Fuel, lipids

Fuel. Lipids are an important source of energy in the diet. In quantitative terms, they represent the principal energy reserve in animals. Neutral fats in particular are stored in specialized cells, known as adipocytes. Fatty acids are released from these again as needed, and these are then oxidized in the mitochondria to form water and carbon dioxide, with oxygen being consumed. This process also gives rise to reduced coenzymes, which are used for ATP production in the respiratory chain (see p. 140). 

Haas, M.J. G.J. Piazza T.A. Foglia. Enzymatic approaches to the production of biodiesel fuels. Lipid Biotechnology T.M. Kuo, H.W. Gardner, Eds. Marcel Dekker New York, Basel, 2002 pp. 587-598. 

Chemicals are ubiquitous as air, carbohydrates, enzymes, lipids, minerals, proteins, vitamins, water, and wood. Naturally occurring chemicals are supplemented by man-made substances. There are about 70000 chemicals in use with another 500-1000 added each year. Their properties have been harnessed to enhance the quality of life, e.g. cosmetics, detergents, energy fuels, explosives, fertilizers, foods and drinks, glass, metals, paints, paper, pesticides, pharmaceuticals, plastics, rubber, solvents, textiles thus chemicals are found in virtually all workplaces. Besides the benefits, chemicals also pose dangers to man and the environment. For example ... 

The source of long-chain fatty acids is either dietary lipid or de novo synthesis from acetyi-CoA derived from carbohydrate. Fatty acids may be oxidized to acetyl-CoA (P-oxidation) or esterifred with glycerol, forming triacylglycerol (fat) as the body s main fuel reserve. 

Inherited aldolase A deficiency and pyruvate kinase deficiency in erythrocytes cause hemolytic anemia. The exercise capacity of patients with muscle phos-phofiaictokinase deficiency is low, particularly on high-carbohydrate diets. By providing an alternative lipid fuel, eg, during starvation, when blood free fatty acids and ketone bodies are increased, work capacity is improved. 

Fatty acids are synthesized by an extramitochondrial system, which is responsible for the complete synthesis of palmitate from acetyl-CoA in the cytosol. In the rat, the pathway is well represented in adipose tissue and liver, whereas in humans adipose tissue may not be an important site, and liver has only low activity. In birds, lipogenesis is confined to the liver, where it is particularly important in providing lipids for egg formation. In most mammals, glucose is the primary substrate for lipogenesis, but in ruminants it is acetate, the main fuel molecule produced by the diet. Critical diseases of the pathway have not been reported in humans. However, inhibition of lipogenesis occurs in type 1 (insulin-de-pendent) diabetes mellitus, and variations in its activity may affect the nature and extent of obesity. 

Besides water, the diet must provide metabolic fuels (mainly carbohydrates and lipids), protein (for growth and turnover of tissue proteins), fiber (for roughage), minerals (elements with specific metabolic functions), and vitamins and essential fatty acids (organic compounds needed in small amounts for essential metabolic and physiologic functions). The polysaccharides, tri-acylglycerols, and proteins that make up the bulk of the diet must be hydrolyzed to their constituent monosaccharides, fatty acids, and amino acids, respectively, before absorption and utilization. Minerals and vitamins must be released from the complex matrix of food before they can be absorbed and utifized. 

Lipids Fats and oils Energy storage (reserve fuel) Vegetable oils, animal fats, cholesterol, hormones 1-30... 

Lipids are an essential part of our diet and of our biochemical constitution. Cell membranes need phosphoacylglycerides and cholesterol to maintain their structure and function fat provides thermal and electrical insulation while fatty acids are an important fuel, especially for muscles like the heart. However, have you ever wondered why eating too much carbohydrate causes weight gain How do jam doughnuts and sweet desserts cause unwanted changes in body shape The answer, of course, lies in biochemistry. 

Alicyclic hydrocarbons are saturated carbon chains that form ring structures. Naturally occurring alicyclic hydrocarbons are common (Chap. 1). For example, alicyclic hydrocarbons are a major component of crude oil, comprising 20-67 vol.%. Other examples of complex, naturally occurring alicyclic hydrocarbons include camphor (a plant terpene) and cyclohexyl fatty acids (components of microbial lipids). Anthropogenic sources of alicyclic hydrocarbons to the environment include fossil-fuel processing and oil spills, as well as the use of such agrochemicals as the pyrethrin insecticides (Chap. 1, and references therein). 

The observation of possibly increased IMCL levels in highly trained athletes led to a second focus of research in the field of sports medicine. Besides carbohydrates, lipids are the major fuel of skeletal muscle during work and rest. A variety of questions have to be answered. It was examined, whether exercise of various intensities alters IMCL levels, whether those changes depend on the duration of exercise, and finally, how IMCL are replenished during post-exercise recovery. 

Box 7.1 A short history of experiments leading to the conclusion that long-chain fatty acids are the important lipid fuel... 

The various roles of fats were summarised in the introduction to this chapter. Two questions arise from this what is the advantage of storing lipid as the major energy reserve Why are there three lipid fuels in the blood while there is only one carbohydrate fuel ... 

Ketone bodies are produced in the liver by partial oxidation of long-chain fatty acids arising from the triacylglyc-erol stored in adipose tissue, so that the question arises, why should one lipid fuel be converted into another There are several reasons. 

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