Reducing the ratio between essential fatty acids: C18:2 n–6/C18:3 n–3 down to 5 is recommended by Nutritional Guidelines. We studied the fatty acid (FA) changes in consumers’ plasma following changes in livestock diet. First, a zootechnical study introduced 5% of extruded linseed into the diet of livestock to replace other oleaginous ingredients, and on an iso-nutritional values basis. The products from linseed-fed animals contained more n–3 fatty acids (precursor α-linolenic and derivatives obtained by elongations and desaturations) than control animal products (issued from animals fed without linseed), and more conjugated linoleic acids (CLA). The n–6/n–3 ratio was reduced by 54% in butter, 60% in meat and 86% in eggs. Following this, a double-blind, randomised, cross-over clinical study involving 75 healthy volunteers compared plasma and erythrocyte FA profiles in consumers of animal products (from livestock fed the linseed diet or from livestock fed standard diet). It showed modifications in the FA composition of the experimental human regimen with more C18:3 n–3 (1.65 vs. 0.75 g/day), and more n–3 derivatives. The C18:2 n–6/C18:3 n–3 ratio decreased (7 vs. 15). In volunteers’ plasma, C18:3 n–3 increased in the essay group (0.93 vs. 0.44% of the FA), so did n–3 derivatives and CLA. The n–6/n–3 ratio decreased from 14.3 to 10.2. In erythrocytes, C20:5 n–3 increased in the essay group (0.59 vs. 0.45%) and so did C22:6 n–3. The n–6/n–3 ratio decreased in parallel from 4.2 to 3.8. Without any changes in consumers’ eating habits, foodstuffs from animals fed linseed diets induced significant modifications of human plasma and erythrocyte fatty acid composition (comparable to that noted under the ‘Cretan’ diet) and a sharp increase in CLA.