Fats or lipids, like carbohydrates and sugars, are an important energy or fuel source to meet the caloric needs of the patient. When combined to meet the caloric requirements of the patient, the dietary regimen is referred to a mixed-fuel system, and mimics the normal diet. As a caloric source, there are between 8.3 to 9 kcal per gram of fat or approximately double the amount per gram of carbohydrate. As such, fat is often referred to as a dense caloric source. It is also a vital source of the "essential" (cannot be made by the body) fatty acids. In adults, up to approximately 20 to 30% of the total daily caloric supplement is from fat. As in the above example of a 70 kg patient, the amount of calories from fat sources would be in the range of 350 to 840 kcals or approximately 40 to 70 g of fat per day, with the balance of kcals as carbohydrate. If therapy is provided by a catheter into the stomach or intestine, or if therapy is provided by an intravenous catheter, the fat source can be given as various oily lipids such as soybean, olive, fish and/or coconut oils. When given intravenously, the fat emulsion can be given separately in its manufacturer's container or more commonly mixed by the Pharmacy into a single bag each day with all of the other nutrients, and such a mixture is known as an all-in-one or total nutrient admixture.

In addition to containing essential fatty acids, and being an equivalent energy source as carbohydrates, fats have additional properties that may favorably affect the course of disease during acute illness in patients. Part of the body's response to acute illness includes an "inflammatory response" that includes, for example, fever, sweating, increased blood sugar or glucose, which must be controlled. Some fats contain omega-6 fatty acids (e.g., soybean oil) that, in certain diseases, can worsen the inflammation and complicate the recovery process. Other fats contain omega-9 fatty acids (e.g.., olive oil) that are less inflammatory, while others contain omega-3 fatty acids (e.g., fish oil) that can actually be anti-inflammatory, and be beneficial to the recovery process. This is currently an intense area of investigation at this time.

As intravenous fat emulsion is a mixture of very small oil droplets in water, the stability of this product is essential to its safety upon infusion. When instability occurs, the small oil droplets grow in size to form large fat globules that when given intravenously may clog blood vessels in vital organs such as the lungs and liver causing damage. During acute or critical illness, unstable intravenous fat emulsions may delay recovery, or worse, cause greater harm and complicate the outcome of the patient. To achieve the full benefits of this important source of essential fatty acids, daily caloric source and possible additional clinical benefits, a stable emulsion during the period of intravenous infusion is vital.