The period of adapted starvation

From about three weeks of total starvation onwards, the body appears to be fully adapted to starvation and there is a kind of steady state, in which there is gradual depletion of the body's protein mass (minimised by the mechanisms discussed earlier), and steady depletion of the fat stores. Ketone body concentrations in the blood reach about 6-8 mmol/l, and ketone bodies provide about two-thirds of the metabolic requirement of the brain. Other tissues that require glucose (erythrocytes, renal medulla for instance) produce lactate, which is efficiently recycled, using energy derived from fatty acid oxidation. Thus, the rate of 'irreversible loss' of glucose is minimised. The major fuel flows in this state are summarised on Fig. 8.6.

We can see how the pattern of metabolism is governed by the physico-chemical features of fat and carbohydrate outlined in Chapter 1, so that fat - the most energy-dense fuel store - constitutes the major long-term fuel reserve and metabolism is geared to derive the maximum proportion of energy from fat oxidation. The changes that bring about this metabolic adaptation are mediated in a gradual way by changing concentrations of substrates in the blood, and

Fuel Metabolism Starvation

Fig. 8.6 Major fuel flows in prolonged starvation. Protein (especially that in muscle) and glycerol from triacylglycerol in adipose tissue are the only long-term sources of glucose. The complete oxidation of glucose is reduced by the production of ketone bodies which serve as an alternative fuel, e.g. for the brain. Those tissues that must use glucose (e.g. red blood cells, renal medulla) produce lactate, which is 'recycled' in gluconeogenesis. The major source of fuel for oxidation is thus adipose tissue triacylglycerol (TAG), providing fuel both in the form of non-esterified fatty acids and (via the liver) ketone bodies.

Fig. 8.6 Major fuel flows in prolonged starvation. Protein (especially that in muscle) and glycerol from triacylglycerol in adipose tissue are the only long-term sources of glucose. The complete oxidation of glucose is reduced by the production of ketone bodies which serve as an alternative fuel, e.g. for the brain. Those tissues that must use glucose (e.g. red blood cells, renal medulla) produce lactate, which is 'recycled' in gluconeogenesis. The major source of fuel for oxidation is thus adipose tissue triacylglycerol (TAG), providing fuel both in the form of non-esterified fatty acids and (via the liver) ketone bodies.

by the almost automatic responses of the endocrine system: insulin secretion decreases as the plasma glucose concentration falls, leptin secretion follows, while glucagon secretion increases. The central nervous system is involved in these responses, with mild activation of the adrenal medulla and sympathetic nervous system. However, the involvement of the central nervous system is very much less than in 'acute' situations such as exercise and trauma. A decrease in thyroid hormone secretion, via the hypothalamic-pituitary system, may in turn largely result from the 'peripheral' changes (via leptin).

The adapted state will, we hope, come to an end with refeeding. Otherwise it will continue, usually until weakness of the respiratory muscles leads to inability to clear the lungs properly, and pneumonia sets in and leads to death. There is some evidence that survival is determined by the size of the fat stores: when the fat stores are finally depleted as far as they can be, there is a sudden additional loss of protein and death follows quickly.

Nevertheless, it is worth pondering the ability of the metabolic pattern to adapt to such an extreme situation. We began our tour of metabolic regulation by looking at the changes that occur during normal daily life, with food coming in regularly three times a day. Many of us in the Western world are not used to missing a meal, let alone a day's food: the fact that the body could survive for around two months without any food intake is a clear illustration of the coordinated regulation of metabolism that not only underpins our daily lives, but also allows us to continue in some very extreme situations.

The Most Important Guide On Dieting And Nutrition For 21st Century

The Most Important Guide On Dieting And Nutrition For 21st Century

A Hard Hitting, Powerhouse E-book That Is Guaranteed To Change The Way You Look At Your Health And Wellness... Forever. Everything You Know About Health And Wellness Is Going To Change, Discover How You Can Enjoy Great Health Without Going Through Extreme Workouts Or Horrendous Diets.

Get My Free Ebook


Post a comment