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Polyunsaturated Fats

Also indexed as: Corn Oil, Flaxseed Oil, Hemp Oil, Pumpkin Seed Oil, Safflower Oil, Sesame Oil, Soybean Oil, Sunflower Oil

Flaxseed, soybean, and pumpkin seed oils—types of “PUFAs”—are best suited for salads, sauces, and dips.

Polyunsaturated fats contain large amounts of polyunsaturated fatty acids (PUFAs). Polyunsaturated fatty acids are so-named because, due to the presence of two or more double bonds, there are places along the carbon chain where the fatty acid is not “saturated” with hydrogen. Polyunsaturated fats are liquid at room temperature, and remain in liquid form even when refrigerated or frozen. Polyunsaturated fats are divided into two families: the omega-3 fats and the omega-6 fats.

Varieties

Alpha-linolenic acid (ALA)

ALA is a member of the omega-3 family of fatty acids. It is called an essential fatty acid because the body cannot manufacture it—essential fatty acids must be consumed in the diet. Dietary sources of ALA include flaxseed, soybean, and pumpkin seed oil.

Eicosapentaenoic acid (EPA)

EPA is a member of the omega-3 family of fatty acids. The oils derived from cold-water fish (salmon, tuna, sardines, and cod) are concentrated sources of EPA. To a limited extent, the human body can make EPA from ALA.

Docosahexaenoic acid (DHA)

DHA is a member of the omega-3 family of fatty acids. This fatty acid is found in cold-water fish and in some types of algae.

Linoleic acid

Linoleic acid is a member of the omega-6 family of fatty acids. It is another essential fatty acid. Dietary sources of linoleic acid include sunflower seed, safflower, and corn oil.

Gamma-linolenic acid (GLA)

GLA is a member of the omega-6 family of fatty acids. Borage oil and evening primrose oil are rich sources of GLA.

Buying and storing tips

Polyunsaturated oils can become rancid when exposed to heat, light, and oxygen. As a result, oil-processing methods affect the nutritional content, storage life, and quality of oils. Choosing a high quality cooking oil can be a challenge unless one understands the terms that food manufacturers use to describe the methods by which cooking oils are processed. When purchasing cooking oil, it is important to review the label, and note the method of extraction, and whether the oil is refined or unrefined. Whenever possible, choose expeller-pressed, unrefined oils (see definitions below). Select oils in light-resistant plastic containers, or dark brown or green glass containers.

Extraction Methods

Mechanical (expeller) extraction

During mechanical extraction, an expeller press crushes the seeds, nuts, or vegetables to extract the oil. This pressing is done under intense pressure, and raises the temperature of the oil to 185 to 200°F (85–93.3°C). Typically, nuts and seeds are heated up to 250°F (120°C) before being placed in the expeller; heating makes the pressing more efficient. Some manufacturers produce “cold-pressed” oils—a term typically used to describe oil that was extracted without using additional external heat. This term is also used when cold water is run through the expeller, keeping the temperature of the oil from rising. However, there is no legal or binding definition of “cold-pressed,” so oils may be so labeled even when temperatures were quite high during pressing.

Solvent extraction

Solvent extraction is a more efficient and complete method of oil extraction, and is therefore the preferred method of large cooking oil manufacturers. During solvent extraction, nuts and seeds are cracked to expose the oil, and then combined with a chemical solution containing a solvent (typically hexane). The solvent pulls the oil from the nut or seed. The oil-solvent mixture is then heated to about 300°F (150°C) to evaporate out the solvent.

Refining Methods

Unrefined oils

Once the oil is extracted (either through mechanical or solvent extraction), manufacturers may simply filter the oil to remove some impurities and sell it as unrefined. Unrefined oil retains its full natural flavor, aroma, and color, and many naturally occurring nutrients.

Refined oils

To extend the shelf life of the extracted oil, some manufacturers refine oils. Refining can include as many as 40 different steps, including bleaching, deodorizing, and degumming. Refined oils are clear, odorless, and less flavorful than unrefined oils, and are more suitable for high temperature cooking. All polyunsaturated oils should be stored in the refrigerator or the freezer.

Availability

Corn, safflower, and sunflower seed oil can be purchased at most grocery stores. Flaxseed and hempseed oil are often available only in natural food stores.

Preparation, uses, and tips

Polyunsaturated fats are extremely vulnerable to damage from heat, so they are not suitable for high-temperature cooking. These oils are best used in salad dressings, sauces, and dips. To add flavor to grains and stir-fry dishes, sprinkle the cooked food with flaxseed oil just before serving.

