Changes in food intake and changes in body weight

Calculating in kJ, adipose tissue is:

• 15% water
• 5% protein at 17 kJ /g = 0.85 kJ /gram adipose tissue
• 80% triacylglycerol at 37 kJ /gram = 29.6 kJ /g adipose tissue

Hence 1 gram of adipose tissue yields 30 kJ.

From this we can calculate a theoretical weight change as:

• 33 grams /MJ energy imbalance /day
• 230 grams /MJ energy imbalance /week

Calculating in kcal, adipose tissue is:

• 15% water
• 5% protein at 4 kcal /g = 0.2 kcal /gram adipose tissue
• 80% triacylglycerol at 9 kcal /gram = 7.2 kcal /g adipose tissue

Hence 1 gram of adipose tissue yields 7.4 kcal

From this we can calculate a theoretical weight change as:

• 135 grams /1000 kcal energy imbalance /day
• 945 grams /1000 kcal energy imbalance /week

Assuming an energy requirement of 10 MJ (2500 kcal) /day , this means that total starvation would result in a maximum possible weight loss of:

• 330 grams /day
• 2.3 kg /week

In practice we do not see this theoretical change in weight with changes in food intake because there is adaptation.

As food intake increases above requirements there is adaptation, and the rate of weight gain decreases until the subject is again in energy balance, but with a higher body weight, and a higher energy requirement.

This is because, as food intake increases, so there is:

• a higher energy cost of digestion and absorption of foods as more is eaten
• a higher energy cost of synthesis of triacylglycerol and glycogen, because there is more food available for synthesis of body reserves
• an increased rate of protein turnover as a result of greater availability of energy
• a higher BMR as body weight increases
• a higher energy cost of physical activity with greater body weight because it requires more energy to move a heavier body

As food intake decreases below requirements there is again adaptation, and the rate of weight loss decreases until the subject is again in energy balance, but with a lower body weight and a lower energy requirement.

This is because, as food intake decreases, so there is:

• a lower energy cost of digestion and absorption of foods as less is eaten
• a much lower energy cost of synthesis of triacylglycerol and glycogen because there is less surplus food available for synthesis of body reserves
• a decreased rate of protein turnover as a result of lower availability of energy
• a lower BMR as body weight decreases
• a lower energy cost of physical activity with lower body weight because it requires less energy to move a lighter body

During the first few days of severe energy restriction the rate of weight loss is greater than that calculated from loss of adipose tissue. This is because there is a considerable depletion of glycogen reserves in muscle and liver, and when the glycogen is depleted the water trapped within the glycogen molecules is lost.