Fats get a lot of bad press in the media. There are so many adverts with "
X% fat-free" or "
only 1g of fat per Jaffa Cake" as if that's going to stop you from getting fat when you "
om nom nom" your way through a whole box of the things!
Saturated fats are usually described as "
bad" and
polyunsaturates are usually described as "
good". This is simplistic.
Everything is bad in
excess, even polyunsaturates. The thing about fats is that there are
four basic types (
saturates,
monounsaturates,
ω-6 polyunsaturates and
ω-3 polyunsaturates) and they need to be consumed in
roughly the right proportions for optimum health. Suffice it to say, the majority of people in the West do not eat them in anywhere near the right proportions. So, what exactly are fats?
Fats
Fats are an
ester of
glycerol (
CH2OH-CHOH-CH2OH) &
3 fatty acids, and are also known as
triglycerides (
TG's) or
triacylglycerols (
TAG's).
1 molecule of glycerol +
3 molecules of fatty acid =
1 molecule of triglyceride +
3 molecules of water.
It's the
fatty acids that determine whether a fat is
saturated,
monounsaturated etc. The four different types of fatty acid have a
CH3 at one end and a
COOH at the other. The difference is in the middle section.
Saturated fatty acids have a middle section consisting of
CH2's. Here's a diagram for
Stearic acid (the predominant fatty acid in
beef):-
__H H H H H H H H H H H H H H H H H O
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-O-H
__H H H H H H H H H H H H H H H H H
Monounsaturated fatty acids have
one C=C bond in the middle, which is usually (but not always) 9 from the left-hand end, resulting in monounsaturates often being referred to as
ω-9's, as
ω is the last letter of the
Greek alphabet. Here's a diagram for
Oleic acid (the predominant fatty acid in
olive oil):-
__H H H H H H H H_____H H H H H H H O
H-C-C-C-C-C-C-C-C-C=C-C-C-C-C-C-C-C-C-O-H
__H H H H H H H H H H H H H H H H H
ω-6 polyunsaturated fatty acids have
two or more C=C bonds in the middle, with the last one always being 6 from the left-hand end. Here's a diagram for
Linoleic acid (the predominant fatty acid in
sunflower oil):-
__H H H H H_____H_____H H H H H H H O
H-C-C-C-C-C-C=C-C-C=C-C-C-C-C-C-C-C-C-O-H
__H H H H H H H H H H H H H H H H H
ω-3 polyunsaturated fatty acids have
three or more C=C bonds in the middle, with the last one always being 3 from the left-hand end. Here's a diagram for
Alpha-linolenic acid (the predominant fatty acid in
flax-seed oil):-
__H H_____H_____H_____H H H H H H H O
H-C-C-C=C-C-C=C-C-C=C-C-C-C-C-C-C-C-C-O-H
__H H H H H H H H H H H H H H H H H
These diagrams are
slightly misleading. Where there is a
C=C bond, there are two
H's on the "underside" only of the molecule. This
asymmetry causes the
H's to
repel each other and
bend the molecule into a
V-shape at each
C=C bond.
C=C bonds with
H's on the same side are known as "
cis" bonds. The above molecule is really cis, cis, cis (c,c,c) Alpha-linolenic acid. The other type of
C=C bond is known as "
trans" and looks like the following diagram:-
__H H H___H H___H H___H H H H H H H O
H-C-C-C=C-C-C=C-C-C=C-C-C-C-C-C-C-C-C-O-H
__H H___H H___H H___H H H H H H H H
This is a diagram of trans, trans, trans (t,t,t) Alpha-linolenic acid. As the
H's are on opposite sides of the molecule, they
do not repel each other and the molecule is
straight, as shown above. Note that
saturated fatty acid molecules are naturally straight. Therein lies the problem with
trans-fatty acids. They're
straight, like
saturated fatty acids, but they have
unsaturated bonds, which are prone to
peroxidation. See
http://www.cyberlipid.org/perox/oxid0002.htm WARNING! Heavy-duty organic chemistry!
Our bodies take
trans-fatty acids and incorporate them into
cell membranes as if they were
saturated fatty acids. This results in
atherogenicity (
artery-clogging), damage to the
immune system and other
health problems.
Trans-fatty acids are found in
partially-hydrogenated vegetable oils, so any
cooking/spreading fats which have the word "
hydrogenated" high-up in the ingredients list should be avoided. These are
bad fats.
