Health Effects of Rancid Fat

Posted: August 1, 2012 in Real Life, What is ... ?
Tags: , , , , ,

by Shelly Najjar

Note: an easier-to-read version of this article can be found here.

Stick of butter, Creative Commons-Attribution by Robert S. Donovan

Photo Credit: Robert S. Donovan
(booleansplit) via Flickr

Here’s another post based on a paper I wrote for school. I got interested in the topic because we learned about how oxidation (mentioned more later) is bad, and that rancid fats are oxidized, so it made sense to me that rancid fats would be bad. But, science is complicated, and I wanted to know if this was true, or if I was oversimplifying.

What is rancid fat?

Rancidity is the term used to describe the process and properties of a fat that is stale, smells bad, and is discolored. Scientists  studied rancid fat since before the 1800s, with great progress on discovering the process of rancidity and methods to prevent or slow the process. The consequences of eating rancid fat have also been investigated, sometimes prompted by scientific curiosity and sometimes by outbreaks of illness.

How fats go rancid

Fats can become rancid through oxidation, irradiation, enzymatic lipolysis, and heat. Light and metal ions can also quicken these processes. One of the main ways that rancidity happens is through oxidation, so that is what I’ll talk about here. (For a good overview of the other processes, see Kubow, S. Free Radic Biol Med 12(1):63-81, 1992.)

Lipid (fat) oxidation occurs through a chain reaction process. The stages of this chain reaction are initiation, propagation, and termination. Fats are many carbon molecules linked together. Initiation is the event that begins the chain reaction by removing an electron from a carbon in the fat. (Source: Gropper, Smith, and Groff, 2009 – commissioned link*). This is bad because now the fat interacts with the body differently, including continuing this chain reaction.

How rancid fats get in our bodies

Rancid fats are found in the human diet in places such as cooking oils and fats, deep-fried foods, and some ethnic foods that are purposely made rancid. However, any fat, given the right conditions and amount of time, can go rancid. That means that any food containing fat can become rancid.

This does not mean you should stop eating fat, though. It just means you have to be smarter about how you store fat and what you choose to eat.

Here are some examples of why this is important.

But… we do not see all the same health effects in humans that we do in animals. Human health information on this topic comes from reported cases of toxicity due to eating rancid fat, since it is unethical to experimentally test toxicity on humans. However, the scientific community is involved in describing health outcomes, determining and quantifying exposures, and identifying treatments when cases are reported.

So, what do the human cases show us?

Spain location

Photo Credit: The World Factbook – Public Domain

Case Study #1: Spain, 1981-2†

In the early 80s, one region in Spain experienced an epidemic of what seemed to be pneumonia, but with additional symptoms (Source: Morb Mortal Wkly Rep. 1982 Mar 5;31(8):93-5.). It was eventually discovered to be a new disease, named toxic oil syndrome (TOS), because it is thought to be caused by consumption of adulterated cooking oil. As a result of this disease, in the first two years 356 people died and over 20,000 people were affected.

This disease has three phases: acute, intermediate, and chronic. (Source: World Health Organization, 2006)

  • Acute – fever, rash, muscle pain, and problems with blood vessels and white blood cells; respiratory failure (lungs stop working) is main cause of death
  • Intermediate – muscle pain, muscle wasting, fluid retention, high triglycerides, pulmonary hypertension (high blood pressure in the lungs), liver disease, and sicca syndrome (aka Sjogren syndrome, an autoimmune disorder that stops tear and saliva production and is often accompanied by rheumatoid arthritis); main causes of death were thromboembolism (a blood clot that moves to an area of the body like the heart, brain, or lungs) and pulmonary hypertension
  • Chronic – continuation of the intermediate phase plus nerve pain, scleroderma (connective tissue disease where fibrous tissue, like scar tissue, is made in the skin and other organs, causing tissue hardening and thickening), carpal tunnel syndrome, Raynaud’s phenomenon (cold temperatures or strong emotions cause blood vessel spasms that block blood flow to the fingers, toes, ears, and nose); deaths were caused by respiratory failure, central nervous system infection (infections of brain and spinal cord), and pulmonary hypertension

The people who are still living are at high risk of cardiovascular diseases (heart and blood vessel problems like heart attacks), even though they do have high HDL (“good cholesterol”), which is normally something that protects from having cardiovascular disease.

