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.