n-3 fatty acids (FA) in erythrocyte membranes in the three different dietary groups. Median and quartiles of the percentage of n-3 FA in erythrocyte membranes in control (CONT), additive (ADD) and fish oil (FO) group at week 0, 1, 2, 4, 8, 12, 14 and 16 of experimental feeding. Different uppercase letters indicate statistically significant differences between different time points within the individual groups. Different Lowercase letters indicate statistically significant differences between groups at the same time point. (p ≤ 0.01). The FA included for the calculation of total n-3 FA are listed in additional file 1.