The aroma of mirchi bajjis, samosas and potato chips is nevertheless inviting to anyone who passes by the snack shop. What is snack time without these greasy and fried foods? Kids love their snack time just for the buttery biscuits, chocolatey cakes or fried foods. Adults too look forward to their daily evening ritual of vada pavs, French fries and more. While this is the scenario existing anywhere in the world, in India consumption of fried foods cooked in roadside shops, sold in supermarkets and prepared in restaurants is highly common. Though heart-wrecking, one cannot deny the fact that socioeconomic status of people determines their choice of foods. And, those belonging to the low-income category primarily rely upon fried foods sold in roadside shops for their meals and snacks. Even research shows that 48% individuals consume fried foods 1-6 times per week and snacks made up 21% of the meals consumed primarily constituting of different varieties of fried foods. So, oils constitute an integral part of cooking and with high inflation rates and hike in prices what is the possibility of cooking with oils just once and disposing them off? Almost none. Oils do add fat but what is the effect of eating foods that have been fried using those oils that have been repeatedly used for frying foods not once or twice but a many more times?
Cancers are taking a toll on the lives of people and colorectal cancer (CRC) occupies third position as the commonly diagnosed and leading cause of death in individuals. We blame sedentary lifestyle, obesity and increased intake of fatty foods but a certain group of chemicals called the polycyclic aromatic hydrocarbons (PAHs) have been found to be great contributors to dietary contamination, human consumption and development of gastrointestinal tract (CIT) cancers. PAHs are formed while cooking at high temperatures. Diet is to be blamed for more than 80% CRC cases.
Effects of Repeated Heating of Cooking Oils
There have been no effective studies noting the effects of using repeatedly heating cooking oils (RHCO) in India though there is research data available on animals showing that consuming RHCO-based foods increases the presence of reactive oxygen species (ROS) and thus a decreased radical scavenging activity and oxidative stress. Studies show that use of RHCO induces genotoxicity and carcinogenicity, elevates blood pressure levels, increases cardiovascular disease risk, endothelial dysfunction, impaired vasorelaxation responses, increased LDL levels and atherosclerosis. While some studies in rats exhibit functional changes in blood vessels, serum alkaline phosphatase, aspartate aminotransferase and alanine aminotransferase levels; improper kidney functioning with high blood pressure levels, intestinal damage and malabsorption of glucose there are certain other studies that show no significant damages due to consumption of RHCO-based food in animals.
This might be the scenario on one side but what about the knowledge of those involved in food preparation regarding the ill effects of RHCO? A Kuala Lumpur study tried to determine this in roadside vendors where 100 participants (food outlet operators) were involved in the study. These people were taken through a face-to-face questioning session and results showed that almost 67% agreed that RHCO was not good practice, 69% agreed that it was detrimental to health and 63% accepted that they use RHCO for preparing foods.
An investigation was conducted in India based on the hypothesis that consumption of RHCO leads to intestinal tumor development owing to production of free radicals which generate damage at molecular level. A 28-day oral toxicity study was performed on both male and female rats and these animals were randomly assigned to treatment categories comprising of 1. Control rats (diet only) 2. Treated with unheated cooking oil (UHCO) 3. Rats treated with single heated cooking oil (SHCO) 4. Rats treated with repeated- heated cooking oil (RHCO). Body weights were recorded at the start and end of study and their jejunum, colon and liver were collected for testing purpose. There were no symptoms of mortality or actual death present in these rats at the end of 28 days but there was a drastic change in body weight and organ weight with decreased food consumption rate present in those rats that received 3 RHCO compared to other categories. Colon polyps (mostly adenomas) was present in rats treated with SCHO and 3RHCO. Also, antioxidant enzymes like SOD, GPx and MDA was seen to be higher and, GLU (glucose) and CHOL (cholesterol) levels were significantly higher than usual in 3RHCO-treated rats.
This study showed that consumption of repeated heated oils produces altered hematological levels of blood cells. Eating foods prepared with RHCO poses debilitating health effects. The fat portion of the food and high temperatures of cooking are great risk factors for formation of mutagens and cancer in different parts of the body.
Understanding the Frying Quality of Different Oils
Oils and fats are composed of lipids and triglycerides which contain saturated and unsaturated fatty acids and glycerides. Vegetable oils constitute a part of our daily diet needed for energy levels, fatty acids and fat-soluble vitamins. Corn oil and mustard oil are regularly used for cooking while the latter proving to be heart-healthy due to its minimal amount of saturated fatty acids.
