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Our body does not produce two polyunsaturated fatty acids (PUFA): linoleic acid and alfa-linolenic acid.

I am thinking reasons for it.

Saturated fatty acids have more energy than unsaturated. Saturated fatty acids do not need NADPH and some many other enzymes as unsaturated in beta oxidation. This means that less storage places in the body needed to have energy in the form of saturated fatty acids such as for muscles and heart.

There are also other reasons why our body does not use PUFAs as the primary source of the energy but saturated:

  • PUFAs lower metabolism and interfere with thyroid function
  • PUFAs spontaneously oxidize, speed up the process of glycation, since too much glucose
  • PUFAs decrease mitochondrial respiration - more oxygen and CO2, less lactate
  • PUFAs promote diabetes, cancer, inflammation and biological stress

There are positive sides of the PUFAs when they are used in other way. - PUFAs replace trans fats and saturated fats in certain types of foods. - PUFAs can help your body to eliminate high cholesterol levels - PUFAs decrease the risk of heart disease

There are also some types of essential fats that your body cannot produce on its own - omega-6 and omega-3 fatty acids. PUFAs are used to create these. You can get PUFAs from vegetable oil, fish and nuts for instance. Balanced diet is essential.

Normal lipid metabolism depends directly on food lipids. Both the essential fatty acids and right amounts of PUFAs can be obtained from food. Body converts the essential fatty acids to long PUFAs, which serve as the precursors of prostaglandins and leucotrienes for instance.

There are so many reasons why our body does not produce polyunsaturated fatty acids.

Why our body does not produce polyunsaturated fatty acids?

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I think the framing of this question is wrong. I would guess that our body doesn't produce PUFAs because they were present in adequate amounts in our ancestors' diets, so there was little pressure to gain the ability to synthesize them. The common ancestor of animals probably just didn't possess these enzymes, and we've evolved to make do with what's available in the diet. –  RecursivelyIronic Feb 19 '13 at 19:56
I am interested on the biochemical sides, not on epidemiological. However, good point to bring the epidemiological sides too. –  Masi Feb 20 '13 at 4:22

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up vote 3 down vote accepted

I think the reason is that there would be so much more oxidative stress in our body to convert PUFAs then to saturated ones. Special system should also be needed for the fine regulation.

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