Editorials

Fried foods and the risk of coronary heart disease

BMJ 2012; 344 doi: http://dx.doi.org/10.1136/bmj.d8274 (Published 24 January 2012) Cite this as: BMJ 2012;344:d8274
  1. Michael F Leitzmann, professor1,
  2. Tobias Kurth, director of research2
  1. 1Department of Epidemiology and Preventive Medicine, University of Regensburg, D-93053 Regensburg, Germany
  2. 2Inserm Unit 708, Université Bordeaux Segalen, F-33076 Bordeaux, France
  1. michael.leitzmann{at}klinik.uni-regensburg.de

Frying itself may not be bad as long as the type of oil used for frying is good

In the early 1970s, accumulating evidence showed remarkably low rates of coronary heart disease in various Mediterranean countries, where fat consumption was fairly high but olive oil was the dominant source of dietary fat.1 Prospective cohort studies corroborated and extended those findings by showing that polyunsaturated and monounsaturated fats decrease the risk of coronary heart disease, whereas trans fats and saturated fats increase risk.

Despite much research into the relation between dietary fat and coronary heart disease in recent decades, no prospective study has comprehensively investigated the association between consumption of fried food and subsequent risk of coronary heart disease. The linked prospective cohort study by Guallar-Castillón (doi:10.1136/bmj.e363) and colleagues fills this gap.2 The study comprised 40 757 people (about two thirds of whom were women) and was conducted in five regions in Spain that traditionally have widely varying diets. Participants were interviewed at baseline (1992) about their usual diet and food preparation. Sixty two per cent of study subjects reported using olive oil for frying and the remaining participants used sunflower or other vegetable oils. The consumption of fried foods was not associated with the risk of coronary heart disease during the 12 year follow-up. After adjustment for a large number of potential confounding factors, the hazard ratio of coronary heart disease for frequent versus infrequent intake of fried food was 1.08 (95% confidence interval 0.82 to 1.43).

This result may seem surprising because frying is generally considered an unhealthy way of preparing food. How can it be explained? Before attributing causal effects, methodological factors leading to biased results must be considered. Could inaccurate dietary assessment of intake of fried foods explain the observed null findings? The accuracy of reported consumption of fried foods in this study is not known, but correlation coefficients between dietary histories and 24 hour dietary recalls for monounsaturated and polyunsaturated fat in this cohort are modest to high, ranging from r=0.65 to r=0.89.3 Fried food intake was assessed only once at study baseline, and dietary changes during follow-up could not be taken into account. Furthermore, no information is provided on whether intakes of other types of dietary fat that increase risk of coronary heart disease, such as trans fats and saturated fats, affected the results.

Existing data about the association between consumption of fried food and the risk of coronary heart disease are sparse and the findings inconsistent. A case-control study from Costa Rica comprising 485 cases and 508 controls also reported that frequent intake of fried foods was unrelated to myocardial infarction (odds ratio 1.06, 0.59 to 1.91).4 However, differences in the oils used to fry foods in Costa Rica (palm oil and partially hydrogenated soy oil) may limit direct comparisons with the study from Spain. By contrast, an international case-control study of 5761 cases of myocardial infarction and 10 646 controls from 52 countries found that frequent intake of fried foods was associated with a small increased risk of myocardial infarction (1.13, 1.02 to 1.25), but the types of oils used for frying were unknown.5 In addition, regional differences were not investigated. Two other studies suggested potential adverse effects of fried foods on myocardial infarction and on subclinical atherosclerosis as measured by carotid intima media thickness.6 7

Taken together, the myth that frying food is generally bad for the heart is not supported by available evidence. However, this does not mean that frequent meals of fish and chips will have no health consequences. The study by Guallar-Castillón and colleagues suggests that specific aspects of frying food are relevant, such as the oil used, together with other aspects of the diet.

Frying modifies the nutritional content of foods and the frying medium.8 For example, frying leads to an increase in trans fats and a decrease in unsaturated fats in foods. Frying also increases the energy density of food and makes food more palatable, which may lead to the consumption of larger amounts. Indeed, consumption of fried foods has been found to increase the likelihood of cardiovascular risk factors, including arterial hypertension,9 low concentrations of high density lipoprotein-cholesterol,10 and adiposity.11 However, the intake of fried foods can also have beneficial effects on cardiovascular risk factors. For example, a study from Italy reported that—compared with the simple addition of raw extra virgin olive oil to a mixed meal—frying food in extra virgin olive oil improved post-prandial insulin and C peptide responses in obese insulin resistant women.12

Future studies should therefore characterise fried foods in more detail by including information on the type of oil used for frying, the type of frying procedure performed (deep fried or pan fried), the time and temperature used for frying, and the degree to which oils are reused. Such improvements in dietary assessment should help disentangle the myths from the facts when evaluating the potential effects of fried foods on human nutrition and health. In terms of the practical implications of Guallar-Castillón and colleagues’ study, advice should focus on achieving an appropriate balance of fried foods—such as fish, meat, and potatoes—because these contain considerable amounts of nutrients that affect the risk of coronary heart disease.

Notes

Cite this as: BMJ 2012;344:d8274

Footnotes

  • Research, doi:10.1136/bmj.e363
  • Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Provenance and peer review: Commissioned; not externally peer reviewed.

References