By Luke Fortney, MD
UnityPoint-Meriter McKee Clinic, Madison, WI
Dr. Fortney reports no financial relationships relevant to this field of study.
SYNOPSIS: Frequent consumption of food fried (four or more times a week) in reused oils significantly increases obesity and type 2 diabetes, hypertension, and hyperlipidemia, and is associated with increased risk of cardiovascular disease. What remains unknown, however, is the ideal duration, temperature, and method for safe frying, as well as how often oil can be reasonably reused.
SOURCE: Gadiraju TV, et al. Fried food consumption and cardiovascular health: A review of current evidence. Nutrients 2015;7:8424-7430.
- Not all fried oil is equal. Like the phenomenon of the “French Paradox” there appears to be a protective “Spanish Paradox” with fried foods, particularly when compared with frying techniques in the United States.
- Eating fried food in general should occur infrequently, ≤ 3 times per week.
- When choosing oils for frying, refined olive oil (not extra-virgin) is the sensible choice (see resources for culinary recommendations).
- Fried foods are calorie-dense and contribute significantly to weight gain and obesity, which then increase diabetes, hypertension, and hyperlipidemia, which in turn are the most important factors leading to cardiovascular disease such as heart attack and stroke.
- Avoid reusing oil for prolonged frying, which increases the production of harmful phytosterol oxidation products and polycyclic aromatic hydrocarbons.
- Although specific data and guidelines are lacking, it is sensible to use lower temperatures for shorter periods of time when frying.
What we eat, and perhaps more importantly what we don’t eat, has long been a key question in nutrition research in terms of health promotion and disease prevention. However, in the modern era, patients and clinicians alike often find that dietary guidelines are often incomplete, outdated, or confusing. The standard American diet has long been the standard for how not to eat, and is an example where fad diets and misinformation can lead us astray. For example, the “low-fat” craze of the 1980s and ’90s ushered in years of unhealthy, scattered, and confused eating patterns that only lead to worsening health and obesity in the United States.1 One of the main reasons for this was the substitution of sugar in place of fat (fructose in particular) in many processed foods, as well as widespread use of the now maligned trans-fats.
Evidence pointing toward this concerning health decline comes from temporal comparisons of U.S. adults between 1988-1994 and 2001-2006, which found significant increases in body mass index (BMI) and alcohol consumption, while exercise and fruit/vegetable intake decreased. Overall, general adherence to a healthy lifestyle decreased by nearly half (15% to 8% of all U.S. adults between these time periods).1 This is particularly concerning given that the larger European Prospective Investigation Into Cancer and Nutrition study (EPIC) found that the incidence of type 2 diabetes mellitus (T2DM), cardiovascular disease (CVD), and cancer could be significantly reduced by simply not smoking, maintaining a reasonable weight (BMI < 30 kg/m2), getting at least 3.5 hours of exercise per week, and eating a reasonably healthy diet.2
From the nutrition aspect of this health decline, much more has been learned about the beneficial qualities of healthy fats such as olive oil. As the old medical adage states: Nutrition is just as much about the quality of calories as it is about the quantity. When it comes to fats, it is becoming clearer that both the type of fat and how it is used in food preparation are very important, particularly with high-heat frying.
Dietary intake of fats used for grilling, pan-frying, and deep-frying is one of the major ways to be exposed to polycyclic aromatic hydrocarbons (PAHs) through use of high cooking temperatures. PAHs are important because they contribute to lifetime cancer and CVD risk.3 In addition, frying foods in oil also results in formation of phytosterol oxidation products (POPs), which are similarly cumulatively deleterious to health over time. What’s interesting and relatively new information is the observation that pan-frying and longer frying times (e.g., 30 vs 60 minutes vs 120 vs 240 or more minutes) generates more POPs.4 The longer oils and fats are exposed to the higher cooking temperatures used in frying, the more POPs and other deleterious compounds are generated.4
While it may seem obvious that fried food consumption is deleterious to health, its actual effects on cardiovascular and cancer-related deaths are still subjects of debate. What has remained surprisingly unknown is to what extent frying contributes to and causes disease in human beings. A recent study approached this question by reviewing the evidence, and found that there is an association between fried food consumption and CVD, particularly with hypertension, hyperlipidemia, diabetes, and obesity.5 However, most of the data come from questionnaires as the main tool to capture fried food intake and are limited to case-control and cohort studies. Interestingly, the review by Gadiraju et al identified only a small number of studies that, to date, have reported an actual association between frequency of fried food intake and risk of CVD (such as coronary artery disease and heart failure). What’s surprising is the lack of follow-up studies to confirm these associations.
