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The Role of Lipids
Alyce Newton, MS, RD, LDN, CNSC
Caption: PN bag containing lipids
According to The American Heritage® Stedman’s Medical Dictionary (on Dictionary.com), lipids are "Any of a large group of organic compounds that are oily to the touch and insoluble [cannot be dissolved] in water. Lipids include fatty acids, oils, waxes, sterols, and triglycerides. They are a source of stored energy and are a component of cell membranes.”
What are oils?
Oils are fats that are liquid at room temperature. Fats come from many different plants, such as vegetables, grains, seeds, nuts, and fruits (i.e., avocados, olives, and coconuts). Oils also come from fish.
What are the functions of lipids in our bodies?
Fatty acids are stored as triglycerides in our adipose tissue. They serve as a high-calorie energy reserve. Fatty acids are involved in the formation of cell membranes; are components of our bodies’ inflammatory and immune responses; carry fat-soluble vitamins; are involved in hormone and enzyme production; and provide our bodies with essential fatty acids that our bodies cannot produce.
What are essential fatty acids (EFAs)?
Essential fatty acids (EFAs) are fats that are essential for growth, development, and function. Food sources for EFAs include leafy vegetables, seeds, nuts, whole grains, and vegetable oils (i.e. corn, safflower, soybean, sesame, sunflower). Our bodies cannot make two long chain fatty acids that are considered EFAs. These must be acquired in the diet, through tube feeding, or intravenously. These are:
• Omega-6 EFA, linoleic acid (LA), which should comprise a minimum of 2% of total calories
• Omega-3 EFA, alpha-linolenic acid (ALA), which should comprise a minimum of 0.25% of total calories
When fat-free (lipid-free) parenteral nutrition (PN) is provided, essential fatty acid deficiency (EFAD) may be observed in adults and older children after four weeks, and in infants and young children in less than one week. One of the predominant symptoms of EFAD is a dry, scaly skin rash. Biochemical markers of deficiency include an elevated triene:tetraene (T/T) ratio in the blood, which may occur when insufficient intravenous (IV) linoleic acid is provided for an extended period of time.
How are lipids absorbed?
After you eat a meal that contains fat, your digestive tract breaks the fat down into chylomicrons to transport fatty acids and fats from your gastrointestinal tract to your liver, adipose tissue, heart, skeletal muscle, and other organs. If this process is bypassed, as with PN, a chylomicron-type emulsion is required to allow the body to safely absorb the IV fat. Lipid emulsions are designed to function similarly to chylomicrons.
What is contained in the IV lipids for PN?
Commercial IV lipid emulsions contain one or more types of oils, glycerin, and a phospholipid emulsifier. They may also contain vitamin E, vitamin K, phytosterols, and cholesterol.
FDA Approved IV Lipid Emulsions
• Soybean oil lipid emulsion (100% soybean oil)—contains 53% LA and 8% ALA.
• Combination lipid emulsion (olive oil 80% and soybean oil 20%)—contains 18.5% LA, 2% ALA; not yet commercially available in the United States.
Not FDA Approved Lipid Emulsions*
•Medium chain triglyceride (MCT) emulsions—contain 26–35% LA and 4–5% ALA. The MCT emulsions contain either a 50–50 blend of MCT oil and soybean oil, or are "structured” emulsions containing both MCT and fatty acids obtained from soybean oils joined together.
• Combination three oils lipid emulsion (soybean oil 40%, MCT oil 50%, and fish oil 10%)—contains 26% LA and 3.4% ALA.
• Combination four oils lipid emulsion (soybean oil 30%, MCT oil 30%, olive oil 25%, and fish oil 15%)—contains 21.4% LA and 2.5% ALA.
• Fish oil lipid emulsion (100% fish oil)—contains 4.4% LA and 1.8% ALA. Fish oil emulsion was originally formulated to supplement soybean oil lipid emulsion and enhance the omega-6 to omega-3 profile of PN patients receiving soybean oil emulsion.
*Limited use in U.S. may be allowed for products not approved in U.S., and has been available for certain of these emulsions, under compassionate use Investigational New Drug (IND) application.
Hise M, Brown JC. Lipids. In: Mueller CM, ed. The A.S.P.E.N. Adult Nutrition Support Core Curriculum. 2nd ed. Silver Spring, MD: The American Society for Parenteral and Enteral Nutrition; 2012: 284-297.
Physicians Committee for Responsible Medicine: http://pcrm.org/health/health-topics/essential-fatty-acids. Accessed 20 April 2015.
Vanek VW, Seidner DL, Allen P, et al; Novel Nutrient Task Force, Intravenous Fat Emulsions Workgroup; American Society for Parenteral and Enteral Nutrition (A.S.P.E.N.) Board of Directors. A.S.P.E.N. position paper: clinical role for alternative intravenous fat emulsions. Nutr Clin Pract. 2012; 27:150-192.
Alyce Newton, MS, RD, LDN, CNSC, is Medical Science Liaison for Fresenius Kabi USA, LLC.
Editor’s note: More details about the specific types of IV lipid emulsions are available in a table created byTrisha Furhman and published by Dietitians in Nutrition Support (DNS) in Support Line (Nov 2014,vol 36, no 6), posted with permission at www.oley.org/documents/IV_Fat_Emulsion_Guide.pdf.
LifelineLetter, May/June 2015