# Rancidity

Category: Foods & Substances

Rancidity of oils occurs when they are exposed to oxygen, in the body just as in the bottle. Harmful free radicals are formed, and oxygen is used up.

8 passages · 1 author · 1997–2022 · Most-cited: [Ray Peat](https://bioenergeticoracle.com/md/voices/ray-peat/index.md)

Canonical page: https://bioenergeticoracle.com/concepts/rancidity

## Synthesis

**Rancidity** is the harmful oxidation of unsaturated fats, a process Ray Peat described as the oil essentially *respiring*—consuming oxygen and breaking down into toxic compounds even at room temperature. [Source 3, 5] This spontaneous degradation occurs because **polyunsaturated fatty acids (PUFAs)** are chemically unstable; their multiple double bonds readily react with oxygen, initiating a self-propagating chain reaction that produces free radicals. [Source 7] Peat demonstrated this dramatically by placing a tube from a bottle of vegetable oil into water, observing the water being drawn up as the oil consumed oxygen "just like there was a little animal in the bottle." [Source 3, 5] This process accelerates sharply at body temperature, meaning ingested rancid oils continue oxidizing aggressively within warm, oxygen-rich tissues. [Source 3, 4]

The physiological consequences of consuming rancid fats are profound and multi-systemic. Once inside the body, these oxidizing lipids compete directly with mitochondrial respiration, effectively *sucking up oxygen* that cells need for energy production. [Source 3] The free radicals generated attack cellular components, including **DNA** and **proteins**, causing structural and functional abnormalities. [Source 7] Peat argued that this oxidative interference is a core mechanism by which unsaturated oils block thyroid hormone secretion, transport, and tissue response, leading to a generalized hypothyroid state and consequent estrogen excess. [Source 7] Furthermore, he noted that the smaller, denser **LDL particles** become rancid much more quickly due to their greater surface area relative to their fat content, linking rancidity directly to vascular damage and the immunological changes that drive heart disease. [Source 1]

Peat identified specific oils as particularly dangerous due to their high PUFA content and rapid rancidification. Standard liquid vegetable oils like soy, canola, corn, safflower, and cottonseed are 50–60% PUFA and oxidize readily. [Source 3, 8] **Fish oil** and **algae oil** are even worse, having evolved for cold-water organisms; at room or body temperature, their highly unsaturated fats break down with extreme speed, making them potent immunosuppressants whose anti-inflammatory effect comes from toxic breakdown products that interfere with the immune system. [Source 3, 6, 8] Even whole grains and seeds, while containing nutrients in their germ, harbor enough polyunsaturated fat to go rancid—a fact Peat noted was the very reason the Chinese invented grain refining thousands of years ago, to remove the germ and prevent this nutritional damage. [Source 8] He advised that seeds like hemp and pumpkin are "probably fairly rancid by the time you ingest them anyway." [Source 2]

In contrast, highly saturated fats resist rancidity. Peat highlighted **coconut oil** as uniquely stable, showing no evidence of rancidity after a year at room temperature; he proposed that its saturated fats act as an antioxidant by spatially separating the small percentage of unstable unsaturated molecules, preventing the destructive chain reactions. [Source 4] This stability explains why coconut oil reduces the body's need for vitamin E and offers both direct and indirect antioxidant protection. [Source 4] Olive oil, at only 8–11% PUFA, is much safer than seed oils, and butter, at roughly 3% PUFA, is safer still. [Source 3, 7] Peat recommended storing even these safer oils in a cool, dark place—ideally a refrigerator—to minimize any oxidative deterioration. [Source 3] The overarching principle is that *saturated fats are protective*, while the rancidity of unsaturated oils is a fundamental driver of cancer, liver cirrhosis, and heart disease, conditions Peat stated cannot even occur experimentally without unsaturated fats in the diet. [Source 7]

## People also ask

### How does rancidity interfere with thyroid function?

Peat argued that oxidizing lipids from rancid fats consume oxygen needed by mitochondria and generate free radicals that block thyroid hormone secretion, transport, and tissue response, leading to a hypothyroid state and estrogen excess.

### Why is coconut oil more resistant to rancidity than vegetable oils?

Coconut oil is highly saturated, which makes it chemically stable; Peat noted it showed no rancidity after a year at room temperature and proposed its saturated fats act as an antioxidant by separating unstable unsaturated molecules to prevent chain reactions.

### What makes fish oil especially harmful once it goes rancid?

Fish oil’s highly unsaturated fats, evolved for cold-water organisms, break down extremely fast at body temperature into toxic compounds that Peat described as potent immunosuppressants, interfering with the immune system to produce an anti-inflammatory effect.

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## Cited passages

Passage numbers match the `[Source N]` markers in the synthesis above.

