INTESTINAL ABSORPTION OF LIPIDS IN DAIRY CATTLE
Microbial phospholipids are digested in the small intestine and contribute to the pool of fatty acids that are processed and absorbed through the intestinal wall. The bile secreted by the liver and the pancreatic juice (rich in enzymes and bicarbonate) are mixed with the contents of the small intestine. These secretions are essential to prepare the lipids for absorption by forming water miscible particles called micelles that can enter the intestinal cells. In the intestinal cells, a major portion of fatty acids are bound to glycerol (coming from blood glucose) to form triglycerides. Triglycerides, some free fatty acids,cholesterol and other lipid-like substances are coated with protein to form triglyceride-rich lipoproteins (TG-rich LP) also called chylomicrons or very low density lipoproteins. The TG-rich LP enter lymph vessels and flow to the thoracic duct (the junction of the lymphatic system with the blood system) where they enter the blood system. In contrast to most nutrients absorbed from the gastro intestinal tract, the absorbed lipids enter the general circulation directly and are used by all body tissues without a preliminary processing by the liver.
UTILIZATION OF DIETARY LIPIDS BY THE UDDER
About half the fat in the milk is derived from the uptake of fatty acids by the mammary gland. These fatty acids come primarily from the triglyceride-rich lipoproteins formed during the intestinal absorption of lipids. An increase in long chain fatty acids (i.e., acids made of more than 16 carbons) in the diet increases their secretion in milk, but it also inhibits the synthesis of short- and medium-chain fatty acids in the mammary tissue. Thus, the marked depression in fat secretion when cows are fed low fiber diets can becompensated only partially by increasing fat in the diet.
THE ROLE OF LIVER AND FAT MOBILIZATION
During periods of under feeding or in early lactation, cows meet their energy demand by mobilizing fat from adipose tissues to obtain energy in addition to that provided by the diet. Fatty acids coming from the triglycerides stored in the adipose tissues (located primarily beneath the hide, in the abdomen and over the kidneys) are released into the blood. Mobilized fatty acids are taken up by the liver where they can be used as an energy source or be converted to ketones that may be released in the blood and used as an energy source by many tissues. The liver does not have a high capacity to form and to export TG-rich LP and the excess mobilized fatty acids are stored as triglycerides within the liver cells. The fat deposited in the liver contributes to development of metabolic disorders (e.g., ketosis and fatty liver) in early lactation.
ADDED LIPIDS IN DAIRY RATIONS
Lipids contain about 2.25 times more energy than carbohydrates. Also, lipids are sometimes referred to as a “cold” nutrients because during digestion and utilization by the body they produce less heat than carbohydrates and proteins. Thus, increasing lipids in dairy cow rations may have several potential benefits:
* Increase the caloric (energy) density of the ration, especially when intake may be limited as in high forage diet;
* Limit the need for carbohydrate-rich concentrates which are usually required in early lactation when a cow is in negative energy balance;
* In hot weather, lipids may help to reduce the heat stress of a lactating cow.
Feed intake and milk production responses vary greatly according to the type of lipids added in a diet. Cows should not be fed more than about 1.5 kg/day of lipids in addition to the lipids present in feedstuffs. This amount of lipids translates into a total of about 6 to 8% lipids in the diet before negative effects become evident. Milk production is maximized when lipids comprise 5% of the dietary dry matter. Added dietary fat usually decreases milk protein by about 0.1%. In addition, excess lipids may depress feed intake, milk production and milk fat composition.