Nutritional Highlights

Corn oil, 1 Tbsp (14g)
Calories: 120
Protein: 0.0g
Carbohydrate: 0.0g
Total Fat: 13.6g
Fiber: 0.0g

Flax oil, 1 Tbsp (14g)
Calories: 135
Protein: 0.0g
Carbohydrate: 0.0g
Total Fat: 15g
Fiber: 0.0g

Hemp oil, 1 Tbsp (14g)
Calories: 135
Protein: 0.0g
Carbohydrate: 0.0g
Total Fat: 15g
Fiber: 0.0g

Safflower oil, 1 Tbsp (14g)
Calories: 120
Protein: 0.0g
Carbohydrate: 0.0g
Total Fat: 13.6g
Fiber: 0.0g
*Good source of: Vitamin E 5.8 IU

Sunflower oil, 1 Tbsp (14g)
Calories: 120
Protein: 0.0g
Carbohydrate: 0.0g
Total Fat: 13.6g
Fiber: 0.0g

*Foods that are an “excellent source” of a particular nutrient provide 20% or more of the Recommended Daily Value, based upon United States Department of Agriculture (USDA) guidelines. Foods that are a “good source” of a particular nutrient provide between 10 and 20% of the USDA Recommended Daily Value. Nutritional information and daily nutritional guidelines may vary in different countries. Please consult the appropriate organization in your country for specific nutritional values and the recommended daily guidelines.

Health benefits and concerns

Atherosclerosis

People who eat diets high in alpha-linolenic acid (ALA), which is found in canola and flaxseed oil, have higher blood levels of omega-3 fatty acids than those consuming lower amounts; this may confer some protection against atherosclerosis. In 1994, researchers conducted a study in people with a history of heart disease, using what they called the “Mediterranean” diet. The diet was significantly different from what people from Mediterranean countries actually eat, in that it contained little olive oil. Instead, the diet included a special margarine high in ALA. Those people assigned to the Mediterranean diet had a remarkable 70% reduced risk of dying from heart disease compared with the control group during the first 27 months. Similar results were also confirmed after almost 4 years. The diet was high in beans, peas, fish, fruit, vegetables, bread, and cereals, and low in meat, dairy fat, and eggs. Although the authors believe that the high ALA content of the diet was partly responsible for the surprising outcome, other aspects of the diet may have been partly or even totally responsible for decreased death rates. Therefore, the success of the Mediterranean diet does not prove that ALA protects against heart disease.

Constipation

Some doctors recommend 15ml per day of flaxseed oil to help relieve constipation, though there is little scientific research to support this approach.

Cystic fibrosis (CF)

The impaired digestion of fats in CF often leads to a deficiency of essential fatty acids (EFAs). This deficiency may in turn lead to a lowered immune function, which makes children with CF more susceptible to respiratory infection. EFA deficiency can be reversed by supplementation with corn oil (1 gram per 2.2 pounds [1 kg] body weight per day), safflower oil (1 gram per 2.2 pounds [1 kg] body weight per day), linoleic acid (7.7 grams per day), and eicosapentaenoic acid (EPA from fish oil) (2.7 grams per day). EPA supplementation was particularly effective. In a double-blind trial, six weeks of supplementation with 2.7 grams per day of EPA led to reduction in sputum and improvement in lung function in children with chronic respiratory infection due to CF.

High cholesterol

Doctors and researchers are interested in alpha-linolenic acid (ALA), the special omega-3 fatty acid found in large amounts in flaxseeds and flaxseed oil. ALA is a precursor to the fatty acid EPA (also found in fish oil) which protects against heart disease. To a limited extent, ALA converts to EPA within the body. However, unlike EPA, ALA does not lower triglyceride levels (a risk factor for heart disease).

High triglycerides

Double-blind trials have consistently demonstrated that fish oil containing EPA and DHA (docosahexaenoic acid) lowers triglyceride levels. The amount of fish oil used in much of the research provided 3 grams per day of omega-3 fatty acids. To calculate how much omega-3 fatty acid is contained in a fish-oil supplement, add together the amounts of EPA and DHA. For example, a typical 1,000-mg capsule of fish oil provides 180mg of EPA and 120mg of DHA (total omega-3 fatty acids=300 mg). Ten of these capsules would contain 3,000mg of omega-3 fatty acids. Other sources of omega-3 fatty acids, such as flaxseed oil, do not lower triglycerides. While flaxseed oil has other benefits, it should not be used for the purpose of reducing triglycerides.

Benign prostatic hyperplasia (BPH)

Despite the lack of good published research, many doctors have been impressed with the effectiveness of essential fatty acids (EFAs) in cases of BPH. A typical recommendation is 1 Tbsp (15mL) of flaxseed oil per day, perhaps reduced to 1 or 2 tsp (5 to 10mL) per day after several months. Because taking EFAs increases the requirement for vitamin E, most doctors recommend taking a vitamin E supplement along with EFA supplementation.

Psoriasis

Some doctors have reported success using flaxseed oil (usually 1 to 3 Tbsp [15 to 45 mL] per day) for psoriasis, although there have been no published trials to support that observation.

Rheumatoid arthritis (RA)

Many double-blind trials have proven that omega-3 fatty acids in fish oil partially relieve symptoms of RA. The effect results from the anti-inflammatory activity of fish oil. Many doctors recommend 3 grams per day of EPA and DHA, an amount commonly found in 10 grams of fish oil. Positive results can take 3 months to become evident. In a double-blind trial, however, flaxseed oil was not effective for RA. This is probably because the omega-3 fatty acids found in fish oil (EPA and DHA), unlike the omega-3 found in flaxseed oil (i.e., alpha-linoleic acid), are effective agents against RA.

Health benefits and concerns for fats and oils
Many health benefits and concerns associated with this food are applicable to other fats and oils. Read about health benefits and concerns for fats and oils for a full description.