(http://www.drmcdougall.com/res_vegetable_fat_med.html)
Vegetable fats have been recommended for better health since the
mid-1960s when we were asked to replace butter with margarine, and lard
with corn and safflower oil in order to lower our cholesterol and reduce
our risk of dying of heart disease. By the mid-1970s researchers had
discovered margarine raises cholesterol even more than butter, and even
though vegetable oils might reduce our risk of heart disease they would
greatly increase our risk of cancer, and make us fat.
Now, in the 1990s leading health experts are advising us to liberally
use olive oil, a monounsaturated fat, and the polyunsaturated omega-3
fats, like fish and flaxseed oils. These oils are touted as miracle
tonics able to relieve suffering from arthritis to cancer. Have we
finally got the right message on the use of oils? The truth is there can
be some benefits, but like the margarine and corn oils recommended with
impunity in the past, these oils also have serious drawbacks.
ESSENTIAL FATS
Only plants can create two types of polyunsaturated fats called
essential fatty acids (FE) known as omega-3 and omega-6 fats (w-3 and
w-6 fats). They are considered essential because we cannot make either
so both must be present in our foods. All other fatty acids can be
synthesized by man from any excess of dietary energy. However, just
because other fatty acids are considered unessential because we can make
them does not mean they are unimportant. For example, arachidonic acid,
derived from linoleic acid, is the major precursor for those very
important and powerful hormones, known as eicosanoids.
Linoleic acid is the most common kind of w-6 fat consumed by people.
Another w-6 fat often talked about is gamma linolenic acid. Alpha
linolenic acid is the most common w-3 fat consumed. Eicosapentaenoic
acid is an w-3 fat made from alpha linolenic acid and found in large
concentrations in fish oils. Linoleic acid is found mainly in vegetable
seed oils, and the main dietary source of alpha linolenic acid is leaves
and some seeds.
There are three important functions of EFA:
The most important is as part of phospholipids in all animal
cellular membranes--a deficiency of EFA results in the formation
of faulty membranes.
A second is the transport and oxidation of cholesterol; as a
result EFA tend to lower plasma cholesterol.
A third function is as precursors of tiny, but powerful
hormones, known as eicosanoids (prostaglandins, leukotrienes,
and thromboxanes), which are only formed from EFA.
EFA DEFICIENCY
Deficiency of EFA in experimental animals causes lesions mainly
attributable to faulty cellular membranes, such as sudden failure of
growth, scaliness of the skin, increased water loss by a change of skin
permeability, impaired fertility, kidney abnormalities, increased
susceptibility to infection, and weaknesses in the cardiovascular
system. In man, pure deficiency of EFA has been studied mostly in
persons fed intravenously. However, sensitive tests have found
deficiencies in elderly patients, people with fat malabsorption
diseases, and after serious accidents or burns. EFA deficiency does not
occur in people following low-fat diets, because these diets are high in
vegetable foods, rich in EFA.
Through the intake of large amounts of animal products, hydrogenation of
vegetable oils, milling, and selection of w-3 poor foods, we have been
systematically depleting our intake of EFA. A relative deficiency is
also caused by large intakes of saturated animal fats and synthetic
trans fats (as found in margarine and shortenings) common in Western
diets. This deficiency of EFA plays an important part in the causation
of atherosclerosis, coronary thrombosis, multiple sclerosis,
complications of diabetes mellitus, hypertension, and certain forms of
cancer.
EFA REQUIREMENTS
Feeding diets containing as little as 0.1 to 0.5% of the calories as
linoleic acid is sufficient to correct all signs of essential fatty acid
deficiency. However, for optimal health higher intakes are recommended.
Various factors affect the dietary requirement of EFA. Animal
experiments and epidemiological studies lead to a recommendation that
the intake of w-6 linoleic acid should be decreased to as low as 2-4 %
of the calories and that of w-3 fats be increased to levels higher than
w-6 linoleic acid for the prevention of chronic diseases prevailing in
the industrialized countries (Proc Soc Exp Biol Med 200:174, 1992).
Since plants synthesize these fats they are the original and obvious
source of all EFA. If animals, say fish, have significant amounts of EFA
in their tissues it is because they ate plants, like algae, which
originally made the EFA. Natural oils contain combinations of varying
amounts of both w-6 and w-3 fats, as well as several saturated and
monounsaturated fats. Essential fatty acids are found in significant
amounts in various plants:
Linoleic: Safflower, sunflower, hemp seed, soybeans, walnuts,
pumpkin, sesame, flax.
Alpha linolenic: Flax, hemp, canola (rapeseed), soybeans,
walnuts, green leafy vegetables, purslane, perilla.
Gamma linolenic: Borage, black currant seed, primrose.
Eicosapentaenoic: Cold water marine fish.
