LIPIDS

Lipids are defined as biological compounds soluble in organic solvents. The major categories are fatty acids, triacylglycerols (triglycerides), phospholipids, glycolipids (sphingolipids), sterols (such as cholesterol), and other iso-prenoids, fat-soluble vitamins, and waxes--each of which are composed of numerous subspecies. There are more than 1,000 different lipid species.

The term "fat" refers to triacylglycerols, which are composed of three fatty acid molecules attached to a glycerol backbone. The energy equivalent of fat is approximately 9 Calories per gram, which is more than twice the number for an equal weight of carbohydrate or protein. Triacylglycerols are stored primarily in adipocytes (fat cells). Triaglycerols (and other lipids) are transported in circulation as part of lipoproteins (lipids plus protein). The different types are: chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL).

Lipids serve a variety of functions. Triacylglycerols are used for energy storage and insulation. Phospholipids, glycolipids and cholesterol are key building blocks of membranes and lipoproteins; they can also be converted to other highly bioactive compounds (such as platelet-derived growth factor, prostaglandins, diacylglycerols, ceramides, and steroid hormones) that participate in cell signaling. Cholesterol is converted to bile acids, which help emulsify food for digestion. Fat-soluble vitamins are involved in vision and cell regulation (A), calcium utilization (D), coagulation (K) and protection of membranes from oxidative damage (E and carotenoids).

Deficiencies: Only a few classes of lipids are required in the diet. These are the "essential" fatty acids (linoleic acid and alpha-linolenic acid), fat-soluble vitamins (vitamins A, D, E and K), and a number of other lipids that are mostly required under special circumstances, such as early in life or when digestion and utilization are not functioning fully.

Clinical uses: Enteral and parenteral formulations often contain lipids as sources of energy and essential fatty acids; supplements rich in unsaturated fatty acids (fish oils) have been used in treating atherosclerosis. Both are conducted under professional care.

Diet recommendations: The 1995 Dietary Guidelines for Americans recommends that intake of total fat be limited to no more than 30 percent of total calories by choosing lower fat options and increasing the amount and variety of grain products, fruits, and vegetables. Selection of foods based on the Food Pyramid also reduces intake of saturated fat and cholesterol. Overall caloric intake should be matched to one's caloric needs combined with physical activity to maintain overall fitness. The limitation of fat percentage does not apply to infants and toddlers below the age of 2 years, but after that age, children should gradually adopt a diet that contains no more than 30 percent of calories from fat by about 5 years of age.

Toxicity: The major problem of lipid excess is the growing problem of overweight and obesity. This concern has been addressed in the Dietary Guidelines. Toxicity is also seen if vitamin A or D are consumed in excess of the Recommended Dietary Allowance (RDA).

Recent research: Major areas of ongoing research are to understand better the roles of genetics, diet, and lifestyle (especially physical activity) in the relationships between diet and obesity, cardiovascular disease, and cancer. Food industries are involved in the development of foods that contain less total fat and a lower percentage of saturated fat.

For further information:

Shils, M. E., Olson, J. A., Shike, M. & Ross, A. C., eds. (1998) Modern Nutrition in Health and Disease, 9th ed., Williams & Wilkins, Baltimore, MD.

For more specific information about particular lipid classes, consult recent publications in:

The Journal of Nutrition http://www.nutrition.org
The American Journal of Clinical Nutrition http://www.ajcn.org.

For the 1995 Dietary Guidelines, contact:
http://www.nalusda.gov/fnic/dga/dga95.html

Prepared By

Alfred H. Merrill, Ph.D.
Director, Emory Center for Nutrition
and Health Sciences
Emory University School of Medicine
4113 Rollins Research Center
Atlanta, GA 30322-3050
Phone: 404-727-5978
FAX: 404-727-3954
Email: amerril@emory.edu

Rachel Shireman, Ph.D.
Professor, Department of Food Science
and Human Nutrition
University of Florida
303 Food Science Building
Gainesville, FL 32611
Phone: 352-392-1991
FAX: 352-392-9467
Email: RBS@dean.acprog.ifas.ufl.edu