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. Foods that are a “good
source” of a particular nutrient provide between 10 and 20% of the Recommended Daily
Value.
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.
The information presented in this website is for informational purposes only and was created by a team of U.S. registered dietitians and food experts. Consult your doctor, practitioner, and/or pharmacist for any health problem and before using any supplements, making dietary changes, or before making any changes in prescribed medications.
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