Based on this and other reviews, there are significant gaps in the current medical literature that include a lack of information on the type of oils used for frying, temperature and duration of frying, how often oils were reused, whether food was fried at home or in a restaurant, actual frying procedures, and the influence of other lifestyle factors. From a research standpoint, there is an overall lack of understanding about other contributing dietary patterns with frying, in terms of risk stratification for what foods might be more harmful when fried.5
In terms of scientific certainty and understanding, the best that can be stated to date about the health effects from fried food is that frequent consumption is significantly associated with incidence of T2DM (see article on page 35), but only moderately with developing CVD. However, most of this association appears to be mediated directly by increasing obesity, which then drives incidence of diabetes, hypertension, and hyperlipidemia.6
Recently, two large, long-term, prospective trials involving two cohorts of U.S. men and women concluded that there is enough evidence to suggest a higher risk of developing at least one chronic disease of lifestyle when fried foods are consumed four or more times a week, but much more high-quality research is needed to clarify the actual, specific, adverse impact on health as well as mechanisms of action.6
Healthy Oils for Frying? The Mediterranean Effect and the ‘Spanish Paradox’
There is emerging evidence to suggest that the quality and type of oils used for frying may be important, as well as specific cooking techniques. The Spanish cohort of the EPIC study helped address these questions by prospectively evaluating the associations between fried food consumption and CVD among 40,757 adults from 1992 to 2004.7 Ultimately, in Spain, where olive oil and sunflower oil are predominantly used for frying, the consumption of fried foods was not associated with coronary heart disease or with all-cause mortality.
The authors of this large study cited the potential mitigating effects of sunflower and olive oils compared to others. For example, olive oil is less prone to oxidation at standard frying temperatures than other fats. Similar results were found in a non-fatal myocardial infarction study based in Costa Rica, where palm and soy oil are commonly used for frying (both are liquid at room temperature).8
On the other hand, frying solid fats, such as lard and butter (compared to the liquid nature of plant-based olive and sunflower oil at room temperature), is also known to increase levels of harmful trans-fats and POPs through the high-heat cooking process. In addition, the frying in particular (compared to boiling) in some foods, such as eggs, has been found to increase the production of angiotensin-converting enzyme inhibitory peptides (which help lower blood pressure) as well as reduce paraoxonase activity (an enzyme that inhibits oxidation of LDL “bad” cholesterol). What’s more, and not surprisingly, reused oil has been shown to impair arterial endothelial function more than previously unused oil.9 However, the frying process is complex, and still not yet completely understood in terms of health consequences, with many laboratory studies showing that fried foods affect the body in many ways, which are particularly difficult to anticipate.
The authors of the Spanish cohort of the EPIC study also pointed out that most fried foods in the Spanish-Mediterranean diet are prepared at home with fresh ingredients and oils compared to reused oils common to many fast food and other restaurants in the United States. Consumption of fried processed snacks, which are also high in salt and sugar, is relatively low in Spain compared to the United States, where consumption of fast food is often a proxy for slower, home-cooked food. That being said, these and other variables were controlled and accounted for in this and other high-quality studies.
Adverse Risk of Fried Food: The ‘Standard American Diet’ Strikes Again
To emphasize this point, in the Nurses’ Health Study (NHS) in the United States, a prospective cohort of 121,700 female nurses, as well as 51,529 men from the Health Professionals Follow-Up Study (HPFS) were followed over many years. Cahill et al found that fried food consumption was significantly associated with the risk of incident T2DM and coronary artery disease, as mentioned above.6 These associations remain significant after controlling for demographic, diet, and other lifestyle factors such as smoking. This was the first study, interestingly, to actually examine and report these adverse health associations with frequency of fried food consumption. These two prospective U.S. studies also did not find any correlation with types of foods being used and consumed with frying (e.g., red meat and potatoes). Furthermore, similar to the Spanish study, fried foods consumed away from home (i.e., at a restaurant and fast food) were more strongly associated with diabetes and heart disease.
The authors attributed these disparities to the advanced, deleterious deterioration (increased polymerization, oxidation, and hydrogenation in the formation of unhealthy trans-fat, as well as increased degradation of healthy polyunsaturated fat) of reused fats from the frying process as well as larger portion sizes in the United States compared to Spain and other places in the world. Not surprisingly, both the HPFS and NHS studies pointed to increases over time in BMI, hypertension, T2DM, and hyperlipidemia with increased frequency of fried foods.
- King DE, et al. Adherence to a healthy lifestyle habits in US adults 1988-2006. Am J Med 2009;122:528-534.
- Earl S, et al. Healthy living is the best revenge: Findings from the European Prospective Investigation Into Cancer and Nutrition-Potsdam Study. Arch Intern Med 2009;169:1355-1362.
- Duan X, et al. Dietary intake of polycyclic aromatic hydrocarbons (PAHs) and associated cancer risk in a cohort of Chinese urban adults: Inter- and intra-individual variability. Chemosphere 2015;144:2469-2475.
- Ramadan MF. Oxidation of beta-sitosterol and campesterol in sunflower oil upon deep and pan frying of French fries. J Food Sci Technol 2015;52:6301-6311.
- Gadiraju TV, et al. Fried food consumption and cardiovascular health: A review of current evidence. Nutrients 2015;7:8424-8430.
- Cahill LE, et al. Fried-food consumption and risk of type 2 diabetes and coronary artery disease: A prospective study in 2 cohorts of US women and men. Am J Clin Nutr 2014;100:667-675.
- Guallar-Castillon P, et al. Consumption of fried foods and risk of coronary heart disease: Spanish cohort of the European Prospective Investigation into Cancer and Nutrition study BMJ 2012;344:e363.
- Kabagambe EK, et al. Individual saturated fatty acids and nonfatal acute myocardial infarction in Costa Rica. Eur J Clin Nutr 2003;57:1447-1457.
- Williams MJ, et al. Impaired endothelial function following a meal rich in used and cooking fat. J Am Coll Cardiol 1999;33:1050-1055.