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Despite numerous studies, the importance which the tissue or the composition of the diet may have in the biological distribution of each fatty acid is not well known. To determine the importance of tissue origin and dietary fatty acids in the fatty acid composition of cell phospholipids, 54 male adult rats were fed isocaloric diets for one month varying only in their fatty acid compositions. The fat component of the six experimental groups was derived from olive oil, sunflower oil, fish oil, soybean oil, palmitic acid, or 82% palmitic acid plus 18% soybean oil, supplying the essential fatty acid. The fatty acid composition of phospholipids from thymocytes, pancreatic exocrine, muscle and adipose tissues was studied by gas-chromatography. The tissue of origin was a more important source of variation than diet in the fatty acid content of the cell phospholipids except for palmitic acid (16:0), eicosapentaenoic acid (20:5 n-3), and docosahexaenoic acid (22:6 n-3). This study points out the complexity of the interrelations between different families of fatty acids and of the specificity of each tissue to changes in the composition of dietary fatty acids, as well as the inconvenience of speaking from the dietary point of view of groups of fatty acid families based on the position of the double bond, since their individual behaviour, including saturated fatty acids, is very different in the face of dietary manipulation. The study also highlights the different behaviour of each of the fatty acids in relation to the others in the diet in each of the tissues, a circumstance which should be taken into account when evaluating the biological effects in both epidemiological and experimental studies.

We undertook a systematic evaluation of the lipid, protein, calorie, and fatty acid composition in 35 species of fish, shellfish and molluscs commonly consumed throughout the four seasons of the year in Andalusia, Spain. Using a portion of muscle tissue the following were measured in each study unit: total lipids (extraction using Folch's method and gravimetry), protein concentration (Kjehldal's method), total calories (direct calorimetry), and composition of fatty acids (gas chromatography). The lipid, protein, and different fatty acid concentrations found are presented in table form. There was a high degree of inter-species variability in the concentration of lipids and the various fatty acids. There was also a high degree of intra-species seasonal variability in some cases. The relative proportion of fatty acids was not independent of the total concentration of lipids, independently of the season studied. This systematic study of a large group of species shows that the cataloguing of fish as 'white' or 'blue' depends especially on the time of year they are captured. For example, in spring the mackerel (Scomber scombrus), a fish considered traditionally to be 'blue' (fatty), has the same lipid concentration as the dover sole (Solea vulgaris), commonly considered to be 'white' (little fat), and the sea pike (Merluccius merluccius) has a higher lipid concentration in autumn than the jack mackerel (Trachurus trachurus). Even greater differences existed when the fish were classified according to their richness in n-3 fatty acids. These differences in the total lipid concentration and the composition of fatty acids, as well as the inter-relations between them, may, under certain circumstances, be important for the calculation of dietary calories and nutritional values, and may explain the differences found between the various tables of food composition, as well as the divergent results in epidemiological studies on the association between fish in diets and various diseases, such as diabetes or ischaemic cardiopathy.