Note: an easier-to-read version of this article can be found here.
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 – I get commissions for purchases made through that 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.
- In laboratory experiments, rancid fat seems to have effects on health, whether through the results of live animal studies or test tube studies using molecules and cells.
- Rancid fat can destroy vitamins, which could lead to deficiency. (This would be an indirect health effect of eating rancid fat, since the thing that harms you is the deficiency, rather than the fat itself.) (Source: Pavcek PL, Shull GM. J Biol Chem 146(2):351-5, 1942.)
- Animal studies using rodents, pigs, and dogs show vitamin, protein, and fat deficiencies because of rancid fat. (Source: Quackenbush F. J Am Oil Chem Soc 22(12):336-8, 1945.)
- Other health effects of rancid fat observed during animal studies include malnutrition, anemia, diarrhea, hair loss, dermatitis, swelling of the lips and eyes, kidney and liver bleeding, gastric papilloma (growths in the stomach), reproductive failure and loss of offspring, cancer, and death. (Sources: Greenberg SM, Frazer AC. J Nutr 50(4):421-40, 1953 and Totani N, Burenjargal M, Yawata M, Ojiri Y. J Oleo Sci 57(3):153-60, 2008.)
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?
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.
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.
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.
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.**
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