Fat is a macro-nutrient – a nutrient that provides calories, which we accordingly consume in relatively large quantities (compare vitamins, of which we consume very small amounts).
Dictionary Definition of “fat”
- a natural oily substance occurring in animal bodies, especially when deposited as a layer under the skin or around certain organs
- [Chemistry] any of a group of natural esters of glycerol and various fatty acids, which are solid at room temperature and are the main constituents of animal and vegetable fat
There are about 9 Calories in a gram of pure fat.
Butter is about 80% fat, due to the water content.
Oils, like olive oil, are 100% fat.
Fats are esters of glycerol and fatty acids.
Fats are in the family of chemicals known as “lipids”.
“Lipid” definition: organic compounds that are fatty acids or their derivatives and are insoluble in water but soluble in organic solvents
“Lipid” origin: from Greek “lipos” meaning “fat”
Types of Fat
Fat is mostly made up of fatty acids.
Fatty acids can be grouped into three classes: saturated, mono-unsaturated, poly-unsaturated
These classes are named according to the chemical structure – “saturation” indicates how full of hydrogen atoms the fatty acid molecule is.
Fats exist as combinations of these three classes in various proportions.
SFA, Saturated Fatty Acids
Named so because the fat molecule is saturated with hydrogen atoms.
Thus, there are no carbon-carbon double bounds in the backbone of the molecule.
By having this uniform molecular structure, SFA form solids more readily – the backbones of the molecules line up alongside each other more easily since there are no kinks in the chain.
SFA therefore tend to be solid at room temperature.
SFA are typically from animal sources.
Examples: butter (milk fat), lard (pig fat), tallow (cow fat); coconut oil
There are long-standing health concerns over SFA consumption.
These concerns stem from the effects of SFA consumption on blood cholesterol levels, and then the assumption that those changes contribute to heart disease risk.
The Logic for “SFA => Hearth Disease Risk”
SFA have been shown to increase total blood cholesterol levels.
If you assume that total blood cholesterol is a useful indicator for heart disease risk, then it will seem that consuming SFA promotes heart disease.
But, cholesterol measurements and their use as cardiovascular disease risk indicators are not that simple, and the deeper you look, the less SFA seem to be a risk.
MUFA, Mono-Unsaturated Fatty Acids
The molecule is one pair of hydrogen atoms short of being full – “mono-unsaturated”.
There is one double bond in the carbon backbone.
Naturally occurring MUFAs are in the cis molecular configuration, where there is a kink in the chain.
Since MUFAs are also typically plant sourced, it follows that the fats from plants tend to be liquid at room temperature – hence “plant fats” sounds odd, because we generally call them “oils” (liquid fats).
Examples of mostly-MUFA oils: olive oil, safflower oil, rapeseed (“canola”) oil
Apparently some sunflower oils are very high in MUFA, but…*
PUFA, Poly-Unsaturated Fatty Acids
More than one pair of hydrogen atoms short of saturation.
Multiple double bonds.
Because oils are liquid at room temperature, they go rancid relatively quickly – they have a shorter shelf life than the more stable SFA and PUFA.
Fats go rancid when they are broken down by exposure to: air, light, moisture, bacteria
Food manufacturers can chemically process PUFAs to increase the degree of hydrogen saturation – this is called hydrogenation.
Hydrogenation changes the chemical structure of fatty acids, increasing the number of hydrogen atoms in the molecule and thus replacing some double bonds with single bonds.
Hydrogenation therefore converts some MUFA to SFA, and some PUFA to MUFA or SFA (or just more-saturated PUFA).
This conversion changes the physical properties of the fatty acids, including their melting (temperature) range, so they are closer to solid than liquid at room temperature; it improves shelf life and stability (heat resistance).
Unlike simpler substances that have distinct melting points (specific temperature at which it melts/solidifies), fats and other polymeric substances have melting ranges (temperature ranges throughout which they exist as varying proportions of solid and liquid, “semi-solids”) – think of butter and spreads.
A higher melting range will make a substance more solid at room temperature.
This is a desirable trait for baking.
In summary: You can take a liquid “vegetable” (seed) oil, high in PUFA, which is cheaper than a naturally-more-solid-and-stable animal fat, and process the oil to make it more solid and stable and give it a longer shelf life (i.e. turn it into margarine), so you can have cheap fats that are good to bake with and that you can store for a long time.
