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* The figures are rounded to the nearest whole number; therefore the content of an individual fatty acid of less than 0.5% is not recorded. The figures across a row rarely sum to 100% because each sample contains a large number of components, each contributing less than 1%. Thus, samples for ruminants (cow, goat, lamb) contain a wide variety of odd- and branched-chain fatty acids. Hen's egg yolk contains about 1% 20:4n-6, which is not normally found in other storage fats and is not listed individually in this table. Data on human milk composition are presented in Table 4.6. Human adipose tissue (1) is a sample from British adults, whereas human adipose tissue (2) is a sample from Israeli adults, whose diet contains more linoleic acid than the British diet. Pig adipose tissue (1) is a sample taken from pigs given a normal cereal-based diet, whereas pig adipose tissue (2) is a sample from pigs given diets to which have been added oilseeds rich in linoleic acid. Contrast the linoleic acid content from storage fats of simple-stomached animals (pig, poultry, human) with those from ruminants in which much of the polyunsaturates in the animals' diets have been reduced by biohydrogenation in the rumen. Note also that whereas fish oils are mainly of interest because of their content of n-3 polyunsaturates, cod, herring and mackerel are particularly rich in long-chain monoenes.

* The figures are rounded to the nearest whole number; therefore the content of an individual fatty acid of less than 0.5% is not recorded. The figures across a row rarely sum to 100% because each sample contains a large number of components, each contributing less than 1%. Thus, samples for ruminants (cow, goat, lamb) contain a wide variety of odd- and branched-chain fatty acids. Hen's egg yolk contains about 1% 20:4n-6, which is not normally found in other storage fats and is not listed individually in this table. Data on human milk composition are presented in Table 4.6. Human adipose tissue (1) is a sample from British adults, whereas human adipose tissue (2) is a sample from Israeli adults, whose diet contains more linoleic acid than the British diet. Pig adipose tissue (1) is a sample taken from pigs given a normal cereal-based diet, whereas pig adipose tissue (2) is a sample from pigs given diets to which have been added oilseeds rich in linoleic acid. Contrast the linoleic acid content from storage fats of simple-stomached animals (pig, poultry, human) with those from ruminants in which much of the polyunsaturates in the animals' diets have been reduced by biohydrogenation in the rumen. Note also that whereas fish oils are mainly of interest because of their content of n-3 polyunsaturates, cod, herring and mackerel are particularly rich in long-chain monoenes.

energy liberated by oxidation of substrates is not used for chemical work and is released instead as heat.

The UCP of brown adipose tissue is a member of a family of mitochondrial transporter proteins. It is now known as UCP1 as several closely related proteins have been found in animals and plants, and it is suggested that they may also play a role in energy dissipation as heat. The mammalian proteins are known as UCP2 (expressed in many tissues), UCP3 (expressed mainly in skeletal muscle) and UCP4 (expressed only in brain). The closely related protein found in some plants may play a role in warming up tubers for sprouting.

3.3.2 Milk triacylglycerols provide a supply of energy for the needs of the new-born

Milk produced in the mammary gland can be regarded as a store of energy for the new-born animal. The cream fraction of milk consists of milk fat globules composed primarily of triacylglycerols, with smaller amounts of cholesterol, fat-soluble vitamins and other hydrophobic lipids, some of which may be environmental contaminants. The milk fat globule is surrounded by a unit membrane containing mainly protein, phospholipid and cholesterol and has an average diameter of 1-2 p,m, although some may attain sizes up to 10 ^m. The fat droplets found in the mammary cell have no discernible membrane. They are formed within the endoplasmic reticulum membranes and migrate to the apical regions of the mammary secretory cell where they are enveloped in the plasma membrane. A neck of membrane is pinched off and the resulting vesicle is expelled into the lumen as a milk fat globule. Like the triacylglycerols in adipose tissue, their synthesis and breakdown in the mammary gland is controlled by hormones the secretion of which is in part governed by the nutritional status of the animal (Section 3.6).

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