Frying Does not Just Oxidize Oils, It will Also Decimate the Tocopherol & Tocotrienol Content of the Oils and Can Thus More Than Double the Oxidative Burden on Your Body

Fried butter on a stick - I bet there are more peroxides in the crust than in the butter beneath it.
Yesterday we have taken a look at the saturated fat content of the diet and its effects on body composition, insulin resistance and inflammatory markers (see "Study Shows Doubling Saturated Fats Would Yield More Benefits Than Halving Them"). Today, we are going to look at the effects of a way not just Americans, but more or less half of the globe likes to process their foods on the oils and subsequent health of someone who would consume these oils on a regular basis. By now, you will probably already know into which direction this post is heading. Right, we are talking about frying, about lipid oxidation and about the health effects of oxidized soy bean and palm oil the #1 "choices" in processed foods.

Oxidized frying oils and their effects on our health

The paper by Jaarin and Kamisah, two researchers from the Department of Pharmacology at the Universiti Kebangsaan Malaysi in Malaysia is certainly not the only, maybe not even the latest study dealing with this issue, what I like about it though, is the fact that they don't use some sort of standardized oxidized oil, but actually went through the following unquestionably not unrealistic procedure to obtain their frying oils:
"A kilogram of sweet potato slices were fried in a stainless steel wok containing two and half litres of palm oil or soy oil for 10 minutes at 180°C. Upon completion of the frying process, once heated oil was obtained. The process was repeated four times to obtain five times heated oil with a cooling interval of at least five hours. The food quantity was proportionately adjusted with the amount of vegetable oil left. No fresh oil was added between the frying processes to make up for the loss due to uptake by the frying materials." (Jaarin. 2012)
After the oils had been heated, a small quantity was extracted and the peroxide value, fatty acid composition and vitamin E content measurements (another strength of the study, most other studies discard the fatty acid composition and vitamin E content).
Figure 1: Peroxide levels (data expressed relative to fresh palm oil) of palm and soy oil after frying at 180°C for 10minutes once or five times (based on Jaarin. 2012)
If you take a look at the peroxide values in figure 1 you see that the peroxide values in both the palm and the soy oil increased significantly after only one heating process (10min at 180°) already and -- at least in the case of soy oil -- exceeded the established maximally allowed value (red line). The palm oil, on the other hand is slightly below the margin. Now that obviously does not mean that the palm oil was still 100% healthy, while the soy oil was toxic waste - it's rather an artifice due to the arbitrarily set 10meq/kg limit for lipid peroxides in oils that are meant for human consumption (the Japanese must by the way be 'peroxide proof' because their laws allow concentrations up to 30meq/kg!).
Figure 2: Relative fatty acid composition of the fresh, once or five times heated oils (based on Jaarin. 2012)
A mechanistic explanation for the fact that the soy oil presents a higher total peroxide level than the palm oil can easily be derived from the data in figure 2. As you can see,...
"[...t]he fresh soy oil contained about five times more polyunsaturated fatty acid compared to the palm oil. It seemed that five times heating had reduced about 10% of the polyunsaturated fatty acid content in the soy oil. The content of monounsaturated fatty acid in the fresh palm oil was higher than that of the fresh soy oil. Palm oil had a quite balanced ratio of saturated and unsaturated fatty acids, whereas more than 70% of soy oil fatty acid was unsaturated (polyunsaturated and monounsaturated). This unique fatty acid composition of palm oil renders its stability against oxidative insult." (Jaarin. 2012)
Now, both the oxidation process during heating as well as the in vivo oxidative effects of the consumption of those oils does not depend on their fatty acid makeup and oxidation status, only, but is also affected by the amount of antioxidants, in particular vitamin E, the respective oil brings to the table.
Figure 3: Relative amount of tocopherols and tocotrienols that remained in the oils after frying (left) and effects of the consumption of a diet containing 15% of the oils + 85% standard rodent chow for 4 months on male and ovariectomized rats that received additional +2% cholesterol in their diets on thiobarbituric acid reactive substances (TBARS), astandard marker of lipid oxidation (right;(based on Jaarin. 2012)
In that, the vitamin E content is not only important to protect you from the effects of the peroxides that are already in the oil, they are at least as important to protect the polyunsaturated fatty acids you consume from being oxidized in your body.

Against that background, is should be obvious that the profound reductions in both tocopherols and tocotrienols will augment the negative effects, so that it is actually not that surprising that the amount of thiobarbituric acid reactive substances (TBARS = standard marker of lipid oxidation) in the blood of the rodents which consumed the diets with 15% of the five times heated palm and soy oil more than doubled.

Figure 4: While olive oil is more stable than corn and soy, it's not 'oxidation proof either' what's also intriguing is the amount of oxidation that is induced just by exposing the oil to air or air and light for 30 days! (Naz. 2012)
Putting things into perspective: While I would hope that no one of you will use the same oil for frying twice or thrice, let alone five times, I am not so sure about the foods you would be served at the university canteen, let alone the fast food store around the corner. Against that background, it is however still at least unsettling to see that even when you are frying potatoes or meats in a pan at home, the T-BAR levels rise significantly.

I assume that no one of you will use soy oil and only few will be cooking with palm oil, but I know that many people still use olive oil in a hot pan for several minutes. And while I do not have data from the same study, the data in figure 5 would suggest that olive oil is probably as susceptible to heat as palm oil (if you go by the relations of palm:soy in the study at hand and olive:soy in the study by Naz et al.). If we go by this rule of thumb estimate, baking your potatoes in olive oil may not kill you, but clearly isn't the best way either.

Coconut oil!? No, unfortunately even the wonder oil does not come to a rescue. At least if we go by the data Matthäus obtained in 2007, it produces about as much peroxides as palm oil during the frying process (Matthäus. 2007). That's better than soy, and way below the critical margin, but still not without consequences on the overall oxidative burden you are exposing yourself to, when you consume significant amounts of fried foods on an everyday basis - and since I know that you do only have your occasional piece of fried grass fed butter on a stick, you won't have to be afraid anyway ;-)

Suggested reads:
  • Pimp my virgin olive oil - Discusses among other things how the polyphenols stabilize the vitamin E and render EVOO more heat stable than regular olive oil.
  • Vitamin(s)! E - A brief reminder that there is more than alpha-tocopherol + some evidence that delta tocopherol is king, when it comes to protect dietary oils

  • Matthäus, B, Use of palm oil for frying in comparison with other high-stability oils. Eur. J. Lipid Sci. Technol. 2007;109: 400–409. 
  • Jaarin K, Kamisah Y. Repeatedly Heated Vegetable Oils and Lipid Peroxidation. Intech. 2012.
  • Naz S, Siddiqi R, Sheikh H, Sayeed SA. Deterioration of olive, corn and soybean oils due to air, light, heat and deep-frying  Food Research International, Volume 38, Issue 2, March 2005, Pages 127–134.
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