Monday, 22 December 2008

Fats: Spawn of Satan or Dogs' Doodads?

From http://ajcn.nutrition.org/content/65/5/1628S.full.pdf

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 http://4.bp.blogspot.com/-fdLiVnZlbqA/UcHC376NkBI/AAAAAAAAAmo/J1SDPyV4S84/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

2 comments:

ProudDaddy said...

In a perfect world, would you want to tweak your fat consumption recommendations to consider chain length; e.g., more saturated okay if MCT?

Nigel Kinbrum said...

I don't consider it necessary to tweak things to the n'th degree.

I've just updated this post as there were a couple of dead links in it.