1. What is oxidation?
2. What fats does oxidation happen to?
3. When do the fats that I eat get oxidized?
4. What happens when I eat oxidized fats?
5. Studies on consumption of oxidized oils and oxidized lipid blood levels
6. Atherosclerosis and Oxidized Lipids
7. An Assault on Health
8. How do I minimize my consumption of oxidized fat?
9. Conclusion
I want to start by talking about cooking fats. You know, the stuff that you throw in the pan before you toss in your meat or veggies. Depending on how grew up, cooking fat might be anything from butter to coconut oil to "I can't believe it's not butter". Quality fats are a great fuel source, although this isn't the blog for that. Today we are talking about what happens when you eat oxidized fats or more simply put, "rancid" fats.
What is oxidation?
Simply put, oxidation is a process where fatty acids, specifically polyunsaturated fatty acids (PUFAs) break down through their interaction with oxygen (6). The sciency name for this is called lipid peroxidation, but let's make it simple and say that "the fats are going rancid." The result of this process is the production of advanced lipid oxidation endproducts (ALEs). ALEs include the end products of lipid peroxidation including aldehydes like malondialdehyde (MDA) and other compounds like 4-hydroxyonenal (HNE) (3).
Why does any of this matter? It is known that these ALEs are toxic to cells by binding to proteins and interfering with their functions well as toxic to genes within the cells which may cause genetic mutation (3,6). We will dive into this in a bit, so hold your horses!
What fats does oxidation happen to?
Oxidation is most likely to happen to fats that contain high amounts of omega-6 PUFAs like soybean oil, vegetable oil, peanut oil, cottonseed oil, and canola oil. It can even happen to fats that contain a high amount of omega-3 fatty acids like fish oil! (Considering the freshness, storage, and packaging of your fish oil is very important, popping those old fish oil in the cabinet might do you more harm that good!). By the way, I don't recommend consuming any of the omega-6 oils that I just rattled off. Fats that contain more saturated fatty acids like coconut oil, butter, tallow, or lard don't oxidize near as quickly as the unsaturated fatty acids, because it takes a higher temperature to break down saturated fatty acid bonds. Additionally, fats like olive oil do not oxidize as quickly as PUFAs due to the high concentration of monounsaturated fatty acids and high antioxidant concentration (1).
When do the fats that I eat get oxidized?
When you put them in the oxidation machine! Totally kidding. The longer that a fat is cooked with and exposed to the air, the faster it oxidizes. In a study that we will look at below, researchers oxidized the fat that they were using simply by exposing it to the air (4). If you add heat to the equation, you will further increase the rate of oxidation. This should make you start to question restaurants and deep fried foods like french fries.
Let's bring it together. We know that fats get rancid (oxidized), we know this happens more to fats that contain a lot of PUFAs like soybean oil, canola, or vegetable oil, and we know that the longer we cook with these fats and the hotter that they get (restaurants), the more toxic ALEs they produce!
What happens when I eat oxidized fats?
When oxidized fats are consumed, they are absorbed through the GI tract and absorbed into the lymphatic system or into the blood stream. Some of these ALEs initiate an infammatory response in the circulatory system as well as in the organs like the liver, kidney, lung, and gut. It is very plausible that the repeated offenses of consuming highly oxidized fats is an assault against your body. (3)
Furthermore, a publication from 2002 (and I am sure that there are others) suggests how the role of oxidized fats and lipid oxidation products consumed in the diet may contribute to the disease state of atherosclerosis (hardening of the arteries). The abstract states how "these ingested substances can have deleterious cardiovascular effects in both humans and experimental animals." (2).
Hmm, clearly oxidized fats can't be very beneficial for us. Let's dive in further and take a look at a study where some test subjects ate oxidized corn oil.
Studies on consumption of oxidized oils and oxidized lipid blood levels
In a study from 1994, researches fed healthy human subjects corn oil that either contained a low, medium, or high amount of oxidized lipids. In order to make the highly oxidized oil, they exposed the corn oil to air for 6 to 8 weeks. They confirmed the level of oxidation by using several markers including measuring for peroxides that are produced through lipid oxidation (5).
The researchers make it a point to note that although they have a 'highly oxidized' corn oil group, the amount of oxidation in this corn oil is only about 5% of the fatty acids in the sample, which may be less than the amount of oxidation that happens during cooking or deep frying with these oils in our restaurants today.
So what happened after these guys pounded the oxidized corn oil? (Such a bad idea in my opinion, but for the cause of science nonetheless!). They went on and measured their blood to see if these oxidized fatty acids were getting into their blood system. You guessed it, those fed the corn oil with a low amount of lipid oxidation had low amounts of oxidized lipids in their blood. ( they actually measured the oxidized lipids in the serum chylomicrons, which are little fat transporters in the blood stream). Those fed the corn oil with a high concentration of oxidized lipids had a roughly five-fold increase in oxidized lipids in their blood stream! (5).
Atherosclerosis and Oxidized Lipids
We know that having oxidized lipids in the blood stream is not good! You can actually get this measured today through an oxLDL test and it is useful for monitoring the progression of atherosclerosis.