There are
naturally-occurring trans-fatty acids made by
bacteria in the
stomachs of
ruminant animals, like
Conjugated Linoleic Acid (CLA). This looks a bit like the diagram below:-
__H H H H H_____H___H H H H H H H H O
H-C-C-C-C-C-C=C-C=C-C-C-C-C-C-C-C-C-C-O-H
__H H H H H H H___H H H H H H H H H
This has one of the
C=C bonds shifted to the left and also has
one cis bond and one trans bond, so the molecule is
always bent. CLA has possibly beneficial properties, but human trials show
mixed results.
It's certainly not artery-clogging, so don't let anyone put you off eating
butter from
grass-fed cows (e.g.
Anchor or
Kerrygold butter) by saying that it has
trans-fats in it.
CLA is a harmless trans-fat.
Saturated fat consumption should be
about 10% of total calories. This is because, even though saturated fats are
not essential (our bodies can synthesise them), this guarantees adequate
synthesis of
sex hormones. Total
polyunsaturate consumption should be
about 5% of total calories, with a
ω-6:ω-3 ratio of between 1:2 and 4:1. As
ω-3's are found in greater quantities than
ω-6's only in
flax-seeds (a.k.a.
linseeds) and
oily fish, and many people eat
way too little or
no oily fish (and who, other than
body-builders and some
vegetarians/
vegans, eats
flax-seeds?), the
ω-6:ω-3 ratio in the
West is
~20:1. This is due to the widespread consumption of
meats,
eggs &
milk from
grain-fed animals,
grains,
nuts and
seeds. There are high rates of
heart disease and other
inflammatory diseases in the West, as
ω-6's end up in series 1 & 2 prostaglandins, and
series 2 prostaglandins are pro-inflammatory.
ω-3's end up in
series 3 prostaglandins, which are
anti-inflammatory.
So eat up yer
oily fish if you're not
vegetarian or
vegan. Otherwise, eat up yer
ground-up flax-seeds!
Monounsaturates can make up
about 15% to 35% of total calories, depending on
activity levels. From the histogram in
Everyone is Different,
sedentary people, on average,
burn twice as much energy from fats as from carbohydrates.
So, if
25% of energy comes from
protein say,
25% of energy can come from
carbohydrates, and
50% can come from
fats, i.e. a
2:1 ratio of
fats:carbs. The cyclists at the left-hand end of the histogram in Chapter 1 would do best on
25% protein,
15% carbohydrate,
60% fat, when
sedentary, whereas the cyclist at the right-hand end of the histogram would do best on
25% protein,
60% carbohydrate,
15% fat, when
sedentary.
When active, more carbs are needed by everyone.
Which
fats contain which
fatty acids? See
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgQcYggz8ORx7mdTT7_ngR-jKOG-2FB4sm5HZWFyJF7xFYlNn-XCkbs4tSIuFFaJFVUdFTw13Q5M2-Adu_jeVnu3O37NhPlK_XWrxIACJ_QT1KmVElpl1YoLO4nfBBoqJNI5FQlhSDh/s640/comparison-of-fats.gif for a
Comparison of Dietary Fats.
For
high-temperature cooking,
saturates are the
least likely to
oxidise (when they're on fire, they're oxidising!), followed by
monounsaturates, then
ω-6's, with
ω-3's being the
most likely to
oxidise. An
oil doesn't have to be
smoking, to be
oxidising. Alpha-linolenic acid
oxidises to
varnish at
room temperature without smoking, which is why
linseed oil is used to varnish
cricket bats and
dilute putty. The best
non-animal fat for
high-temperature cooking is
Coconut Oil, followed by
Palm Oil and then
Olive Oil.
Extra-Virgin Olive Oil (
EVOO) has a lower
smoke point than
refined Olive Oil (due to higher levels of
free fatty acids), but has higher levels of
polyphenol antioxidants, which makes it
heart-healthy.
Oils high in
polyunsaturates shouldn't be
heated to temperatures greater than 100°C, as
polyunsaturates can change from the
cis configuration to the
trans configuration at
102°C. See
http://www.harricksci.com/sites/default/files/pdf/application_notes/FatIR_App-Notes_ATR_Oil-Analysis.pdf
