Adipose tissue fuel reserves

The size of fuel reserves, especially triacylglycerol, varies considerably in both male and female adults. It is assumed that the amount of adipose tissue in a 70 kg male is 15 kg, of which 75% is triacylglycerol. Data from Cahill (1970) Wahren (1979). 

Fuel Reserves in Adipose Tissue Triacylglycerols, with their hydrocarbon-like fatty acids, have the highest energy content of the major nutrients. 

The fuel reserves of a healthy adult human are of three types glycogen stored in the liver and, in relatively small quantities, in muscles large quantities of triacylglycerols in adipose tissues and tissue proteins, which can be degraded when necessary to provide fuel (Table 23-5). 

Fatty acids stored in adipose tissue, in the form of neutral TAG, serve as the body s major fuel storage reserve. TAGs provide concentrated stores of metabolic energy because they are highly reduced and largely anhydrous. The yield from complete oxidation of fatty acids to CO2 and H2O is nine kcal/g of fat (as compared to four kcal/g of protein or carbohydrate, see Figure 27.5, p. 357). 

Adipose Tissue Maintains Vast Fuel Reserves in the Form of Triacylglycerols The Liver Is the Central Clearing House for All Energy-Related Metabolism ... 

Adipose Tissue Maintains Vast Fuel Reserves in the Form of Triacylglycerols... 

Adipose tissue triglyceride, fuel reserve, 31t Aerobic exercise... 

The motion of the body-mass set point had led scientists to postulate that there must be a homeostatic signal from the fuel stores of the body to the brain. This has been referred to as a lipostat which can communicate the level of fuel stores to the brain to alter the appetite and metabolic rate control centers in the brain. In 1995, the hormone leptin was discovered. It is secreted by white adipose tissue at a rate that is proportional to the size of the fat reserves. The hypothalamus in the brain influences energy intake and expenditure, and it possesses receptors for leptin. Binding of leptin reduces food intake and increases whole body BMR. 

Under normal conditions, the processes shown in Figure 3-2 are tightly coupled, so that the oxidation of metabolic fuels is controlled by the availability of ADI which, in turn is controlled by the rate at which ATP is being utilized in performing physical and chemical work. Work output, or energy expenditure, thus controls the rate at which metabolic fuels are oxidized, and hence the amount of food that must be eaten to meet energy requirements. As discussed in section 5.3.1, metabolic fuels in excess of immediate requirements are stored as reserves of glycogen in muscle and liver and as fat in adipose tissue. 

By contrast, if the intake of metabolic fuels is greater than is required to meet energy expenditure, the body will spend more time in the fed state than the fasting state there will be more accumulation of nutrient reserves than utilization. The result of this is an increase in body size, and especially an increase in adipose tissue stores. If continued for long enough, this will result in overweight or obesity, with potentially serious health consequences — see Chapter 6. 

In the fasting state, which is the normal state between meals, these reserves are mobilized and used. Glycogen is a source of glucose, while adipose tissue provides both fatty acids and glycerol from triacylglycerol. Some of the relatively labile protein laid down in response to meals is also mobilized in fasting, and the amino acids are used both as a metabolic fuel and, more importantly, a source of citric acid cycle intermediates for gluconeogenesis. 

The cause of obesity is an intake of metabolic fuels greater than is required for energy expenditure, so that excess is stored, largely as fat in adipose tissue reserves. The simple answer to the problem of obesity is therefore to reverse the balance reduce food intake and increase physical activity and hence energy expenditure. 

At more moderate levels of exercise (e.g. gentle jogging or walking briskly), plasma non-esterified fatty acids provide the major fuel. This means that, for weight reduction, when the aim is to reduce adipose tissue reserves (section 6.3), relatively prolonged exercise of moderate intensity is more desirable than shorter periods of more intense... 

Adipose tissue depots animals store their reserve fuel in a specialized tissue... 

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