No treatments have been successful long term, probably because scientists are still unsure of what exactly caused the disease. It is similar to an autoimmune disorder (where the body attacks itself) triggered by the oil. The oil was deceptively sold as olive oil and possibly developed toxic compounds when it was processed with excessively high heat to remove the dye that had been added to mark the oil for industrial use only. (Source: Patterson R, Germolec D. J Immunotoxicol 2005;2(1):51-8.)

†Please note: It has come to my attention that the suspected toxic compounds created with the high heat in this case are not the same thing as rancid fat. However, I am leaving this case in this post to show that it is very important to be cautious when refining and storing oils.

India Location

Photo Credit: The World Factbook – Public Domain

Case Study #2: India, 1992

A group of 45 children were hospitalized with vomiting, abdominal pain, and diarrhea, which prompted an investigation. (Source: Bhat RV, et al. J Toxicol Clin Toxicol 1995;33(3):219-22.) The investigation turned up a total of 71 children and 9 adults who were affected by eating rancid cream-filled biscuits the children had found in the street and shared with their families. Most children ate 0.5 to 2 biscuits (because they were bitter), and were discharged from the hospital within 24 hours; one girl ate 12 biscuits and remained in the hospital for 7 days.

There were two types of biscuits, both rancid, but the pineapple flavoring of one covered the taste of the rancidity. The biscuits had an inner and outer wrapper which contained different information. The inner wrapper said the biscuits expired almost 6 years before they were consumed, and the outer wrapper said they had expired 3 years before. It appeared that the biscuits had been packaged for export, but no one knew why they were in the street. All the hospitalized children were treated successfully, and the researchers decided that the cause of the illness was the oxidative rancidity of the cream inside the biscuits.

Taiwan Location

Photo Credit: The World Factbook – Public Domain

Case Study #3: Taiwan

Despite low rates of smoking, lung cancer is the leading cause of death for women in Taiwan; similar patterns have been noticed in Chinese women living elsewhere. (Source: Ko YC, et al. Am J Epidemiol 2000;151(2):140-7.) Researchers suspected inhaling cooking oil fumes increased cancer risk, so they observed non-smoking lung cancer patients and compared them to randomly selected community members and other non-cancer hospital patients.

They found that cooking frequency and methods were related to lung cancer.

  • Cooking more meals increased the risk of developing lung cancer.
  • There was also an association between lung cancer and the temperature of the oil at the time the food was added.
  • Women who experienced eye irritation during cooking were more likely to develop lung cancer than those who did not have eye irritation.
  • There was a higher risk of lung cancer if the women waited to use the oil until it was hot enough to produce fumes.
  • If the women used a fume extractor, the risk of lung cancer was reduced (but not completely gone, which the authors suggested meant the air still had some fumes from the oil that were not being removed).

The researchers attribute these findings to the high level of carcinogens found in high temperature cooking oil fumes, which the women were inhaling.

The reason this occurs was explained in another study (Totani N, et al. J Oleo Sci 2007;56(9):449-56.), which showed that compounds are released from the oil through the steam created from deep-frying foods containing water, which vaporizes in the hot oil and rises to the surface, taking with it volatile compounds (compounds that evaporate easily). The amount of these compounds increases rapidly as heating time increases, because oils are only stable to certain temperatures for limited amounts of time. When heated beyond the point at which they are stable, more of these compounds (which can cause oxidative damage to cells) are created (a process that can be considered a type of rancidity). Since heated oil not used for deep-frying contained more of these compounds, the researchers think the steam created during deep-frying is an essential part of removing these compounds from oil (preventing people from eating them), and moving them into the air (so people can breathe them instead). Breathing these compounds was associated with a higher risk of lung cancer, most likely due to the damage in the lungs caused by the compounds in the fumes from the deep-frying oil.

