In the blood, cholesterol is carried by particles
known as lipoproteins. There are two types of lipopro-
teins: low-density lipoprotein (LDL) and high-density
lipoprotein (HDL). When LDLs supply more choles-
terol than is needed, they deposit it on the walls of
arteries, forming plaque. Atherosclerosis is a disease
that results from the buildup of these fatty deposits,
restricting the flow of blood, increasing blood pres-
sure, and increasing the likelihood of a heart attack or
By contrast, HDL particles carry excess cholesterol
from the tissues back to the liver, where it is
converted to other substances or eliminated.
For these reasons, doctors and nutritionists
recommend diets that lead to a high amount
of HDLs and a low amount of LDLs in the
Studies have shown that eating too much
polyunsaturated fat lowers both HDL and
LDL, but consuming monounsaturated fatty
acids can lower LDL while raising HDL. So
monounsaturated fatty acids, which are found in
high amounts in olive oil, avocado, and fish (especially
salmon and mackerel), acts as a happy medium.
Keeping your HDL high and your LDL low by making olive
oil and other sources of monounsaturated fatty acids a staple in
your diet can help prevent the arter-
ies from narrowing. By contrast, an
excessively high ratio of linoleic acid
(and other omega- 6 fatty acids) to
α-linolenic acid (and other omega- 3
fatty acids) can lead to health condi-
tions that affect the immune and
nervous systems over time.
Armed with this new perspective
on the chemistry of fatty acids, Jack
knew exactly what to order when the family went out to the newest sea-
food restaurant in town: grilled salmon, topped with a slice of avocado,
brown rice, and a salad with balsamic vinegar and olive oil.
Healthy-Fat Foods. WebMD Slideshow: http://www.webmd.com/diet/ss/
slideshow-healthy-fat-foods [accessed Sept 2015].
Dietary Fats: Know Which Types to Choose. Mayo Clinic: http://www.mayoclinic.
[accessed Sept 2015].
Lipids: An Introduction. VisionLearning: http://www.visionlearning.com/en/
library/Biology/2/Lipids/207 [accessed Sept 2015].
David Warmflash, M.D., is a science writer who lives in Portland, Ore. His
latest ChemMatters article, “Tooth Decay: A Delicate Balance,” appeared in the
October/November 2015 issue.
There are mainly two compounds created from
α-linolenic acid, called eicosapentaenoic acid (EPA)
and docosahexaenoic acid (DHA), which are used
to make other compounds that have roles in the
immune and nervous systems. So, when the production of these compounds is disrupted, it can
lead to a range of conditions over many years. For
instance, the cell membranes of nerve cells have high
proportions of DHA, and animal studies have shown that
depletion of DHA in the brain can result in learning deficits.
Finding a healthy balance
Having learned all of this chemistry,
Jack figured out three possible ways
for dealing with the larger proportion
of linoleic acid compared to α-linolenic
acid in food. One solution would be to
reduce the dietary intake of linoleic acid,
but that is difficult, given that many
foods contain a large amount of it.
Another solution would be to
compensate for the high levels of
linoleic acid in the diet by eating as
much α-linolenic acid as possible. For
instance, Jack’s mother is doing this by
sprinkling flax seeds on her salad and
her baked potatoes, because flax seeds
contain a lot of α-linolenic acid.
Some people are taking a third
approach. In addition to eating more α-linolenic acid, they try to consume
EPA and DHA in high amounts, because doing so can bypass the need to
make these fatty acids from α-linolenic acid in the first place. A good source
of EPA and DHA is fish and other seafood. The oils in fish are loaded with
EPA and DHA, so Jack persuaded his family to eat more seafood.
Olive oil, cholesterol, and
Keeping a healthy ratio of linoleic acid to α-linolenic acid—two polyunsaturated fatty acids—is not the only way to stay healthy, Jack learned.
that lead to a high
amount of HDLs and
a low amount of
LDLs in the blood.
Figure 4. Chemical structures of (a) linoleic acid; and (b) α-linolenic acid.
Note that both structures have cis double bonds.