These nutrients have already been encountered earlier in the discussion, so they will only be mentioned briefly here. Essential fatty acids are particular fats that the body needs, but cannot produce endogenously. Semi-essential ones, on the other hand, can be produced within the body from precursors, but only in limited quantities that are insufficient to cover requirements. Both classes of lipids must therefore be ingested exogenously, i.e. through the diet. Essential fats are:

1. α-linolenic acid: essential fat of the ω-3 series (read omega-3);
2. linoleic acid: essential fat of the ω-6 series (read omega-6).

Semi-essential ones, on the other hand, are

1. eicosa-penta-enoic acid: semi-essential fat of the ω-3 series;
2. docosa-hexa-enoic acid: a semi-essential fat of the ω-3 series;
3. arachidonic acid: semi-essential fat of the ω-6 series.

Function

In the context of a high-fat diet (and therefore unsuitable for functional metabolic restoration diets), they favour the preferential consumption of lipids as the primary energy source.

Negative notes

As important as it is to get essential and semi-essential fatty acids in the diet, they are still high-calorie nutrients. It is therefore recommended that they be taken in the right amount. Supplementation products intended to provide a supplement of essential fats rarely pose this problem, but it is necessary to ensure that they do not contain other potentially toxic substances, such as environmental contaminants (not uncommon in supplements derived from fishery products) and various peroxides.

Form and dosage

Supplements of essential and semi-essential fatty acids are commercially available both in the form of oils and in the form of tablets. As far as dosage is concerned, it is first necessary to ensure that intake is commensurate with actual needs. In other words, it must be ensured that taking the supplement does not lead to an excessive intake of fat compared to the parameters set when planning the diet. Secondly, it is necessary to take into account the type of fats contained in the product and the proportions between them: it is estimated that the optimal ratio between ω-3 and ω-6 is between 4:1 and 8:1 in favour of the latter. Unfortunately, it has also been found that in Western countries the balance is much more shifted in the direction of ω-6  than would be recommended (often reaching a ratio of 10:1). It is therefore a good idea to ensure that the supplement product chosen contains only ω-3 and, more specifically, that it does not contain arachidonic acid. Finally, there are recommendations concerning the ratio of EPA and DHA: the ideal product should contain them in the ratio of 2:1 in favour of the former.