Energy balance and changes in body weight

When energy intake is greater than energy expenditure (positive energy balance) there is increased storage of excess metabolic fuel, largely as adipose tissue; similarly, if energy intake is inadequate to meet expenditure (negative energy balance), there is utilization of reserves of adipose tissue.

Adipose tissue consists of 80% triacylglycerol (with an energy yield of 37 kJ/g) and 5% protein (energy yield 17 kJ/g) — the remaining 15% is water. Hence, adipose tissue reserves are equivalent to approximately 30 kJ/g or 30 MJ/kg. This means that the theoretical change in body weight is 33 g/MJ energy imbalance per day, or 230 g/ MJ energy imbalance per week. On this basis, it is possible to calculate that, even with total starvation, a person with an energy expenditure of 10 MJ/day would lose only 330 g body weight per day or 2.3 kg/week.

These calculations suggest that there should be a constant change of body weight with a constant excessive or deficient energy intake, but this is not observed in practice. As shown in Figure 5.5, with positive energy balance the rate of weight gain is never as great as would be predicted, and gradually slows down, so that after a time the subject regains energy balance, albeit with a higher body weight. Similarly, in negative energy balance weight is not lost at a constant rate; the rate of loss slows down, and (assuming that the energy deficit is not too severe) levels off, and the subject regains energy balance, at a lower body weight.

A number of factors contribute to this adaptation to changing energy balance:

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theoretical

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"" positive energy balance

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time figure 5.5 Predicted and observed changes in body weight with energy imbalance.

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  • As more food is eaten, so there is an increased energy cost of digestion and absorption.
  • When food intake is in excess of requirements, a greater proportion is used for synthesis of adipose tissue triacylglycerol reserves, so there is a considerably greater diet-induced thermogenesis. Conversely, in negative energy balance there will be considerably less synthesis of adipose tissue reserves.
  • The rate of protein turnover increases with greater food intake (section 9.2.3.3) and decreases with lower food intake.
  • Although adipose tissue is less metabolically active than muscle, 5% of its weight is metabolically active, and therefore the BMR changes as body weight changes, increasing as body weight rises and decreasing as body weight falls.
  • As shown in Figure 5.4, the energy cost of physical activity is markedly affected by body weight so, even assuming a constant level of physical activity, total energy expenditure will increase with increasing body weight. Furthermore, there is some evidence that people with habitually low energy intakes are more efficient in their movements, and so have a lower cost of activity.

Figure 5.5 shows that in the early stages of negative energy balance the rate of weight loss may be greater than the theoretical rate calculated from the energy yield of adipose tissue. This is because of the loss of relatively large amounts of water associated with liver and muscle glycogen reserves, which are considerably depleted during energy restriction.

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