Some fats are very resistant to fatty acid peroxidation, whereas others are vulnerable. These reactions play an important role in our health, and can be affected greatly by the temperature at which the fatty acids are. Thus, cooking with the correct fats can help to lower your intake of damaged fats.
Here I will explain the chemistry and implications of it on health.
Lipid Peroxidation 101
To begin with, fatty acids are composed of two parts:
- A hydrocarbon tail, which is hydrophobic, aka fat
soluble (butane used as an example below). This chain can be anywhere from 4
to 28 carbons long and naturally occurring fatty acids only contain tails with
an even number of carbons.
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| Butane |
- A carboxylic acid group, which is hydrophilic, aka water
soluble. (shown below) *The "R" group simply means any carbon chain.
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| Carboxylic acid group |
Put together, these two above examples make butyric acid, a fatty acid commonly found in butter.
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| Butyric Acid |
Carbon atoms will form 4 bonds usually, and as it pertains
to this discussion, they always do.
Fatty acids can be broken into two main groups:
- Saturated: Every carbon in the hydrocarbon tail has as
many hydrogens bonded to it as possible(aka the carbons are 'saturated' with
hydrogens), meaning there are no carbon-carbon double bonds.
- Unsaturated: There are one or more carbon-carbon double
bonds(C=C), resulting in some carbons that are 'unsaturated', in that they
don't have as many hydrogens on them as could be possible. In nutrition, this
category is often divided further into monounsaturated fatty acids (MUFAs,
containing only one C=C bond) and polyunsaturated fatty acids (PUFAs,
containing two or more C=C bonds)
Double bonds change a few things about fatty acids,
including the structure(which affects melting point) and the rate of lipid
peroxidation.
Stearic Acid, a saturated fat commonly found in animal
products such as beef. Note the straight nature of the hydrocarbon chain; this
lends itself to fitting together well with other fatty acid molecules, so that
they can pack densely together, which results in a higher melting point.
Saturated fats are usually solid at room temperature.
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| Stearic Acid |
Oleic acid (the main MUFA in olive oil), an 18-carbon MUFA
is shown below. It has one C=C bond (notice the C=C bonded carbons only have 1
hydrogen attached to each of them, rather than 2 as is normal on saturated
carbons)
 |
| Oleic Acid |
Also, notice how the C=C bond makes a kink in the chain,
making this type of fatty acid less straight than saturated fatty acids. This
kink is responsible for the difference in the melting point between things like
butter (mostly saturated) and vegetable oil (mostly unsaturated). The more C=C bonds a fatty acid has, the more kinks
in the chain and therefore the molecules cannot layer together as well. This
reduces the inter(between)molecular forces, and lowers the melting point.
Carbon-carbon single bonds can rotate freely around the bond
axis, but carbon-carbon double bonds are locked in position (the double bond
has to be broken in order for it to rotate).
Here is an example of a polyunsaturated fatty acid, Docosahexaenoic
acid (aka DHA), a major component of fish oil, with 6 double bonds. Its
structure doesn't allow for the molecules to pack tightly, and therefore it has
a lower melting point than more saturated fatty acids of the same length(increasing length raises melting point due to increased Van der Waals forces).
 |
| Docosahexaenoic Acid (DHA) |
Now that we know how double bonds affect structure, what do
they have to do with cooking?
It turns out, that the hydrogens on the carbon adjacent to
the C=C bond, referred to as allylic hydrogens, are highly susceptible to
reacting by free radical mechanisms. When a hydrogen is taken, there is left a
lone electron (this is what we call a radical) on the carbon where the hydrogen
was. Because the free radical formed next to a C=C bond is surprisingly stable;
it doesn’t take that as much energy to cause the formation of a free radical as
it would without the presence of a double bond.
The double bond can shift between two resonance states
(essentially states of electron distribution), and stabilize the fatty acid
radical by spreading out the distribution of the free radical over two carbons
instead of one. Essentially, there is only a half free radical on each carbon
that shares the radical, and this sharing stabilizes the radical, lowering the
energy needed to achieve this state.
By lowering the energy needed to reach the radical state,
the reaction occurs at a much higher rate.
This is where cooking comes in. It is accepted that in
general, the rate of a reaction in chemistry about doubles for every 10 degree Celsius increase in temperature. This is because the rate of collisions between
molecules about doubles for each 10 degree increase in temperature.
In cooking, the increased heat causes a rapid increase in
the formation of fatty acid free radicals by the process shown below. The lipid
radical continues to react (propagation) with other lipids until either two radical react
(highly unlikely unless radicals are present in very high concentrations) or it
reacts with an antioxidant.
 |
| Lipid Peroxidation |
The consumption of lipids that have undergone peroxidation
will result in the absorption of these compounds. These compounds continue to
react with fatty acids in your body, causing major damage to cell membranes,
hormones, cholesterol, and more.
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(a) Initiation of the peroxidation process by an oxidizing radical X · ,
by abstraction of a hydrogen atom, thereby forming a pentadienyl radical. (b)
Oxygenation to form a peroxyl radical and a conjugated diene. (c) Peroxyl
radical moiety partitions to the water-membrane interface where it is poised
for repair by tocopherol. (d) Peroxyl radical is converted to a lipid
hydroperoxide, and the resulting tocopherol radical can be repaired by
ascorbate. (e) Tocopherol has been recycled by ascorbate; the resulting ascorbate
radical can be recycled by enzyme systems. The enzymes phospholipase A2 (PLA2),
phospholipid hydroperoxide glutathione peroxidase (PH-GPx), glutathione
peroxidase (GPx) and fatty acyl-coenzyme A (FA-CoA) cooperate to detoxify and
repair the oxidized fatty acid chain of the phospholipid. (from Buettner 1993).
|
The production of free radicals increases oxidative stress on the
body, which is known to play a key role in the progression many chronic
inflammatory conditions, including diabetes, cancer, heart disease, obesity,
etc[3]. Oxidative stress increases inflammation in the body [1],[2](inflammation
increases oxidative stress as well), which also reduces insulin sensitivity[4],[5],[6],
a key marker in diabetes.
Summary:
- Saturated fats do not undergo lipid peroxidation at anywhere near the rates that unsaturated fats do.
- You do not want to eat fats that have undergone lipid peroxidation
- Cooking increases the rate of lipid peroxidation
For a practical guide on what fats to choose, check out FAQs: What Are Safe Cooking Fats & Oils?
2. Oxidative stress, antioxidants, and
endothelial function.
3. Oxidative stress and diseases - Wikipedia
4. Inflammation and insulin
resistance.
5. Obesity, inflammation, and insulin
resistance.
6. Insulin sensitivity: modulation by nutrients
and inflammation
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