THE BENEFITS OF EFA:
Heart Disease:
There is evidence that EFA in the diet, especially of the w-3 variety,
protects against atherosclerosis and its related thrombotic
complications, such as a heart attack (Eicosanoids 1989;2(2):69-99).
Mechanisms probably involve the eicosanoids and a decrease in the
tendency of platelets to adhere together, a decrease in blood viscosity,
and a decrease in fibrinogen with a resulting decrease in tendency for a
blood clot (thrombus) in the heart artery to form (Am J Clin Nutr
54:438, 1991). After feeding alpha linolenic acid the arteries of obese
subjects have become more compliant (elastic), which indicates a
decreased risk of a heart attack (Aterioscler Thromb Vasc Biol 17:1163,
1997). However, there are other factors EFA change that may increase the
tendency for heart trouble, as you will learn below.
Arthritis:
Eicosanoids produced by EFA and their derivatives cause suppression of
the immune system which has been found to be particularly beneficial to
people suffering from inflammatory arthritis, like rheumatoid arthritis.
Thirty-seven patients with rheumatoid arthritis and active synovitis
were treated with 1.4 g/d gamma linolenic acid in borage seed oil or
cotton seed oil (placebo). Gamma linolenic acid reduced the number of
tender joints by 36% and swollen joint count by 28%. Patients given a
placebo showed no change or showed worsening of disease. (Ann Intern Med
119:867, 1993 ). Other studies have shown similar benefits from gamma
linolenic acid (Arthritis Rheum 39:1808, 1996). Treatment with alpha
linolenic acid, however, has not been shown to help victims of
rheumatoid arthritis (Rheumatology International 14:231, 1995).
Diabetic Neuropathy:
People with diabetes often develop pain, numbness, and burning in their
feet after years of disease. This condition, known as diabetic
neuropathy, has benefited from gamma linolenic acid therapy. For
example, 111 patients with mild diabetic neuropathy were given a dose of
gamma linolenic acid of 480 mg/day. The change over one year was more
favorable than the change with placebo. Treatment was more effective in
relatively well-controlled rather than in poorly-controlled diabetic
patients. (Diabetes Care 16:8, 1993).
QUESTIONABLE BENEFITS EFA
Research has suggested EFA to benefit many other conditions, however,
when put to the test by well-designed studies (double-blind,
placebo-controlled) their effectiveness has failed to be confirmed.
Placebo-controlled trials of EFA supplementation in atopic dermatitis,
which avoided the methodological and analytical problems of previous
studies, found no effect of EFA supplementation in atopic dermatitis
(Lancet 341:1557, 1993; Clin Exp Dermatol 19:127, 1994). Evening
primrose oil and fish oil in the treatment of psoriasis was studied in
thirty-seven patients in a double-blind parallel trial and no
significant improvement in clinical severity of psoriasis was seen (Clin
Exp Dermatol 1994 19:127, 1994).
In a randomized, double-blind, crossover trial 27 women diagnosed with
PMS were treated with EFA and placebo, and treatment did not reduce
premenstrual symptoms (Obstet Gynecol 81:93, 1993).
EFA treatment has been claimed to benefit many other problems, including
migraines (Cephalalgia 17:127 1997), Alzheimer's disease (Med Hypotheses
39:123, 1992), and tardive dyskinesia (Psychiatry Res 27:313, 1989).
However, properly designed studies have yet to be done to confirm or
refute the claimed benefits.
TOO MUCH OF A GOOD THING
Olive oil and omega-3 fatty acids have been promoted to prevent and
treat diseases, however, supplementation with large doses of these
pharmacologically active substances in the wrong setting can do harm.
Greater Risks of Heart Disease:
Most people have assumed olive oil to be protective against heart
disease because of the low incidence of heart disease in Mediterranean
countries and that EFA also prevent heart disease. However, research
indicates otherwise. A study on humans conducted by David Blankenhorn,
M.D., and his associates compared the effects of different types of fats
on the growth of atherosclerotic lesions inside the coronary arteries of
people by studying the results of angiograms taken one year apart (JAMA
263:1646, 1990). The study demonstrated that all three types of
fat--saturated animal fat, monounsaturated (olive oil), and
polyunsaturated (EFA)--were associated with a significant increase in
new atherosclerotic lesions. Most importantly, the growth of these
lesions did not stop when polyunsaturated fats of the w-6 type (linoleic
acid) and monounsaturated fats (olive oil) were substituted for
saturated fats. Only by decreasing all fat intake--including poly- and
monounsaturated fats--did the lesions stop growing.
Dietary polyunsaturated fats (EFA), both the w-3 and w-6 types, are
incorporated into human atherosclerotic plaques; thereby promoting
damage to the arteries and the progression of atherosclerosis (Lancet
344:1195, 1994). In part, this is because these oils are easily
oxidized, forming free radicals that damage the arteries. Most research
indicates w-6 type EFA are much more damaging to the arteries than w-3
type EFA (Am J Clin Nutr 49:301, 1989).