### Source 1 — One Radio Network: Ray Peat with Patrick Timpone — January 17, 2022

Ray Peat · Interview · Jan 17, 2022

> **Patrick Timpone:** For the rancid fats because they got them in there because they've not been polished out. Right. Wow. This guest recommended that people could do a test. I wanted to ask you about this NMR lipoprotein panel. Are you familiar with this? NMR? Yes, sir.
>
> **Ray Peat:** Nuclear magnetic resonance. I don't know if that's a particular blood test.
>
> **Patrick Timpone:** It's supposed to show the different size particles of the... of the different cholesterol and things like that in there. But you've not heard of this one, NMR. Oh, it actually measures the number of small LDL particles.
>
> **Ray Peat:** Oh. Yeah. Yeah, the smaller particles get rancid much more quickly because their surface is greater in in relation to the fats.
>
> **Patrick Timpone:** So we don't want small LDL particles. We want big ones, big puffy ones?
>
> **Ray Peat:** Yeah, there's a close connection between heart health and egg eating helps to keep the particles big. And so eggs have a heart protective effect in reducing the number of ultra small fat particles.
>
> **Patrick Timpone:** Interesting. So in general, we kind of put together the idea of the particles are big, they could clog up things, but it's just the opposite because of the rancidity factor.

### Source 2 — Ask the Herb Doctor: The hormones behind inflammation

Ray Peat · Interview · Dec 17, 2021

> ## Caller: Seeds and Rancidity
>
> **Caller:** What is your opinion on eating hemp seed and pumpkin seed?
>
> **Ray Peat:** They are not necessarily the worst for the PUFA content, but I wouldn't say it's something you should do regularly because of the breakdown products. They're probably fairly rancid by the time you ingest them anyway.

### Source 3 — Original Audio Interview: Ray Peat Interview - One Radio Network - March 15, 2021

Ray Peat · Interview · Mar 15, 2021

> **Patrick Timpone:** What are the biggest ones that people consume on-going in our culture? Would that be like the Wesson oil?
>
> **Ray Peat:** The liquid oils and the fish oil is a newer addition, but it’s especially bad. Fish oils and algae oils. The seed oils like cottonseed oil, linseed, soy, canola, and so on. In the lab when I was working on the effects of estrogen on those fatty acids in the body I bought a bottle of one of those oils, Wesson, one of the liquid oils. I had my apparatus set up for measuring animal oxygen consumption and I just put a rubber tube attached to the top of a quart of vegetable oil and put the other end in a beaker of water and came back a couple of hours later and the water had been drawn up the tube. The oil itself, a bottle full of oil but with the cap off and the bottle with the liquid oil had moved the water, was consuming oxygen just like there was a little animal in the bottle respiring. So the oil constantly when exposed to air is becoming rancid. And that was at room temperature that the bottle of oil was sucking up the oxygen, imagine at the body temperature of 98 degrees how fast that rancidity process is going to be consuming the oxygen in your tissues. That makes it act like estrogen to compete against your mitochondrial energy production. Your fat cells begin sucking up the oxygen and then the polyunsaturated fats reaching the mitochondrion will slow, destroy damage its ability to use oxygen.

### Source 4 — From PMS to Menopause: Female Hormones in Context

Ray Peat · Book · 1997

> ### 29. COCONUT OIL
>
> **Ray Peat:** I have already discussed the many toxic effects of the unsaturated oils, and I have frequently mentioned that coconut oil doesn't have those toxic effects, though it does contain a small amount of the unsaturated oils. Many people have asked me to write something on coconut oil. I thought I might write a small book on it, but I realize that there are no suitable channels for distributing such a book--if the seed-oil industry can eliminate major corporate food products that have used coconut oil for a hundred years, they certainly have the power to prevent dealers from selling a book that would affect their market more seriously. For the present, I will just outline some of the virtues of coconut oil. The unsaturated oils in some cooked foods become rancid in just a few hours, even at refrigerator temperatures, and are responsible for the stale taste of left-over foods. (Eating slightly stale food isn't particularly harmful, since the same oils, even when eaten absolutely fresh, will oxidize at a much higher rate once they are in the body, where they are heated and thoroughly mixed with an abundance of oxygen.) Coconut oil that has been kept at room temperature for a year has been tested for rancidity, and showed no evidence of it. Since we would expect the small percentage of unsaturated oils naturally contained in coconut oil to become rancid, it seems that the other (saturated) oils have an antioxidative effect: I suspect that the dilution keeps the unstable unsaturated fat molecules spatially separated from each other, so they can't interact in the destructive chain reactions that occur in other oils. To interrupt chain-reactions of oxidation is one of the functions of antioxidants, and it is possible that a sufficient quantity of coconut oil in the body has this function.

