B2 High-Intermediate UK 460 Folder Collection
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Now why have we insisted in making all those distinctions in the structure of fatty acids?
saturated and unsaturated; mono and polyunsaturated, cis and trans, omega-6 and omega-3? Well,
because these structural differences result in very different behaviors of lipids in our
body. So the quality of lipids in our diet very much depends on the balance between these
different groups of fats. Let’s start examining them from a nutritional point of view.
Saturated fatty acids are mainly although not exclusively found in food of animal origin.
They are nutritionally non-essential. Remember, this doesn’t mean that they are not important,
just that if necessary we can build them ourselves. Our body can build saturated fats from glucose
and from some amino acids. Stearic acid, the saturated fatty acid 18 carbons long, is preferentially
incorporated into triglycerides for storage in our adipose tissue. Based on what you know,
you should be able to figure out why it wouldn’t be possible to use unsaturated fats for storage:
at body temperature, they would melt. However an excess of saturated fatty acids
in our diet is detrimental because many of them are atherogenic, they promote endogenous
synthesis of cholesterol, they raise LDL cholesterol - soon we will learn why this is bad - and
they promote the process of atherosclerosis putting us at risk for cardiovascular disease.
For these reasons, we don’t want to eat too many of them.
However also keep in mind that not all saturated fatty acids are equally atherogenic. It’s
not true that all saturated fats are bad. In fact, some of them are pretty much neutral.
The above mentioned stearic acid, for example, is quite innocent, it’s used for storage
and it can also be easily converted to the monounsaturated oleic acid, so it doesn’t
harm anybody. Palmitic acid, C16, is more or less neutral. But then there are other
that are really BAD, such as the medium chain lauric, C12, and miristic C14, which are highly
atherogenic.
This slide shows you some dietary sources of saturated fats. As you can see, most of
them are animal. Moving from left to right, the quality of their fat gets worse, due to
the highest presence of those “bad” fatty acids such as lauric and miristic.
Eggs have a lot of stearic so they are pretty much innocuous, then we have chicken meat,
milk and dairy, red meat, and then on the very right, you find some vegetable sources
of a lot of saturated fats, coconut and palm. We normally don’t use them in the kitchen,
but the food industry likes their oils very much because they are cheap and have interesting
technological properties, so coconut kernel oil, palm oil and palm kernel oil are commonly
used as ingredients in many products. Palm oil is what we make from the pulp of the fruit,
the yellow part, and then palm kernel from the white kernel. Not only they are very rich
in saturated fats, but they have the worst type of saturated fats, lauric and miristic.
We refer to palm kernel and coconut kernel oils as “tropical fats”, they are bad
and we don’t want too much of them in our diet, so make sure to look for them on the
ingredient lists and avoid food that contain them whenever possible. Palm oil is a little
bit better because it has more palmitic acid. Cocoa butter, from cocoa, also contains saturated
fats but it’s mostly stearic, so it’s much better.
Let’s move on to the monounsaturated fats. They are also not nutritionally essential
because they can be built from the corresponding saturated fatty acids of the same length,
so for example stearic C18 can be converted into oleic because our body can introduce
the unsaturation in position nine. Even if they are not essential, we still should
try to get them from food and especially oleic acid which is probably the best possible fatty
acid in our diet because it is very beneficial to our health. It lowers blood cholesterol,
improves the HDL to LDL ratio in our bloodstream - again we will understand this better soon
- and in general it exerts a protective action against cardiovascular disease.
But don’t think that all monounsaturated fats are equally as good. For example, erucic
acid, C22, is toxic for us. It was once abundant in rapeseed oil, which is why it couldn’t
be marketed for human consumption for a long time, but today we have developed a genetically
modified version of rapeseed that builds oleic instead of erucic, we call it canola and it
is now ok for human consumption.
We find oleic acid in olives and olive oil, which should really be our main dietary sources
of this good fat. Olive oil is an excellent oil, it has a rich flavor, it doesn’t easily
get oxidized so it’s quite stable during storage, and cooking, although to maximize
its health promoting activity it is best eaten raw, so that its good polyphenols are not
destroyed. We also have a lot of oleic acid in macadamia
nuts, avocados, canola oil, peanuts, and of course peanut butter. About half the fat in
peanuts is polyunsaturated, the other half is oleic. Be careful however, peanut butter
is one of those products in which the food industry likes to add palm oil for color,
flavor, texture and stability, but this also adds saturated fats. Go for the old-fashioned
version whenever possible. Finally, we also find some oleic acid in sesame seeds and then
nuts and seeds in general, although polyunsaturated fats are prevalent.
Now let’s move now to the polyunsaturated fats. Some of these molecules are extremely
important. They have key structural functions as part of cell membranes phospholipids, especially
in those tissues with complex systems of signal transmission, such as our brain and nerves,
the retina in our eyes, and the outer structure of our skin.
As if that wasn’t enough, they also have key regulatory functions ad precursors of
the eicosanoids, a class of hormone-like substances that we will explore later, and that are master
regulators of countless areas of our metabolism, including immunity, inflammation, blood pressure,
blood fluidity and stomach acidity.
We already know that the most relevant polyunsaturated fats belong to two different families, the
omega 6 and the omega 3 family. In the omega-6 family, we know that linoleic
acid, C18:2, is essential. There’s no way our body can make it starting from something
else, because we do not have any enzyme to introduce unsaturations in position 6. However
we can modify the chain length, so all the other omega-6 are not essential because they
can be derived from linoleic acid, once we have that.
Luckily it is not very difficult to get linoleic from food because it’s quite common,. Its
richest sources are nuts and seeds, and of course the oils we make from them, such as
corn oil or sunflower oil.
In the omega-3 family, alpha-linolenic is the other essential fatty acid in our diet.
Again, all the other omega-3 can be derived from it, however this time there’s a couple
more problems. The first one is that alpha-linolenic is not as widespread in food as linoleic,
so we may very well not be getting enough in the first place. And then on top of that,
the conversion of alpha-linolenic into the other omega-3 fatty acids is very slow. And
this is the reason why it is recommended to have a direct intake of these two other omega-3
fatty acids, EPA eicosapentanoic and DHA docosaesanoic, although they are not strictly essential,
but they are very important for their structural and regulatory functions. They are precursors
of some important eicosanoids, and they affect lipid metabolism to lower blood triglycerides
and improve blood HDL to LDL cholesterol ratio.
The richest source of alpha-linolenic acid are flaxseeds and chiaseeds, and of course
flaxseed oil but this gets very easily oxidised so it must be stored very carefully and rancidity
is a problem. Another good source of alpha-linolenic acid are walnuts, and then there’s some
in soy, soybean oil, rapeseed, and rapeseed oil, as well as some type of grass that can
be fed to hens to make omega-3 enriched eggs or cows to have omega-3 enriched milk.
EPA and DHA are mainly found in fish, especially fatty fish such as salmon, anchovies and sardines.
We said it’s advisable to have a direct intake of these fatty acids, and two to three
servings of fish a week will provide us with all the EPA end DHA we need. For those who
don’t eat fish, some seaweeds are rich in DHA, and then of course there are supplements,
or we can make do with the precursor alpha-linolenic from flaxseeds and walnuts.
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Nutrition Steps 5.6 - Fat Quality Is What Really Matters

460 Folder Collection
陳秋汝 published on May 24, 2016
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