Tunisia, Algeria, Morocco Location

Photo Credit: The World Factbook – Public Domain

Case Study #4: Tunisia, Algeria, and Morocco

Another type of cancer, nasopharyngeal carcinoma (NPC), was noted to be high in certain populations although rare worldwide. (Source: Feng BJ, et al. Int J Cancer 2007;121(7):1550-5.) People living in North Africa have a high incidence, and a possible relationship between certain foods and the incidence of cancer had been implied. In 2007, researchers performed a large and detailed observational study on this topic, to find specific foods that may be related with increased risk of NPC. Patients with cancer from five hospitals were compared with non-cancer hospital patients and cancer-free friends and family of patients with cancers other than NPC.

Rancid sheep fat and rancid butter were both discovered to increase risk of NPC. A preserved meat dish called quaddid (dried meat stored in oil) was also found to increase risk, which supports the findings of the earlier studies. One explanation the authors provided for some of these findings is that rancid butter has a certain compound that can activate the Epstein-Barr virus in white blood cells. Activation of this virus is a major risk factor for NPC, and may be an example of indirect toxic effects of rancid fat.


So, what do you do about all this information? Should we quit eating fat because it might become rancid? Do we need to be concerned about every fat-containing food?

Case studies #1-2 involved unintentional exposure, and #3-4 addressed exposure through common cultural food practices. Through these and animal studies, many recommendations have been formed to prevent the negative human health effects caused by rancid fat. The recommendations fall into two major categories: 1) prevent (or slow) the process of rancidity and 2) decrease the effects rancid fat has on the human body.

Here are three things you can do to protect yourself from the effects of rancid fat:

  • Avoid fat or fat-containing products that have a rancid or stale smell.
  • Store oils and fats correctly.
    • Since light and heat can start the oxidative process, fats and oils should be stored in cold, dark places away from sources of heat such as the stove top.
  • Consume antioxidant-containing foods such as dark green vegetables
    • Antioxidants, whether natural or synthetic, have been shown to decrease the amount of oxidative damage to lipids and prevent the formation of other chain reaction initiation factors, as well as preventing oxidation of vitamins like biotin. (Source: Pavcek PL, Shull GM. J Biol Chem 146(2):351-5, 1942.) Fruits and vegetables are great sources of natural antioxidants.

**Note: an easier-to-understand version of this article can be found here.**

Shelly Najjar, MPH, RDN is a Registered Dietitian Nutritionist and wellness coach at Confident Nutrition. You can find her on Facebook, Twitter (@ShellyNajjar), and LinkedIn.

I appreciate your support. *Affiliate link = Amazon pays me a small portion of the sale price, at no extra cost to you. I only recommend things that I think are worth buying. You can support me and this blog if you click here before shopping on Amazon, so that a small commission on whatever you buy will be sent to me at no extra cost to you.

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  1. […] a very distinctive and unpleasant odor, and can cause long term health effects per Shelly Najjar of Nutrition Nuts and Bolts. Until it goes rancid, peanut butter is safe to […]

  2. […] a fat becomes (the more double chains that are formed) the more likely the fat is to become rancid. Unsaturated fats are generally looked at to be more healthy than saturated fats. You can find […]

  3. kmmrky says:

    Great info! Well done. Thank You.

  4. 1saint says:

    Thanks Shelly you did good.
    You provided information as you believe to be informational. You did not wrote a thesis here nor do you claim to have a PHD. Bravo!

    We as human being are equipped with sensory organs and most of us with a common sense.

    Scientific study and analysis are tools to measure the risk for both the producer and consumer.(health and money)
    We can build labs and engage in human scientific studies but if you DISLIKE the TASTE and SMELL of rancid foods and oils STAY AWAY from them. Eat fresh and healthy.
    Use your wallet to make statements and RETURN items back if you believe they are rancid. Let your grocer, restaurant, supplier know why you return them. Inform others and be informed.

    Loving one another is the only way to live long and prosper.
    your negative critics have some “inside “work to do.