A recent study in African green monkeys found when saturated fat was
replaced with monounsaturated fat (olive oil), the olive provided no
protection from atherosclerosis (Aterioscler Thromb Vasc Biol 15:2101,
1995).
Furthermore, high-fat meals, in contrast to low-fat meals, can cause
considerable increases in plasma triglycerides and plasma levels of
blood coagulation factors which lead to a blood clot or thrombosis in
the heart artery. One of the most important clotting factors predicting
the risk of a heart attack is factor VII. The five fats tested--rapeseed
oil (canola), olive oil, sunflower oil, palm oil, and butter--showed
similar increases in triglycerides and clotting factor VII after eating.
According to the authors, "These findings indicate that high-fat meals
may be prothrombotic (causing a blood clot leading to a heart attack),
irrespective of their fatty acid composition." (Aterioscler Thromb Vasc
Biol 17:2904, 1997).
Since w-3 EFA cause a variety of changes that both decrease and increase
the risk of a heart attack, the overall impact of consuming these as
free oils will have to be determined by future experiments. Undoubtedly,
the w-6 varieties are artery damaging. Most likely, the heart benefits
of a Mediterranean diet are due to it being a nearly vegetarian diet.
The Mediterranean diet is good in spite of the olive oil (Am J Clin Nutr
61:1321S, 1995).
Higher Cholesterol and More Diabetes from Fish Oils:
Much attention has recently been paid to the possible benefits of
increasing the intake of eicosapentaenoic acid (EPA) by consuming fish
oil. However, this can have adverse effects such as raising LDL "bad"
cholesterol levels in patients with already high cholesterol and causing
a deterioration in glucose tolerance, in other words, making diabetes
worse. (Prostaglandins Leukot Essent Fatty Acids 44:127, 1991). In one
recent study of feeding w-6 alpha linolenic acid to obese subjects
insulin sensitivity and HDL "good" cholesterol diminished, and the
amount of oxidized LDL "bad" cholesterol increased (Aterioscler Thromb
Vasc Biol 17:1163, 1997). In most other studies, however, oils high in
alpha linolenic acid have little effect on cholesterol and triglycerides
(Am J Clin Nutr 65:1645, 1997).
Increased Risk of Bleeding:
As mentioned, one of the benefits of EFA is to decrease the risk of a
heart attack by decreasing the tendency for a blood clot to form by
"thinning" the blood. Alpha linolenic acid is much more effective at
decreasing the tendency of platelets to stick together than linoleic
acid (Euro J Clin Nutr 49:169, 1995. However, when you decrease the
clotting tendency of the blood you also increase the bleeding time and
the risk of a fatal bleed after an accident or death during a
hemmorhagic stroke. (Rheumatology International 14:231, 1995).
Nutritional Imbalances:
When a large amount of one type of nutrient is given then it displaces
the metabolism of other similar type nutrients. For example, high doses
of eicosapentaenoic (fish oil) given to westerners also lower levels of
dihomogammalinolenic acid (DGLA), a substance with a wide range of
desirable cardiovascular and anti-inflammatory actions. (Prostaglandins
Leukot Essent Fatty Acids 44:127, 1991). Proper balance is more likely
the closer the source of EFA is to its natural origin--plant foods.
Immune System Suppression:
EFA of both the w-3 and w-6 types inhibit our immune system--especially
human lymphocyte cell-mediated and the production and activity of immune
substances (Immunology 92:166, 1997). This includes suppression of
natural killer cells, the production of immune substances known as
cytokines (interleukin-1 (IL-1), IL-2, tumor necrosis factor-alpha (TNF-alpha)
and also interferon-gamma production. These immune functions are
important for defending ourselves from viruses, bacteria, and parasites,
and cancer cells.
Obesity: Body fat represents that saved "metabolic dollar" for
the day when food becomes unavailable (which hasn't happened lately).
Vegetable fats, including olive oil and EFA are as easily stored as fat
from cows, pigs, and chickens. When 54 obese women in a Mediterranean
country were studied, they were found to be following a diet low in
carbohydrates (35% of the calories) and high in fats (43% of the
calories). And 55% of the total of these fats came from olive oil (Horm
Metab Res 27:499, 1995).
It has been suggested that certain kinds of EFA might help people lose
weight. However, a 12 week, double-blind evaluation of evening primrose
oil as an antiobesity agent on 100 women found no significant difference
in the weight loss achieved by those taking primrose oil compared with
placebo (Int J Obes 7:549, 1983).
Cancer: Hundreds of studies since 1930 have been done on the
effects of dietary fat on cancer occurrence in experimental animals.
Both animal and vegetable fats have been shown to increase the risk of
animals developing and dying of cancer (Cancer Res 52:2040, 1992). The
risk of spread (metastasis) is also increased with greater fat intake.
