Drastic feed reduction can lead to a potentially fatal metabolic condition, hyperlipemiaThat fat mare out there seems to grow fatter on air. It is getting to the point where you are not sure she will fit through her stall door, and now she is barren for no apparent reason. The last time the veterinarian was on the farm, he told you a good part of her problemmaybe all of itwas her weight. Put her on a diet, he prescribed.
She looks as if she could go indefinitely without feed and still be a blimp, so the temptation is great to just stop feeding her grain or cut her ration back to a handful.
But you cannot. By doing that, there is a potential for hyperlipemia. Although primarily a problem in ponies, it can occur in horses, especially overweight ones, and is extremely serious when it does occur. The recovery rate is low, and the death rate is high.
Hyperlipemia is the term for elevated blood lipids, frequently accompanied by fatty infiltration of the liver. Moderately elevated levels or serum lipids generally occur in horses that are off their feed due to illness or injury. This is termed hyperlipidemia and is of only minor concern, although it does indicate the necessity for nutritional support.
A quick look at lipid metabolism will aid in understanding the cause of hyperlipemia. Adipose tissue (fat) is broken down into fatty acids and triglycerides during periods of feed deprivation or severe reduction. A large portion of the fatty acids are deposited in the liver, where they may be used to provide energy or released back into the circulatory system; some may be retained in the liver. Therefore, from lengthy fasting or starvation, lipids continue to increase in the plasma, resulting in lipemia, and in the liver, resulting in fatty liver.

Clinical signs
Signs of hyperlipemia are related to liver dysfunction, and none are specific, but general signs may include weakness, depression, ataxia, diarrhea, jaundice, ventral edema, a grayish coating on the tongue, and rank, malodorous breath. Death may occur, prior to which the horse goes down on its side and is unable to rise.
A blood sample must be taken and the serum (or plasma) examined. Normally this should be clear, but in hyperlipemia it is yellowish or creamy and opaque. The color is due to an increase in bilirubin, which occurs in anorexia, and the opacity is the result of the excessive circulating lipids.
If this is found, the degree of lipemia may be determined by triglyceride assay of the serum. To determine the level of liver damage, liver function tests must be done. Acidosis is also a frequent finding; bicarbonate levels should be checked. And there is often an associated azotemia (accumulation of nitrogenous waste products in the blood) resulting from reduced renal filtration.
There are several changes found on necropsy, both grossly and histologically, but we need not go into them because correction of the condition by then has become a real challenge.

Treatment
As the cause of hyperlipemia is lack of feed intake for whatever reason, the treatment is to get the horse to eat. Obviously, if the horse is not eating because it is ill, the causative disease must be addressed. Bute or dipyrone may help, but tube and/or intravenous feeding may become necessary. If these conditions are the cause of a crash diet, just raise the grain ration a little.
The acidosis and azotemia must be corrected, also. Administration of sodium bicarbonate will correct the acidosis, but the azotemia may be more of a challenge. The administration of glucose is very helpful.
An attempt must be made to remove excess lipid from the blood. Insulin therapy is probably the course of choice. Bear in mind, though, that the prognosis is guarded to poor.

Prevention
Pretty simple: Maintain adequate feed levels. This makes getting that extra weight off Miss Roly-Poly a slow process, but slow it must be. It is much better to not let her get fat in the first place.