  5. […] Theorie der freien – What Makes a Food-Grade Oil Unstable or Rancid? – Health Effects of Rancid – Free radicals and antioxidants […]

  6. […] A reader sent me this article: Health Effects of Rancid Fat. […]

  7. […] And then just yesterday, a reader sent me this article: Health Effects of Rancid Fat. […]

  8. […] may happen if you eat fats that have gone bad. You can read the post at Nutrition Nuts and Bolts (read it by clicking this link). The post ended up being a little technical, so I’m going to summarize/excerpt […]

  9. lucchase says:

    I’m quite sure rancid fats can cause tiny blood clots and strokes.

  10. […] Regular rancid fat ingestion, even in small amounts, causes a host of long-term health problems from… […]

  11. Theresa Capri says:

    Thanks for this article. I have always heard that rancid fats were bad for your health, but have never really read any research done, ie studies that rats fed rancid oils died of x,y,z etc…and trying to find articles like this on sites such as pubmed etc are not fruitful for me. Your article has been the closest I have found so far….

  12. Steve says:

    Awesome article! I’ve been wondering about this topic and undo for years but this is the first time someone has put the pieces if the puzzle about rancid foods and oils together which shows they can be dangerous! I’ve always been told not to wate food and that rancid foods can’t hurt you if eaten. Thanks for correcting the misconceptions!

    Keep up the great work!

    • Thank you for the kind words. I’m glad this post was so helpful for you. Thanks so much for taking the time to read, comment, and subscribe. I hope to see you around the blog comments in the future.

  13. […] anyone considered the effects of oil supplements that become rancid? Rancid fat is a killer…<rancid fat causes […]

  14. Ebby says:

    I was wondering if rancid fat had any health issues and found your article, but I’m not at all convinced. None of the studies actually showed that rancid fat caused health issues. There was always speculation or even a leap of faith involved. For example, the biscuits were apparently quite old and the oil had become rancid, but did they do any analyses of the biscuits? Were there any biological pathogens present such as Salmonella or E. coli? As to cooking, products of incomplete combustion and nitrosamines are known carcinogens created during most frying/burning activities. In addition, nitrogen oxides, which also have known health effects, are elevated in kitchens and homes.

    Last but not least, your original premise that oxidants are bad for you (a reasonable case for some materials) does not necessarily extend to oxidized materials. Oxidation and oxidized materials are essential for humans. Oxidized hydrogen (water) is essential as is the use of of the oxygen which is, of course, the classic oxidant.

    Nevertheless your recommendations at the end of the article are quite sensible!

    • Hi Ebby,

      Thanks for reading and for taking the time to share your comments. I’m glad you’re so careful about what you read online. I tried to present the little evidence I found in a way that was as accurate as possible, but perhaps I wasn’t as clear as I should have been.

      In the animal studies, where exposure and environment were controlled, it does appear that rancid/oxidized fats are not beneficial, and even harmful. Scientifically controlled studies are useful because in a free living situation, there are so many other factors that can influence the outcome that it is difficult to make a cause and effect conclusion. In controlled studies, many of these extra variables are removed and the relationship of interest is much clearer.

      The case studies do not show cause and effect, as you mentioned, simply because they are not controlled environments. We do not know for sure that the rancid fats are the only things that caused these health effects in the human case studies. However, there is so little published research on this topic that it is what is used if people are interested in learning about the effects in humans. Patterns can be identified from these studies, but we cannot say for sure that if you eat rancid fats, you will for sure have the same health effects as the people studied. It is important to know the other factors that could influence the results, such as the other carcinogens that you mentioned in your comment.

      As for the general premise, I am not saying that all oxidation reactions are bad for us. Water is essential for humans, as you mentioned; too much water in a short period of time can be harmful. Just because something is good in normal amounts doesn’t mean it’s not bad in larger amounts. Also, using the water as an example, water we drink is essential for life, water we inhale can drown us. The difference in where the water is and is supposed to be, similar to how we have oxidation reactions going on in our bodies all the time, as a way of accessing energy molecules, etc, but we also need antioxidants in our bodies to prevent other oxidation reactions that are not supposed to happen from harming us.