There are more incentives to use processed plant oils beyond their being cheaper than animal fats. For example, if you believe SFA are a cardiovascular disease risk, then avoiding animal fats, which are naturally high in SFA, in favour of lower-saturation MUFA seems beneficial. There’s even the apparent bonus of hydrogenated MUFA being more solid and thus better for baking.
However, the health risks of trans fats outweigh those of saturated fats.
The major problem with partial hydrogenation (the incomplete conversion of PUFA and MUFA to SFA) is that the conversion to unnatural variations of the fat molecules is favoured during the chemical processing.
These differently-chemically-structured variants are called trans isomers (hence “trans fats”) and they are contrasted with the natural cis form of the same molecule.
The difference manifests in how the unpaired hydrogen atoms are arranged on either side of any double bonds.
It is relevant to note that a C-C double bond does not allow rotation.
In a trans fat, the hydrogen atoms are arranged on opposite sides of the molecular backbone.
In a cis fat, the H atoms are on the same side of the backbone.
trans – “across”
cis – “on the same side”
Image source: http://ib.bioninja.com.au/
The result is that a trans fat molecule is relatively straight,
whilst a cis fat molecule has a kink in the chain at the double bond.
This makes a trans-MUFA act more like a SFA – more solid, longer shelf life, better heat stability.
The problem is that the human body doesn’t seem to handle trans fats well.
Trans fats show a significantly higher correlation with cardiovascular disease than other fats.
In autopsies, trans fats are found in high proportions in the fatty deposits of atherosclerotic arteries.
Admittedly, this is what implicated dietary cholesterol as a cardiovascular disease risk factor historically, but it is now widely known that consuming cholesterol-rich foods does not result in cholesterol build-up in the artery walls (a major feature of heart disease). But, in the case of cholesterol, every cell in the body needs it, so our bodies make over a gram of it each day and tightly regulate how much of it passes in and out of the body via the gut; whereas, trans fats are unnatural forms of what would be familiar fat molecules, so finding a high proportion of trans fats in atherosclerotic plaques probably should raise suspicions regarding them.
Types of PUFA
Omega-3 PUFA are famously anti-inflammatory, and are found in oily fish.
The DHA and EPA (aka “long chain omega-3”) from oily fish are thought to be the key beneficial forms of PUFA, so the ALA from flaxseeds, walnuts and newer breeds of oilseed rape should not be mistakenly attributed the same benefits.
Furthermore, the body is very inefficient at converting ALA to DHA/EPA.
Examples of oily fish (sources of DHA and EPA): salmon, herring, pilchard, sardine, trout, mackerel
Note that cold-water fish will typically be fattier (= more oil) as they need it for insulation.
Omega-6 PUFA are found in “vegetable” (seed) oils.
A diet unnaturally high in omega-6 PUFA is said to be pro-inflammatory.
Examples of omega-6 sources: soybean, corn (“maize”), sunflower
Note that soybean and corn products are ubiquitous in USA
Note on High Oleic Sunflower Oil, omega-6:-3 ratio, inflammation issues
*Apparently some sunflower oils are very high in MUFA, but… there’s a concern about the high levels of Linoleic Acid (LA), an omega-6 PUFA, in typical sunflower oil, due to their inflammatory effects.
The ratio of omega-6 to omega-3 PUFA in the diet is an important indicator of risk for excessive or insufficient inflammation (a natural bodily response – too much or too little inflammation is bad).
But, consuming excessive amounts of omega-6 PUFA can block the effects of any omega-3 consumed in lesser amounts in an attempt to balance out the inflammatory effects, since the two classes of PUFA compete for resources within the body.
Therefore, to benefit from the anti-inflammatory effects of omega-3 PUFA, you should aim to reduce omega-6 consumption whilst increasing omega-3 consumption via oily fish, to achieve a ratio of between [4:1 and 1:4] for [omega-6 to omega-3].
As I recall however, there’s no definitive evidence for these ratios – they’re just observations from the diets of healthy populations.
For reference, the average ratio in a typical Western diet is around 15:1 – so we might aim to halve the seed-sourced PUFA we consume, whilst doubling the fish-sourced PUFA, to get a “healthier” ratio.
Find sources that show high omega-6 and/or high 6/3 ratio promote inflammation.
some links here – https://www.wikiwand.com/en/Omega-6_fatty_acid
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