Medical brain notes 38

  Introduction Definition and conceptualization of energy balance   The average adult human consumes close to 1 000 000 calories (4000 MJ) per year. Despite this huge energy intake, most healthy individuals are able to strike a remarkable balance between how much energy is consumed and how much energy is expended, thus resulting in a state of energy balance in the body. This accurate balance between energy intake and energy expenditure is an example of homeostatic control and results in maintenance of body weight and body energy stores. This regulation of energy balance is achieved over the long term despite large fluctuations in both energy intake and energy expenditure within and between days. The accuracy and precision by which the body maintains energy balance is high-lighted by the fact that even a small error in the system can have detrimental consequences over time. If energy intake chronically exceeds energy expenditure by as little as 105 kJ/day, then, over time, a pe...

  Components of energy balance Energy intake   Energy intake is defined as the caloric or energy content of food as provided by the major sources of dietary energy: carbohydrate (16.8 kJ/g), protein (16.8 kJ/g), fat (37.8 kJ/g), and alcohol (29.4 kJ/g). Energy storage   The energy that is consumed in the form of food or drinks can either be stored in the body in the form of fat (the major energy store), glycogen (short-term energy/carbohydrate reserves), or protein (rarely used by the body for energy except in severe cases of starvation and other wasting conditions, or be used by the body to fuel energy-requiring events. Energy expenditure   The energy that is consumed in the form of food is required by the body for metabolic, cellular, and mechanical work such as breathing, heart beat, and muscular work, all of which require energy and result in heat production. The body requires energy for a variety of functions. The largest use of energy is needed to fuel the basa...

  Energy intake Sources of dietary energy As mentioned , the sources of energy in the food we eat include the major macronutrients: protein, car-bohydrate, and fat, as well as alcohol. Carbohydrate and protein provide 16.8 kJ of energy for each gram; alcohol provides 29.4 kJ/g, whereas fat is the most energy dense, providing 37.8 kJ/g. Note that 4.2 kJ is defined as the amount of heat that is required to raise the temperature of 1 liter of water by 1°C. The energy content of food can be measured by bomb calori-metry, which involves combusting a known weight of food inside a sealed chamber and measuring the amount of heat that is released during this process. Thus, 1 g of pure fat would release 37.8 kJ during its complete combustion, whereas 1 g of pure carbohy-drate would release 16.8 kJ. Thus, if the gram quanti-ties of any type of food are known, the energy content can easily be calculated. For example, if a protein-rich nutrition snack contains 21 g of carbohydrate, 6 g of fat, ...

  Factors influencing food intake Digestive factors Several factors in the digestive system exert a short-term influence over food intake. The presence of food  and drink in the stomach and intestine and the resul-tant pressure that they exert may  regulate food intake. This effect is known as gastrointestinal distension. In addition, the stomach produces a hormone called cholecystokinin (CCK) in response to food intake, which may, in turn, regulate food intake. Furthermore, when subjects have fat or carbohydrate infused directly into the small intestine, they report feelings of satiety. This suggests that factors in the intestine regulate food intake. Indeed, receptors in the intes-tine have been identified that recognize the presence of specific macronutrients; these receptors are linked to the brain and therefore can communicate directly with the central nervous system, resulting in regula-tion of energy balance. In addition, other gastrointes-tinal hormones, such as g...

  Energy expenditure Concept of energy expenditure The process of energy expenditure and the oxidation or combustion of food for energy in the body is analogous to a woodstove that burns wood to release heat in a controlled fashion. In this analogy, large chunks of wood are fed to the stove and the wood is gradually combusted in the presence of oxygen to release carbon dioxide, water vapor, and heat. Similarly, in the body, the food consumed is oxidized or combusted in the presence of oxygen to release carbon dioxide, water, and heat. When ingested food is used for energy, however, the release and transfer of energy occur through a series of tightly regulated metabolic path-ways in which the potential energy from food is released slowly and gradually over time. This process ensures that the body is provided with a gradual and constant energy store, rather than relying on a sudden release of energy from an immediate combustion of ingested food. As a simple example of how the body us...