What Is Bad About Deep Frying?

by Victoria Mazmanyan | Last updated on October 03, 2024

Medically reviewed by David Alanakyan

Summary

Deep frying is a cooking method in which food is submerged in hot oil, usually between 350°F and 375°F (175°C and 190°C). This technique results in a crispy exterior and a soft interior, making it popular for a variety of dishes.

During deep-fat frying, the oil serves as both a heating medium and a substance that is absorbed by the food, leading to an increase in the food's overall fat and calorie content.

Deep-frying causes the loss of most vitamins, such as vitamins A, E, and B1, while not significantly altering mineral content.

This cooking method decreases overall protein content by destroying amino acids, such as lysine. Deep frying also increases the percentage of resistant starch while keeping the total starch content the same.

Thermal reactions during deep-frying can produce harmful substances, known as 'process toxicants,' typically found in very small amounts. These include acrylamide, furan, nitrosamines, heterocyclic aromatic amines, and compounds such as 3-MCPD and glycidol esters.

Deep frying with culinary oils rich in unsaturated fatty acids, such as refined, non-genetically engineered natural corn, sunflower, or soybean oils, can lead to the formation of more harmful chemicals called lipid oxidation products.

Overall, cooking oils with high levels of saturated fats are better options for deep frying compared to oils with high unsaturated fats and free fatty acids, such as linoleic acid (5).

Overconsumption of deep-fried food has been linked to an increased risk of various diseases, such as cancer, obesity, high blood pressure, heart disease, diabetes, atherosclerosis, hypercholesterolemia, and neurological diseases.

Table of contents

Introduction

What is Deep Frying?

What Happens to Food During Deep Frying?

Deep Frying and Food Toxicants

Deep Frying and Vitamins

Deep Frying and Minerals

Deep Frying and Protein

Deep Frying and Carbohydrates

Deep Frying and Fats

Health Impact of Deep Frying

Cancer

Cardiovascular Health

Obesity and Type 2 Diabetes

References

Introduction

Deep frying is a popular cooking method known for producing crispy, flavorful foods. However, as is often known, it comes with several health and nutritonal concerns that can outweigh its appeal. While the technique is widely enjoyed, particularly in fast food, it's important to consider the potential downsides associated with deep frying.

In this article, we will examine the scientific evidence regarding the health effects of deep frying. By doing so, we aim to provide a comprehensive understanding of the associated public health concerns and whether they are justified.

What is Deep Frying?

Deep frying, also known as deep-fat frying, is a cooking method where food is completely submerged in hot oil, usually between 350°F to 375°F (175°C to 190°C).

Deep frying differs from pan frying or shallow frying in that it uses more oil, cooks at a higher temperature, and requires a shorter cooking time. Pan frying is generally considered a healthier option compared to deep frying; however, more research is needed in this area.

What Happens to Food During Deep Frying?

Deep frying results in a crispy exterior and a soft interior, making it popular for a variety of dishes. Frying food also offers benefits by reducing the risk of microbial spoilage and extending product shelf life through the thermal destruction of microorganisms and enzymes, as well as by lowering the water activity on the food's surface (1).

During frying, the heat causes moisture in the food to evaporate as steam, significantly reducing the food's volume, moisture content, and weight. Throughout the deep-fat frying process, various chemical reactions take place, including the denaturation of proteins, Maillard reaction, hydrolysis, sugar dehydration, polymerization, lipid oxidation, and starch gelatinization. These reactions are responsible for the browning of the food, as well as the formation of its distinct aroma and texture (2).

During deep-fat frying, the oil serves as both a heating medium and a substance that is absorbed by the food, leading to an increase in the food's overall fat content (2).

Deep Frying and Food Toxicants

Thermal reactions during deep-frying can also produce harmful substances, known as 'process toxicants,' typically found in very small amounts. These include acrylamide, furan, nitrosamines, heterocyclic aromatic amines, and compounds such as 3-MCPD and glycidol esters (3).

These compounds can lead to DNA damage, including changes to nucleotides and major chromosoms. Genotoxic substances typically begin their harmful effects by forming carcinogen–DNA adducts, which happen when a carcinogen or a part of it binds directly to a nucleotide in the DNA (1).

The peroxidative degradation of unsaturated fatty acids (UFAs), especially polyunsaturated fatty acids (PUFAs), leads to the formation of lipid oxidation products (LOPs), including cytotoxic and genotoxic aldehydes, along with their precursors like lipid hydroperoxides, epoxy-fatty acids, and various secondary or tertiary LOPs.

These harmful compounds are commonly found in culinary oils rich in unsaturated fatty acids, such as refined, non-genetically-engineered natural sunflower, soybean, or corn oils, particularly when these oils are subjected to high-temperature frying at around 180°C or stored at room temperature for extended periods (4).

Overall, cooking oils with low levels of free fatty acids and unsaturated fats, especially linoleic acid, are better options for deep frying than oils with high levels of unsaturated fats and free fatty acids (5).

That being said, according to the USDA, most cooking oils, such as peanut, safflower, soybean, grapeseed, canola, corn, olive, sesame seed, and sunflower oils, have smoke points that are higher than those required for deep frying, making them safe for this purpose (6).

Reusing frying oils can contain more harmful substances, including cancer-causing carbonyl compounds, monoepoxides, and specific aldehydes produced from the degradation of linoleic acid (2). Fat digestibility can also be affected when using reused oils or fats (9).