Another study took polyunsaturated fatty acids, heated them for 20 minutes, and fed them to rabbits. It turned out that the rabbits who ate the heated oil were more atherogenic (more fatty deposits in the arteries) than those who ate the oil unheated (5).
Furthermore, another bunch heated up olive oil and this was not associated with atherosclerosis because olive oil is mostly monounsaturated, and these aren't oxidized like the polyunsaturated fatty acids! (5)
An Assault on Health
What does this mean for deep fried food, fast food, or foods cooked in most kitchens across America? Unfortunately, these places are most likely using cheap industrial seed oils or vegetable oils like corn oil, soybean oil, canola oil, or vegetable oil to prepare our food. Not only are these oils high in omega-6 polyunsaturated fatty acids that promote inflammation in and of themselves, but they are extremely prone to the oxidation that was discussed above. This is like a two-fold assault to our bodies. First we get hit with inflammatory omega 6's and if that wasn't bad enough, we had to oxidize them and make them rancid and potentially contribute to atherosclerosis. The thing is, the rancidity of the oils only gets worse the longer the oils are used and I don't think any of us want to know how long the oil has been in the fryer in many restaurants.
How do I minimize my consumption of oxidized fat?
What can you do to keep your fats as minimally oxidized as possible? The first thing that you can do is to not use the oils that are easily prone to oxidation. As I mentioned earlier, most of these unstable oils are also loaded with omega-6 polyunsaturated fatty acids which promote inflammation. We get enough of these in our diets even when we are trying to eat clean, so I do not advocate cooking with canola, soybean, vegetable, corn, safflower, or peanut oil.
The next beneficial tip is to know that healthier fat options like olive oil and your saturated fats like coconut oil are more stable than the PUFAs that I listed above and thus require a greater amount energy (need to be heated to a higher temperature) to become oxidized.
The next step is to keep your fats away from heat when you are not cooking with them. The cooler the better. If you have fats like tallow, butter, or lard, refrigeration is likely best. The unsaturated oils like olive oil and avocado are probably fine at room temperature, but you will notice that they are always in a dark bottle, which gets me to my next tip.
Keep your oils out of the light. Put them in a dark place like the refrigerator or freezer (tallow, lard, butter, etc), or in a cupboard (olive oil, avocado oil). You don't need the sun to help oxidize your oils any faster. The next step is to keep them air tight. Remember in the study above how they oxidized the corn oil just by exposing it to the air? Make sure that you've got a tight lid on your oil.
Temperature is the next variable. It plays a huge role in not only the production of oxidized lipids, but also in volatile gases produced in your kitchen (in another blog!). The best thing to do is to cook using the lowest heat possible to achieve your desired results. Plus, burnt food isn't many peoples favorite anyway!
Lastly, it is suggested that by consuming antioxidants within the diet, this could potentially mitigate some of the lipid oxidation and production of ALEs, therefore reducing the absorption of harmful ALEs from oxidized fats. Sources high in antioxidants include fruits, vegetables, red-wine, or tea (3).
Conclusion
Oxidation of lipids occurs when fats are exposed to oxygen and undergo a chemical reaction that produces advanced lipid oxidation endproducts (ALEs) (3). Polyunsaturated fatty acids (PUFAs) like soybean, vegetable, canola, or peanut oil have the weakest stability compared to monounsaturated (MUFAs) or saturated fats, and are much more prone to oxidation (1). These oils are used in many of our western restaurants and fast food chains today. By eating foods containing oxidized fats, these oxidized fats are absorbed through our GI tract and enter our circulation (5). Oxidized fats may contribute to atherosclerosis, the accumulation of fatty streaks within the arteries (5). You can minimize your exposure to oxidized fats by not cooking with oils high in PUFAs like soybean, canola, peanut, or vegetable oil, by storing your fats in cool, dark places, and cooking at a reasonable temperature without burning your food or making the room smoky!
1. Katragadda, Harinageswara Rao et al. 2010. Emissions of volatile aldehydes from heated cooking oils. Food Chemistry. 120: 59-65.
2. Cohn, Jeffrey S. 2002. Oxidized fat in the diet, postprandial lipaemia and cardiovascular disease. Curr Opin Lipidol. 13(1): 19-24.
3. Kanner, Joseph. 2007. Dietary advanced lipid oxidation endproducts are risk factors to human health. Mol Nutr Food Res. 51(9): 1094-1101.
4. Negre-Salvayre, A et al. 2009. Advanced lipid peroxidation end products in oxidative damage to proteins. Potential role in diseases and therapeutic prospects for the inhibitors. British Journal of Pharmacology. 153(1): 6-20.
5. Staprans, Ilona et al. 1994. Oxidized Lipids in the Diet Are a Source of Oxidized Lipid in Chylomicrons of Human Serum. Arteriosclerosis and Thrombosis: A Journal of Vascular Biology. 14: 1900-1905.
6. Min, B and Ahn, U. 2005. Mechanism of Lipid Peroxidation in Meat and Meat Products - A Review. Food Sci. Biotechnol. 14(1): 152-163.
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