Most of the effects occur during the promotion stage rather than at the
time of initiation of the cancer (when it begins).
Linoleic acid found in large amounts in corn and safflower oils is the
strongest promoter of cancers of all the fatty acids. Olive, fish,
flaxseed and other w-3 essential fatty acids have been shown to inhibit
the growth of cancers in animals when fed as pure fatty acids. However,
after the addition of small amounts of linoleic acid (like corn oil)
they lose some or all of their ability to block tumor growth (Am J Clin
Nutr 66:1523S, 1997). Therefore, it appears that a small amount of
linoleic acid must be present for a fat to be cancer promoting. Of
course, this small amount of linoleic acid will be in all natural human
diets.
The reason some studies have shown olive oil to be cancer promoting and
others have not is probably because of the varying amounts of linoleic
acid in commercially available olive oils. There appears to be some
balance between w-3 and w-6 fatty acids that is ideal for tumor
inhibition, unfortunately that ratio varies with different experimental
models. Because all of the types of fatty acids have been found to be
cancer promoting under some circumstances, prudence would dictate that
all fats, regardless of who labels them "good fats," be kept to a
minimum in your diet.
Heating Oil:
The heating of oil can produce cancer-causing byproducts. The lung
cancer incidence in Chinese women is among the highest in the world, but
tobacco smoking accounts for only a minority of the cancers. Chinese
women are exposed to indoor air pollution from wok cooking. Cancer
causing chemicals from heating cooking oils are dispersed into the air.
In a recent experiment several cooking oils and EFA were heated in a wok
to boiling (J Natl Cancer Inst 87:836, 1995). The oils tested were
unrefined Chinese rapeseed, refined U.S. rapeseed (known as canola),
Chinese soybean, and Chinese peanut, in addition to linolenic, linoleic,
and erucic fatty acids. Cancer causing substances such as 1,3-butadiene,
benzene, acrolein, formaldehyde, and other related compounds were
detected, with emissions tending to be highest for unrefined Chinese
rapeseed oil and lowest for peanut oil. Among the individual fatty acids
tested, heated linolenic acid produced the greatest quantities of cancer
causing substances (1,3-butadiene, benzene, and acrolein). Condensates
from heated linolenic acid, but not linoleic or erucic acid, were found
to be highly cancer causing.
WHAT TO DO?
The safest and healthiest way to get your EFA is in their natural
packages of starches, vegetables, and fruits. Here they are found in the
correct amounts in protected environments surrounded by vitamins,
minerals, fibers, antioxidants, and other phytochemicals to make them
balanced nutrition. If you desire higher concentrations than are present
in these foods then you will want to include more nuts, seeds, and
soybean products in your diet. Remember, these are high fat foods and
can contribute to obesity. Research suggests that there may be a
connection between frequent nut consumption and a reduced incidence of
coronary heart disease (Nutr Rev 54:241, 1996).
Flaxseed (as a whole seed) is one of the richest sources of alpha
linolenic acid and is also a good source of soluble fiber. Consumption
of 50g (1 2/3 ounces) of raw, ground flaxseeds has been shown to
increase the amount of w-3 EFA in the blood and tissues and to lower the
cholesterol by 9% and LDL "bad" cholesterol by 18% (Br J Nutr 69:443,
1993). Blood sugar was also decreased. Even though the benefits of EFA
as oils on cancer growth are questionable, the lignans present in
flaxseed seem to have an antitumor effect when fed at the early stages
of cancer promotion (Nutr Cancer 26:159, 1996). Plant foods are the only
source of phytoestrogens, like isoflavones, coumestans, and lignans,
that are believed to be beneficial for many problems, including
menopausal symptoms, osteoporosis, cancer, and heart disease (Annu Rev
Nutr 17:353, 1997).
Flaxseed is also an excellent laxative. The number of bowel movements
per week is found to increase by 30% with the addition of 50g of
flaxseed daily. The seeds can be added to hot or cold grain cereals and
consumed whole. Uncle Sam cold cereal and Prairie Sun Hot Cereal sold in
the natural foods store have flaxseed. Or it can be ground in a coffee
grinder and applied to almost any dish. About 5 tablespoons of ground
flaxseed daily should have a positive effect. A mixture of ground nuts,
seeds and vegetables, called "The Missing Link," is sold in many natural
food stores or can be ordered by calling (800) 446-2110. Refrigerate
oils and ground seeds because they oxidize easily and become rancid.
For treating some conditions, such as rheumatoid arthritis or diabetic
neuropathy, you may want to try oils high in gamma linolenic acid, such
as primrose oil. The doses used are � to 1 � grams a day of gamma
linolenic acid. This free oil is no longer to be thought of as a
food--it is a medication used to treat symptoms of a disease with both
positive and negative effects.