### Source 5 — Sugar Myths 1 – Sugar and the Link Between Cholesterol.

Ray Peat · Transcript · 2011

> **Andrew Murray:** The initial insult. So just for our listeners to understand in case they are not aware of what this polyunsaturated fatty acids is they are all liquid vegetable oils that are commonly sold and things are fried in, also includes, fish oils and omega 3 and 6 oils, flaxseed oils, hempseed oils. All of these are very unstable products. They can go rancid outside the human body, even if they are at room temperature. Once you ingest them they can go rancid very quickly because we are warm and full of oxygen.
>
> **Ray Peat:** Yeah in an experiment, I put a little clear plastic tube into a cork of a bottle of safflower or corn oil and put the other end in a cup of water and sitting at room temperature, you could see the water being sucked up into the tube as the oil sitting there was just consuming oxygen and becoming rancid even at room temperature, but it is much faster at body temperature.
>
> **Andrew Murray:** Okay so you’re listening to ask the herb doctors on KMUD…SO sugar is the probably the second most maligning substance after cholesterol. So what else are they saying about sugar?

### Source 6 — Good Fats Eluv Radio 2008

Ray Peat · Interview · 2008 · http://l-i-g-h-t.com/files/eluv-good-fats.mp4

> **Ray Peat:** If you have ever had a piece of fat fish laying around at room temperature it's even worse than leaving canola or soy oil or corn oil out at room temperature. The fat gets rancid very quickly. That's because it's evolved to live in cold water at just slightly above freezing temperature. At room temperature, it starts oxidizing quickly. Anyone can do a simple experiment - if you have a quart of an unsaturated vegetable oil, you can put a cork in it and a small rubber tube from the top of the bottle into a glass of water. At room temperature, the oil acts like it's respiring - it's burning oxygen and breaking down, turning into basically a plastic or varnish-like material, and it sucks the water up the tube just like a small organism was breathing in the bottle. Fish oil is even worse than the standard seed oils. Unless, for example, there's an Amazon River fish where the water is very warm in the Amazon, and that fish has fat that's very similar to olive oil and is much safer, but we never get those in the United States.
>
> **Eluv:** What about fish from Florida, being that it's a little warmer?

### Source 7 — From PMS to Menopause: Female Hormones in Context

Ray Peat · Book · 1997

> **Question:** How do they cause oxidative damage?
>
> **Ray Peat:** Unsaturated oils get rancid when exposed to air; that is called oxidation, and it is the same process that occurs when oil paint "dries." Free radicals are produced in the process. This process is accelerated at higher temperatures. The free radicals produced in this process react with parts of cells, such as molecules of DNA and protein and may become attached to those molecules, causing abnormalities of structure and function.
>
> **Question:** What if I eat only organically grown vegetable oils?
>
> **Ray Peat:** Even without the addition of agricultural chemicals, an excess of unsaturated vegetable oils damages the human body. Cancer can't occur, unless there are unsaturated oils in the diet. [C. Ip, et al., Cancer Res. 45, 1985.] Alcoholic cirrhosis of the liver cannot occur unless there are unsaturated oils in the diet. [Nanji and French, Life Sciences. 44, 1989.] Heart disease can be produced by unsaturated oils, and prevented by adding saturated oils to the diet. [J. K. G. Kramer, et al., Lipids 17, 372, 1983.]
>
> **Question:** What oils are safe?

### Source 8 — Ask the Herb Doctor: Sugar I

Ray Peat · Interview · Sep 17, 2010 · https://www.youtube.com/watch?v=pgnzAR_a2MI

> ## Caller: Grains and Food Combining
>
> **Caller:** What about beans and whole grains, like whole grain brown rice or quinoa? Also, a lot of nutritional people claim there's a problem when you combine fruits with proteins and want you to separate them. But what you're saying seems to contradict that.
>
> **Ray Peat:** I think it's good to precede a big protein meal by maybe 15 or 20 minutes with fruits so that you get your liver stocked with some glycogen. That way, you don't respond so violently when you eat pure protein. Regarding whole grains and legumes: they have more nutrients, but only a small amount of minerals compared to fruits. The germs do contain some valuable nutrients, but they also contain enough of the polyunsaturated fats that they go rancid. That was why refining was invented by the Chinese several thousand years ago; they found that the rancidity of the whole grains damaged the nutritional value.
>
> **Caller:** So a good diet would be mostly fruit and protein, staying away from polyunsaturated oils and even the omega-3 oils?
>
> **Ray Peat:** Yes. The virtue of omega-3s is that they are a little less toxic in the sense of causing cancer than the omega-6s, but they tend to break down faster. Their main virtue therapeutically is as an anti-inflammatory agent, but the way they are anti-inflammatory is that their breakdown products interfere with the immune system. In the long run, they are suppressing your immune system, similar to cortisone.

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