FAT METABOLISM IN COWS
At the end of gestation and after calving, every cow experiences a negative energy balance. As a result, they will use their body reserves and break down fat (and muscle) tissue as a source of energy.
When fat is mobilised from the adipose tissue (see Figure 1), the triglyceride structure is broken down and non-esterified fatty acids (NEFA) are transported through the blood to the liver, where they are further metabolised.
The capacity of the liver for creating energy by complete oxidation of NEFA is limited. If oxidation is not complete, ketones are formed, which will appear in the blood, urine, and milk. High ketone levels can result in clinical disease (clinical ketosis) with symptoms of decreased feed intake, milk drop, weight loss, and nervous symptoms. More common is a subclinical presentation of ketosis; ketone levels are elevated, but clinical symptoms are less obvious. Subclinical ketosis is often a herd problem and leads to important economic losses due to reduced milk yield, reduced reproductive results, but also by direct costs associated with the higher susceptibility to diseases. Non-esterified fatty acids can also be removed from the liver again in the form of very low density lipoproteins (VLDL). Choline is essential for the synthesis of VLDL and, therefore, plays a crucial role in fat metabolism. If the transport out of the liver is unsuccessful, an accumulation of fat leads to fatty liver disease and impaired liver function.