Deep Frying

Deep-frying and Reuse of Frying Oil

  • Deep frying is a favorite form of food preparation among Asian Indians and often result in increased consumption of fat particularly trans fat─ the most dangerous form of fat.1 Replacing 2% of food energy from trans fats with unsaturated fats reduced coronary artery disease (CAD) risk by 24 %.2, 3
  •  During deep-frying, the water in the food is evaporated and the void is filled by oil which results in excessive amount of fat absorbed during the process.
  • Polycyclic aromatic hydrocarbons, a class of potentially mutagenic substances are emitted from cooking oils heated at high temperatures. Tocopherols, essential amino acids, and fatty acids in foods are degraded during deep-fat frying.
  • Fats that have been heated for prolonged periods in air contain many dangerous products from oxidation and breakdown of lipids that are toxic to human cells.4
  • Deep-frying is associated with spontaneous hydrogenation and formation of trans fat as well as changes the flavor stability and quality of the oil by hydrolysis, oxidation, and polymerization.5 In fast food restaurants, the cooking oil is usually changed once a week with a periodic adding of fresh oil without removing the oil containing many harmful degradation products.
  • HDL inhibits LDL oxidation primarily through its paraxonase activity; reuse of frying oil reduces paraxonase activity and thus, reducing the ability of HDL to prevent LDL oxidation.6
  • In one study, fast food restaurant cooking oil, just before the weekly change, was compared to unused oil. The repeatedly used oil had 4 times higher peroxide levels, 7 times higher carbonyl levels, and 17 times higher levels of acids.7  Ingestion of a meal rich in fat previously used for deep frying in a commercial fast food restaurant resulted in arterial endothelial dysfunction ─the earliest stage of heart disease.8
  • Reuse of oil used for deep-frying is exceedingly common in India but studies on the harmful effects of reuse of cooking oil have been few or nonexistent. 7


1. World Health Organization. Prevention of cardiovascular disease: A vital investment. World Health Organization, Geneva Switzerland2007.

2. WHO. Prevention of cardiovacular disease. World Health Organization, Genewa Switzerland2007.

3. Mozaffarian  D, Clarke R. Quantitative effects on cardiovascular risk factors and coronary heart disease risk of replacing partially hydrogenated vegetable oils with other fats and oils. European journal of clinical nutrition. May 2009;63 Suppl 2:S22-33.

4. Thompson JA, May WA, Paulose MM, Peterson RJ, Chang SS. Chemical reactions involved in the deep-fat frying of foods. VII. Identification of volatile decomposition products of trilinolein. J Am Oil Chem Soc. Dec 1978;55(12):897-901.

5. Bouchon P. Understanding oil absorption during deep-fat frying. Adv Food Nutr Res. 2009;57:209-234.

6. Enas EA. Indian diet and cardiovascular disease: An update. In: Chatterjee SS, ed. Update in Cardiology Hyderabad: Cardiology Society of India.; 2007.

7. Sutherland WH, Walker RJ, de Jong SA, van Rij AM, Phillips V, Walker HL. Reduced postprandial serum paraoxonase activity after a meal rich in used cooking fat. Arteriosclerosis, thrombosis, and vascular biology. 1999;19(5):1340-1347.

8. Williams MJA, Sutherland WH, McCormick MP, de Jong SA, Walker RJ, Wilkins GT. Impaired endothelial function following a meal rich in used cooking fat. J Am Coll Cardiol. 1999;33(4):1050-1055.

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