Sunday, May 25, 2014

Fat

Fatty Acids

There are four main types of fatty acids, which are classified by the number and structure of carbon-carbon double bonds:

·         Saturated fatty acids (SFA) (no double bonds)
·         Monounsaturated fatty acids (MUFA) (1 double bond)
·         Polyunsaturated fatty acids (PUFA) (>1 double bond)
PUFAs can be further broken down into omega 3s (n-3) and omega 6s (n-6) which refers to the
·         Trans fatty acids (TFA) (≥ 1 double bond in the trans position, mentioned later) 

There are many types of fats, below is a table of some of the common fats categorised by the number of carbons* and double bonds and those in bold tend to be the most commonly mentioned


Foods contain a number of different fatty acids in different proportions, so it’s wrong to say ‘X is a saturated fat’

·         Animal fats have fairly equal proportions of SFA and MUFA, with ruminants and dairyhaving low PUFA and poultry,eggs, pork andfish/seafood having more PUFA
·         Fish/seafood is the best source of long chain omega 3s (LCO3s) such as EPA and DHA but grass-fed/pastured meat, dairy and eggs also contains some (algae also contains DHA)
·         Olive oil is very high in oleic acid (hence the name)
·         Nuts, seeds and seed oils (sunflower, safflower, soybean, cottonseed, canola, etc) are generally high in PUFA from linoleic acid, except canola which is high in MUFA and flax which is high in alpha-linolenic acid
·         Coconut oil is the major source of lauric acid

A notable fat that’s missing from this list is butyric acid (4:0), which is produced by bacterial fermentation of fibre in the large intestine and is also found in dairy.

* Short chain fatty acids (SCFA) are those with < 6 carbons, medium chain with 6-12, and long chain with > 12

PUFAs and Eicosanoids

Of the fats, only linoleic acid and alpha-linolenic acid are essential (because they can’t be synthesised) and so are considered the essential fatty acids, but this misses the point.  The omega 6 and omega 3 families of PUFA have a number of biological functions when DGLA/AA and EPA/DHA are converted to eicosanoids (or docosanoids for DHA) by the cyclooxygenase enzyme family (COX).  In their present form, LA and ALA fulfil no essential functions and need to be converted to DGLA/AA and EPA/DHA (respectively) by desaturation and elongation enzymes (see below).  The conversion of ALA to DHA is so low (~0.5%) that DHA has been suggested to be an essential fatty acid as well [1]

There are a number eicosanoids produced from long chain PUFAs, and each in turn has several biological functions.  As a general rule, those produced from AA are pro-inflammatory, while those produced from DGLA, EPA and DHA are anti-inflammatory.  However, increasing dietary AA, at least in the short term doesn’t seem to increase inflammation [2], while increasing EPA+DHA reduces markers of inflammation [3] [4]


Trans Fats

One of the properties of PUFAs is that the presence of multiple double bonds makes them vulnerable to oxidation (causing them to become rancid more quickly (shorter shelf life)). This was a problem for those promoting PUFA rich vegetable (seed) oils in the early 20th century as being both cheaper and healthier than animal fats.  Fats/oils can be partially hydrogenated to improve shelf life.  Partial hydrogenation is a process that adds hydrogens to the fats, thereby reducing the number of double bonds.  However, partial hydrogenation results in double bonds where the hydrogen atoms are at different sides of the double bond (trans), whereas the hydrogens are at the same side in the double bonds of other fats the (see below).

The trans fats (TFA) produced by this process certainly have a longer shelf life and are more solid at room temperature (mimicking animal fats), but increase inflammation and have a number of deleterious effects, particularly related to cardiovascular disease [5]. A common TFA produced by partial hydrogenation is elaidic acid (18:1 n-9) (an isomer of oleic acid) and these TFA are commonly found in partially hydrogenated oils and certain processed foods.

However, there are trans fats that actually promote health such as vaccenic acid (18:1 n-7), which is present in ruminants and dairy [6].  There are also a family of isomers (same molecules, different structure) of linoleic acid called conjugated linoleic acids (CLA), which have both a cis and a trans double bond (a conjugated double bond).  The most common of these is rumenic acid (18:2cis-9, trans-11), which is also found in ruminants and dairy (hence the name) and seems to promote health as well [7].  Unfortunately the health effects of CLA are often tested using a different isomer of CLA (18:2 trans-10,cis-12)


* Full hydrogenation of linoleic acid (for example) would simply convert it to stearic acid

Fats and Cholesterol

One of the things people are most concerned about with fats is how they affect cholesterol levels, so here you go

Relative to carbohydrate:

·         SFA on average increases HDL-C and LDL-C, but doesn’t significantly change the total cholesterol to HDL-C ratio (perhaps the best blood lipid based risk factor for CVD [8])
·         MUFA and PUFA increases HDL-C and decreases LDL-C and the total cholesterol to HDL-C ratio
·         TFA increases LDL-C and the total cholesterol to HDL-C ratio*

And relative to carbohydrate for other CVD risk factors:

·         SFA, MUFA and PUFA reduces triglycerides, particularly long chain omega 3s
·         MUFA and PUFA, but not SFA (no change) decreases apo B (protein of LDL)**
·         SFA and MUFA, but not PUFA(no change) increases apo A-1 (protein of HDL)**


Whether or not this is meaningful is another story.  I have argued that you can’t simply assume how things willaffect CVD based on how they affect blood lipids (here and here) and that replacing SFA with PUFAmost likely doesn’t reduce CVD (here)

* It’s interesting that this study found TFA didn’t affect HDL-C when so many studies mention that TFA reduce HDL-C by increasing CETP

** The number of LDL and HDL particles is a better risk factor than the amount of cholesterol in those particles

*** The effects of different SFAs is shownhere

**** The effects of different fats is shown here.  Notably replacement of all whole food fats lowers the total cholesterol to HDL-C ratio

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