The difference between saturated and unsaturated fats

Fats (triglyceride lipids) are made up of glycerol and fatty acids; these fatty acids are either saturated, monounsaturated or polyunsaturated. They have the same basic structure (a carbon backbone with hydrogen branches and a carboxyl end); the difference lies in the number of used or spare hydrogen bonds.

Olive oil- mainly mono-unsaturated fat
Olive oil- mainly mono-unsaturated fat

By and large:

  • Saturated fats are solid at room temperature; we see these in meats
  • Monounsaturated fats are liquid at room temperature, but solidify when cool; we see this with olive oil
  • Polyunsaturated fats are liquid at room temperature and when cool; we see this with sunflower oil

Because of this, meat fats are seen as ‘saturated’ and vegetable oils as ‘polyunsaturated’, but all fat-containing foods include some of each type of fat. 

Click here to see which fats you should eat and why.

A carbon atom has four free bonds to which oxygen and hydrogen can join. At one end of the carbon chain, is a carboxyl group (-COOH), giving the acid property of the fat. Any carbon atoms that are missing a hydrogen atom are joined by double bonds rather than single bonds so that each carbon atom participates in four bonds, so it’s how much of the backbone bonds that are used up (or not) that make a fat saturated or unsaturated. Unsaturated fats are less stable and more chemically active than saturated fatty acids (which are relatively stable and inert).

  • Where all the bonds are used up, the fat is said to be saturated.
  • Where all but one are used, leaving one double bond between carbon atoms, the fat is said to be monounsaturated
  • Where there is more than one double bond between carbon atoms, the fat is said to be polyunsaturated

NOTE: Plant and animal cells can modify saturated fatty acids; they ‘insert’ one or more double bonds into the chain by removing two hydrogen atoms; they cannot do this with transfats

Double bonds can occur between any two points in a carbon chain; plants insert double bonds at different points to animals – this has important implications for health and is why we have essential fatty acids.

— Humans rarely insert double bonds into FAs with fewer than 16 carbon atoms in the chain and they cannot insert double bonds into positions closer than seven carbons from the methyl (w) end.

— Plants caninsert double bonds, including at w3 and w6 – hence essential omega 3 and omega 6 fatty acids. Market use of the term ‘polyunsaturated’ usually refers to w6 fatty acids found in popular vegetable oils such as safflower and sunflower.

In unsaturated fatty acids, if the hydrogen atoms are on the same side of the double bonds of the carbon chain, it is said to be in cis- configuration. The hydrogen atoms repel one another and the fatty carbon chain kinks to take up some of the empty space on the side of the molecule opposite the hydrogen atoms; the kinking alters the properties, behaviour and functions of the fatty acid in our body.

If the hydrogen atoms are on the opposite side of the double bonds of the carbon chain then it is said to be in trans- configuration and the fats are known as transfats. The molecule straightens out so they are more stable and have different properties to cis- fatty acids, behaving more like saturated fats. Industrially-produced transfats are harmful to health, as are those formed when vegetable oils are heated to excess; naturally-occurring transfats aren’t.

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