      I do appreciate your comments about the recommendations ending the article. They are good recommendations for general health and preserving flavor of foods regardless of whether you believe the premise.

      I so appreciate you reading and participating in this blog. I hope you continue to enjoy and learn from the articles here.


  15. Winston Peters says:

    Hi Shelly,
    It is hard to find scientific information online regarding the subject of rancidity and health because of all the noise created by alternative medicine quacks. So I was happy to find your article complete with references. However, having looked into “toxic oil syndrome” (Case Study #1: Spain, 1981-2) and checking the CDC reference which you supply, it is clear the outbreak had nothing to do with oil rancidity. Indeed the industrial rapeseed oil originally blamed for the outbreak has been vindicated in a later epidemiological study which points to pesticides on tomatoes ( For me, the inclusion of this irrelevant case is misleading and weakens an otherwise good post.

    • Hi Winston,
      Thanks for reading, and for taking the time to comment. I have updated Case 1 with a disclaimer that it is not the same thing as rancid fat, and included language that communicates less certainty of the cause, although that’s as far as I’m taking it. My concern with The Guardian link was that they did not provide specific references (only “an article” etc). However, I do appreciate your assistance in providing it, and will leave it up for people to read to inform themselves.

  16. Clay says:

    Excellent and fascinating article, Shelley. I will admit, however, that I paused long and hard when I read the part about removing a proton from a carbon. I have to assume you meant removing an *electron* (as in oxidizing), because removing a proton is not so easy, and besides, that would change the carbon into boron. 🙂

    Thanks again for the fascinating article which popped up on google while my partner and I were deciding whether or not to suffer through a bag of rancid walnuts. I think we’ll be cutting back on the cooking heat and improving our kitchen ventilation now too. I know the Lawrence Berkeley Lab has recently been putting out warnings about kitchen air quality.

    • Oops! Thanks for catching that mistake. It’s corrected now.

      I’m glad you are taking the next step and applying health information to make healthy changes in your life. For the future, walnuts (in small portions) are a great snack! You can store them in the freezer to make them last longer, so you won’t have to “suffer through” the next bag. ;P

      Thanks for reading and participating in this blog. If you want to read more you can subscribe to get each new post sent to your inbox or sign up for the RSS feed if you prefer that.

  17. Bob says:

    Only two of your case studies have anything to do with rancid oils. Using false data to prove a point, calls into question the whole article.

    Based on your falsehoods, I’m not sure I should believe the other two case studies. I’ll have to look them up myself and see what they really say.

    • Thanks for your comment, Bob. Although I don’t appreciate my statements being called falsehoods, which implies that I’m lying, I can see that I wasn’t as clear as to why I count cases 2 and 3 as evidence in the argument against rancid oils.

      In case 2, as I mentioned, the investigators decided the cause of the illness was the oxidative rancidity of the cream inside the biscuits. Cream is able to oxidize because of the fat it contains, making this case study appropriate to discuss with this topic.

      As I reread case 3, I realized I forgot to include a “so what” connecting statement between the two articles. It has now been corrected. The thinking behind the hot oil example is that 1) oil that has been heated past the point where it is stable produces volatile compounds in a heat-induced reaction (can be considered a type of rancidity); 2) these compounds are dangerous to health (can cause oxidative damage to cells in our bodies); 3) exposure to steam from water in frying foods allows these compounds to rise to the surface and escape into the air as the steam escapes from the oil; and 4) exposure to these airborne compounds has been associated with a higher incidence of lung cancer.

      I encourage you to look up these articles on your own. It is always a good idea to do your own research, especially when reading things on the Internet. The studies I’ve included are from peer-reviewed journals and I’ve provided all the links within this post.

      Again, thank you for taking the time to read and to comment.

  18. […] seed mixtures can easily go bad, called rancid, from improper storage (for more reading, “Health Effects of Rancid Oil“).  This would include heat and light, to which many pet foods are subjected.  Imagine the […]

  19. […] may happen if you eat fats that have gone bad. You can read the post at Nutrition Nuts and Bolts (read it by clicking this link). The post ended up being a little technical, so I’m going to summarize/excerpt […]

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