato sauce is actually a good addition, as the lycopene in tomato products has been shown to inhibit LDL oxidation and should inhibit similar oxidative protection in PUFAs. Still, even if the soybean oil is pure and true and all its double bonds remain free and clear of oxygen, the absolute amount of omega-6s needs to be monitored. It’s not enough to only focus on avoiding already oxidized PUFA. You have to minimize your overall intake for a couple big reasons.

For one, PUFAs can oxidize in your body. You could eat the finest cold-pressed virgin corn oil (hey, I bet it exists somewhere) and still end up with oxidized LDL, simply because the PUFAs are vulnerable to oxidation in the LDL particle. See, it’s not really the LDL itself that gets oxidized; it’s the fatty acids that the LDL is transporting that oxidize. And if you eat more PUFAs, your LDL is going to carry more PUFAs, and those PUFAs are going to be potentially vulnerable to oxidation. Confounding factors include the antioxidant content of the LDL (vitamin E, CoQ10, mainly), but those antioxidants are going to have to work harder to protect the LDL particles if they’re stuffed with PUFAs derived from your fancy corn oil.

Secondly, the various omega-3 and omega-6 PUFAs “compete” for HUFA (highly unsaturated fatty acid) placement in your tissues. Americans eating standard chow with its 20:1 omega-6:omega-3 ratio will have far more omega-6 in their tissues than omega-3, and, as I’ve mentioned in previous posts, the body makes inflammatory and anti-inflammatory cytokines by drawing upon the tissue PUFAs. Omega-6s and omega-3s make different cytokines. Too much of the former could mean rampant, exaggerated, and chronic systemic inflammation – think heart disease, sore joints, more mental and physical stress.

As for the store bought mayo? Definitely avoid it. The stuff is gross and you’ll use way too much in one sitting. Make your own! Don’t necessarily toss the mackerel this time, though. You should be able to rinse most of the soybean oil off, leaving all that good mackerel fat locked inside. So, yeah, for now – just rinse it off as best you can and keep your eyes peeled for a better brand that uses a better oil.

I’ve read your blog about oils and saturated fat, but found that Rice Bran Oil was not covered. The RBO manufacturers claim it is one of the healthiest oils to use for a variety of reasons including high smoke point. We are using it in our restaurant for deep-frying. Since we are deep frying, I’d like to use the healthiest possible oil that can be obtained at reasonable cost. What do you think?


You pose an interesting question, Mark. Great name, by the way. I’m a big fan.

Rice bran oil is 39% monounsaturated fat, 35% polyunsaturated fat, and about 20% saturated fat. It has a smoke point of 490 to 500 degrees F, making it a popular up-and-coming fry oil for chefs. It’s also fairly rich in vitamin E, an antioxidant called gamma-oryzanol, and several different plant sterols, all of which should offer some protection for the fat against high heat oxidation. That said, it remains fairly high in PUFAs, which are already high in the standard American diet. And unless you plan on switching out the oil every few hours for fresh batches, you’re going to use up those antioxidants and sterols incredibly fast, leading to oxidation and rancidity.

Ideally? I’d go with beef tallow for deep frying. High in saturated and monounsaturated fats, a long track record of quality frying, almost impervious to thermal oxidation, tallow is quality stuff. The only problem is one of cost and labor; I doubt you’d be able to find a source for large quantities of ready-to-go rendered tallow for a manageable price. You might be able to make a good deal with a local producer for raw, unprocessed suet since demand is pretty low, but you’d still have to render it yourself on a regular basis. Depending on the size of your operation and the amount of time/labor you can devote to the rendering of animal fat, it may or may not be realistic or feasible. Your call.

It looks like rice bran oil might be a decent way to strike a balance between your own personal food ethics and the needs of your business. It’s better than corn oil or partially hydrogenated soybean oil – that’s for sure. Just keep a small supply of tallow on hand for personal use.

Thanks for the questions, everyone! If you want a followup question – or you just have something you need to get off your chest – answered, leave it in the comment section!

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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