Fuels for the body

After food is eaten, it is digested in the stomach. The digested nutrients from food are absorbed, mainly in the intestines, into the bloodstream and are used as fuel for the body. When people eat more food than they need, the body stores the extra nutrients as fat, which accounts for weight gain. 

Digestion and absorption are the GI processes that generate usable fuels for the body. Understanding the mechanisms of these processes can enhance rational use of EN support. 

Fatty acids are long carbon chains with a carboxylic acid end. They serve three basic functions in the human body 1. they serve as hormones and intracellular messengers (he. eicosanoids such as prostaglandins) 2. they are components of the phospholipids and glycolipids of cell membranes 3. they act as fuel for the body. The first of these functions will not be tested on the MCAT unless it is explained in a passage. For the second function of fatty acids you should be able to recognize the stnicture of a phospholipid as shown in Lecture 3 of the biology manual. 

As fuel for the body, fatty acids are stored in the form of triacylglycerols. Triacylglycerols can be hydrolyzed to form glycerol and the corresponding fatty adds in a process called lipolysis. Notice that this process is simply the reverse of esterification. In the lab triacylglycerols can be cleaved by the addition of NaOH, a process called saponification. Saponification is the production of soap. 

Fuels for the body are limited to carbohydrates, fats, and proteins. In the American diet, these fuels are consumed in a ratio of approximately 46 42 12 with the recommended ratio being closer to 53 35 12 (.5). Thus in a nongrowing adult, these ratios provide estimates of the fuel use for daily activities. The primary fuels for exercise are carbohydrates and fats. Chapter 3 examines utilization of specific fuels during aerobic exercise. As the amount of daily exercise increases, there is an increased energy expenditure and hence increased need for energy nutrients usually reflected in increased food consumption, decreased body fat, or both (see Chapter 9). 

Overall, fatty acids released from adipose triacylglycerols serve as the major fuel for the body during fasting. These fatty acids are completely oxidized to CO2 and H2O by some tissues. In the liver, mnch of the acetyl CoA generated from p-oxidation of fatty acids is used for synthesis of the ketone bodies acetoacetate and p-hydroxybutyrate, which enter the blood (Fig. 23.17). In skeletal muscles and other... 

Figure 15.11 These foods are fuels for the body. They provide the glucose that is burned to produce 2808 kJ/mol to carry on the activities of life. 

The food we eat provides energy to do work in the body, which includes the growth and repair of cells. Carbohydrates are the primary fuel for the body, but if carbohydrate reserves are exhausted, fats and then proteins are used for energy. 

Diabetes results from a lack of insulin secretion by the pancreas. Without insulin, cells take up glucose very slowly. The lack of insulin results in an inability to use blood glucose for fuel. Consequently, the body behaves as if it were starving even though food is available. The metabolic responses of the untreated insulin-dependent diabetic are essentially the metabolic responses of starvation. 

Because glucose is the preferred fuel for the brain, an individual who experiences a rapid fall in glucose concentration leading to acute neuroglycopenia will initially feel confusion and may progress to coma and even death. In the event that the person survives 3-4 days, the brain can adapt its metabolism to utilize ketone bodies, metabolically derived from acetyl-CoA (see Figure 6.17), as a source of energy. 

It is also a substrate for formation of glutamine, which is an important fuel in the body the reaction is catalysed by glutamine synthetase. 

They stimulate response of the whole body (e.g. increase the rates of glycogenolysis in the liver and proteolysis in mnscle) to provide fuels for the immune cells (see below for details). 

When in excess, acetyl CoA produced from the (3-oxidation of fatty acids is converted into acetoacetate and D-3-hydroxybutyrate. Together with acetone, these compounds are collectively termed ketone bodies. Acetoacetate and D-3-hydroxybutyrate are produced in the liver and provide an alternative supply of fuel for the brain under starvation conditions or in diabetes. 

What about food—the fuel for your body In order to choose an appropriate and balanced diet, you need to know how much energy each type of food releases when it is digested. Food energy is measured in Calories. (You will learn more about Calories later in this section.)... 

To assess the impact of exercise on the needs for specific nutrients, nutrient functions must be evaluated. At a generalized level, the functions of nutrients are (a) growth or maintenance of the structures of the body (one can consider either macro-structures like muscles and bones, or micro-structures like cell membranes and enzymes), (b) fuels for the energy to run the body processes, (c) fluids and regulation of body fluids, and (d) protection from toxic substances including toxic chemicals, carcinogens, and antigens. 

Fatty acids do not serve as fuel for the brain, because they are bound to albumin in plasma and so do not traverse the blood-brain barrier. In starvation, ketone bodies generated by the liver partly replace glucose as fuel for the brain. 

Carbohydrates, fats, and proteins serve as the major fuels of the body and are obtained from the diet. After digestion and absorption, these fuels may be oxidized for energy. 

The brain then takes up and oxidizes ketone bodies to derive energy. Consequently, the brain decreases its use of glucose, although glucose is still a major fuel for the brain. 

---