Metabolism
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Starving to slim
Key points from this exercise:
The energy yield of adipose tissue can be calculated from the fact that it contains 80% triacylglycerol (at 37 kJ /gram) and 5% protein (at 17 kJ /gram). Therefore 100 g of adipose tissue yields:
from triacylglycerol: 80 x 37 = 2960 kJ,
from protein: 5 x 17 = 85 kJ,
total = 3045 kJ = 3.045 MJ / 100 g adipose tissue = 30 MJ / kg adipose tissue (rounding off).
Total starvation, and assuming energy expenditure of 10 MJ /day, would give a maximum theoretically possible weight loss of 2.3 kg / week.
In the first few days of severe energy restriction there is a considerable loss of water trapped in glycogen reserves that are being depleted, so there may well be a greater loss of weight than that calculated from adipose tissue.
Not all of the weight lost will be adipose tissue. The brain is largely dependent on a supply of glucose, and red blood cells completely so. There will be a loss of muscle tissue to provide amino acids for synthesis of glucose (gluconeogenesis).
Fatty acids can never be a source of substrates for gluconeogenesis (although the glycerol from triacylglycerol is used for gluconeogenesis).
Even if adipose tissue reserves are adequate to meet energy needs for a prolonged period, it is unlikely that anyone would survive starvation for more than about 40 days, because of the loss of protein for gluconeogenesis, and the need for vitamins and minerals to replace losses.
It is possible to measure metabolic rate by measuring oxygen consumption. To first approximation each litre of oxygen consumed is equivalent to 20 kJ, regardless of the fuel being oxidised.
Resting metabolic rate is the energy needed:
to maintain circulation and breathing and generally maintain muscle tone
for active transport of ions across cell membranes and between intracellular compartments
to maintain protein turnover - there is continual breakdown of tissue proteins and replacement synthesis; both processes are energy requiring.
to provide an input of energy for endothermic enzyme catalysed reactions.
Plasma glucose falls somewhat in the fasting state, but then remains more or less constant into prolonged starvation.
Plasma free (non-esterified) fatty acids rise somewhat in the fasting state, but then remains more or less constant into prolonged starvation.
Plasma ketone bodies rise steadily with increasing time of starvation.
The ketone bodies are acetoacetate, hydroxybutyrate and acetone. Acetoacetate and hydroxybutyrate are synthesised in the liver and provide an alternative fuel for muscle, which cannot meet its energy needs from fatty acids alone, and in advanced starvation become a significant fuel for the brain as well.
Acetone is formed by non-enzymic decarboxylation of acetoacetate, and is poorly metabolised. Much acetoacetate is reduced to hydroxybutyrate in the liver before release in to the bloodstream
Urea is the end-product of metabolism of the amino groups of amino acids. The urinary excretion of urea therefore reflects the amount of amino acids being metabolised, with their carbon skeletons being used in starvation (mainly) for gluconeogenesis.