Deep Frying and Vitamins

Vitamin loss occurs primarily due to high temperatures and oxidation during food preparation or prolonged frying, with some vitamins being more sensitive to heat and oxidation. However, both deep frying and shallow frying tend to preserve vitamins B1, B2, B6, and C better than methods like stewing, boiling, or steaming, likely because these vitamins are water-soluble (2).

That being said, deep frying causes vegetables to lose twice as much β-carotene (vitamin A) compared to shallow frying. The heat also oxidizes unsaturated fatty acids, reducing fat-soluble vitamin E levels. Additionally, about 30% of thiamin (vitamin B1) is lost during frying (2).

Deep Frying and Minerals

Studies find there to be no significant loss of mineral content during frying. Because minerals are water-soluble, their alteration primarily occurs during boiling and other cooking methods involving water (2).

Minerals are relatively well-preserved when foods are fried at high temperatures between 165–185°C and for short cooking durations (7).

Deep Frying and Protein

Frying, a dehydrating process, often leads to a higher protein concentration in foods. However, thermal treatment can alter the protein composition through denaturation, destroying certain amino acids, especially lysine, and reducing the overall protein content (2).

Deep Frying and Carbohydrates

Deep frying doesn't significantly affect the total carbohydrate content of foods; however, it does change their composition. For example, deep frying increases the percentage of resistant starch while keeping the total starch content the same (2). Resistant starch is not digested in the small intestine and instead gets fermented in the large intestine, promoting overall gut health (8). However, consuming high amounts of resistant starch can also cause gas and bloating.

The results regarding dietary fiber content are uncertain. In some cases, the fiber decomposes, while in others, it may actually increase due to the formation of melanoidin or other indigestible substances (2, 9).

Deep Frying and Fats

One major concern with consuming fried foods is the increase in calorie intake, as they have higher fat content due to oil absorption and retention. On average, French fries and hamburgers increase energy or calorie intake by 42% and 53%, respectively (10).

The Dietary Guidelines for Americans suggest that no more than 35% of daily calories should come from fat; however, in fried foods, up to 75% of calories can be from fat (10).

Elevated oxygen levels and temperatures during frying can accelerate the oxidation of certain lipids. Trans fatty acids may also form during the frying process, and their consumption is linked to a higher risk of cardiovascular disease (2).

Overall, deep frying increases the calorie and fat content of the food while also changing its fat composition.

Health Impact of Deep Frying

Overconsumption of deep-fried food has been linked to an increased risk of various diseases, such as cancer, obesity, high blood pressure, heart disease, diabetes, atherosclerosis, hypercholesterolemia, and neurological diseases.

Research indicates that consistent consumption of large quantities of energy-dense foods—particularly those high in saturated fats and refined carbohydrates—can lead to weight gain and obesity, thereby increasing the risk of non-communicable diseases (11). This has to be taken into consideration when discussing the health impact of deep-frying, as it is essential to consider not only the cooking method but also the type of food being cooked. Both factors play a crucial role in determining the overall health effects.

Diets designed to prevent or delay neurological disorders include decreased consumption of fried food. Long-term intake of food deep-fried in reused oil has even been correlated with increased neurodegeneration in lab animals (12).

Cancer

Research has shown that deep-fried foods are linked to an increased risk of cancer due to the high levels of carcinogenic compounds formed during the frying process.

The toxin acrylamide that is produced during the thermal changes in deep frying has been deemed as a probable and likely carcinogen by the International Agency for Research on Cancer and other esteemed cancer research organizations (13).

Consuming deep-fried foods has been linked to an increased risk of pancreatic and lung cancers. More broadly, eating fried foods has been associated with a higher risk of oral/pharyngeal, esophageal, and laryngeal cancers (14).

An association has been observed between lung cancer in women and the vapors released during fish frying in both Taiwan and China (2). Additionally, a high intake of deep-fried red meat has been connected to breast cancer in women in China (14).

Conversely, another study found that deep-fried foods, especially chips, but not pan-fried foods, may decrease the risk of pancreatic cancer (15).

Cardiovascular Health

Frying has been linked to an increased risk of heart disease, partly because it raises the total amount of trans fats and decreases the availability of unsaturated fats.

In older adults, fried fish consumption has been associated with several negative cardiovascular effects, including a reduced ejection fraction, increased systemic vascular resistance, and decreased cardiac output (16).

One study compared 165 patients with coronary heart disease to 199 controls. It found that patients with coronary heart disease consumed significantly more deep-fried and shallow-fried foods than the controls, who had much lower intakes (17).

Researchers observed a positive relationship between fried food consumption and heart failure. Specifically, eating more than one serving of fried fish per week at baseline was associated with a 48% increased risk of heart failure (2).

Current evidence on the direct relationship between fried food consumption and high blood pressure is limited and inconsistent; however, multiple studies have found an association between the two (2).

Fried food has also been found to be a risk factor for hypercholesterolemia or high cholesterol levels in the blood (18).

Obesity and Type 2 Diabetes

Many research studies have found a direct correlation between fried food consumption and being overweight. In two large-scale studies, frequent consumption of fried foods was associated with a higher risk of developing overweight or obesity (2).

Similarly, the development of type 2 diabetes has been linked to an excessive intake of fried foods, especially fast foods and fried foods from restaurants (2).

References

Author name: Victoria Mazmanyan

Education: General Medicine at YSMU

Last updated: October 03, 2024

Medically reviewed by David Alanakyan