Using the same oil repeatedly for frying purpose leads to oxidative and thermal reactions that alter the nutritional, physicochemical and sensory properties of the oil. These reactions occur based on the quality of oil used and frying conditions.
A study tried analyzing the effect of temperature on the properties of oils after heating and frying. The viscosity, density and boiling point measurements of oil samples before and after frying were measured. Potatoes were cut into strips and deep fried in the oil at constant temperature at three frying times. Frying leads to thermo-oxidative or lipid oxidation and hydrolytic reactions that impair the quality of the frying oil. Major oxidation products called as hydroperoxides occur in triglycerides which again break down to produce lower molecular weight compounds such as fatty acids, alcohols and ketones resulting in a rancid product. Peroxide value (PV) is an important measure for understanding the rancidity reactions of oils. PV is indicative of the quality and stability of fats and oils. Results showed that both mustard and corn oil expressed low degree of unsaturation at room temperature and the presence of trans hydroperoxide forms of esters present with primary oxidized products. When the sample is treated at higher temperature up to their boiling points, a decrease in temperature indicates an increase in absorbance of peroxides. Both, fresh and used oil expressed similar FT-IR spectra. FT-IR spectroscopy helps in evaluating the degree of oxidation after heating and frying.
This study showed that heating oils repeatedly decreased health effects of it. PV for corn oil increased during first frying gradually decreasing during the second and third frying and in case of mustard oil PV consistently decreased through the three frying.
Fried Sunflower Oil & Bone Mass
We are well aware that consumption of fried foods results in obesity, liver damage and atherosclerosis but their effect on bone mineral content and bone mechanical competence remain untouched territories. A new study has elaborated on the negative effects of sunflower oil-fried foods intake on bone mass and strength during growth in rats.
Three different diets were fed to rats randomly-control diet, SFO (fresh oil) and SFOx (fried oil). The sunflower oil was heated for 6 hours a day, cooled down and the same oil was used continuously for seven days. Every twelve hours, 500 grams of potato was added to the frying oil. Food intake was noted down daily and the end of the 8-week study blood samples were injected. Total skeleton bone mass content (BMC) and bone mineral density (BMD) were assessed in vivo at the end of the study period.
None of the animals expressed significant difference in body weight and length at the beginning of the study but body size varied greatly depending on the type of food consumed. SFOx-fed rats attained minimal body length comparatively. Rats fed on SFO diets containing fresh or fried oils showed no great differences in energy consumption compared to the control group. Serum lipid profile was highly altered in the rats belonging to the SFOx group. Rats fed with SFOx showed higher diminished total skeleton BMC, BMC related to body weight (BMC/W), tibiae BMC/W, femur BMC/W compared to other two rat groups. There was no great difference found among rats fed SFO and C diets in regard to BMC, BMC/W, tibiae BMC/W and femur BMC/W. Strangely, there was no differences in bone mass density (BMD), spine BMC/W, spine BMD, femur BMD and tibiae BMD of rats fed on any of the diets.
Rise in serum cholesterol levels and total cholesterol levels in rats fed SFOx diet are risk factors for bone fractures. Also, low BMC is a serious risk factor for developing coronary artery disease.
All these studies have proved without doubt that repeatedly using the same oil for frying purpose causes adverse effects on the health of the individual. Minimize your intake of fried foods, never reuse the same oil for cooking and if you wish to fry, do it with high-quality edible oils.
Delirious health effects of consumption of repeatedly heated vegetable oil: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5616019/
Evaluation of frying oil quality: https://www.sciencedirect.com/science/article/pii/S1878535214001014
Repeated sunflower oil frying affects bone quality in rats: https://www.foodandnutritionjournal.org/volume7number1/fried-sunflower-oil-intake-affects-bone-quality-in-growing-rats/
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Dr. Nafeesa Imteyaz of First Eat Right clinic, is the Best Dietitian Nutritionist in Bangalore. Best Dietitian Nutritionist in Pune. Best Dietitian Nutritionist in Hyderabad. Best Dietitian Nutritionist in Chennai. Best Dietitian Nutritionist in Mumbai. Best Dietitian Nutritionist in Delhi. Best Dietitian Nutritionist in Kolkata.