Now contrast this with a different day: one in which more widely spaced meals are interspersed with some activity, requiring the use of metabolic fuels. The aim is to show how the overall storage of energy is regulated in such a way that the body's immediate needs are met, and any surplus stored; there is integration not just between different modes of substrate storage, but also between substrate storage and utilisation. Some specific details of metabolism in exercise have not yet been covered; this will be done in Chapter 8.
First, imagine that the subject for this 'thought experiment' is sufficiently health-conscious as to eat a mainly carbohydrate breakfast: cereals and semi-skimmed milk, perhaps, but no bacon and eggs. Such a breakfast is likely also to be lower in energy content than a high-fat breakfast.
The disposition of glucose and amino acids will be much as described earlier, although there may be a sharper peak in glucose (and hence insulin) concentration, depending upon the amount and type of carbohydrate eaten. Release of non-esterified fatty acids from adipose tissue will be suppressed, leading to preservation of the adipose tissue triacylglycerol store.
At about an hour after this breakfast, our health-conscious subject sets out for some exercise - nothing strenuous, perhaps a swim or brisk walk for an hour, or even cycling to work. What effects will this have on substrate flow? Clearly the skeletal muscles will require more substrate in order to produce the energy required. For relatively gentle exercise, the mechanisms involved in supplying this are basically those which have already been covered, but they are accompanied by some physiological changes: increased activity of the sympathetic nervous system increases the heart rate and strength of pumping, and blood flow through the exercising muscles increases, delivering more substrate. This in itself leads the exercising muscle to take up more glucose from the plasma. In addition, depending on how strenuous the exercise is, sympathetic nervous activity and increased adrenaline in plasma may gently switch on fat mobilisation in adipose tissue. Thus, the rise in plasma glucose concentration following the meal is diminished (or the decline from the peak concentration is accelerated), and the correspondingly lower glucose concentrations are accompanied by lower insulin concentrations and less conservation of fat stores.
Any suppression of glucagon secretion by the high glucose concentration is somewhat relieved, and glucagon concentrations may rise a little, stimulated also by the sympathetic nervous system (this will be covered in more detail in Chapter 7). The general hormonal tone is changed from one of high insulin/ glucagon ratio and intense substrate storage, to one in which substrate storage is lessened and substrate is diverted instead to the working muscle.
It is probably unnecessary to labour the point by following our health-conscious subject much further. Clearly, a low-fat lunch will be superimposed on a much less storage-primed metabolic system. An afternoon walk will divert yet more substrate into oxidation rather than storage. The net result at the end of the day is that less substrate will have been stored, more oxidised. This is not in the least surprising - the first law of thermodynamics (the law of 'conservation of energy') tells us that if more substrate has been oxidised, less can have been stored. But the laws of thermodynamics do not enable us to see how the body achieves this, diverting substrates into different pathways and between tissues to maximise the amount stored when nutrients are available, whilst making energy available for activity as required.
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All you need is a proper diet of fresh fruits and vegetables and get plenty of exercise and you'll be fine. Ever heard those words from your doctor? If that's all heshe recommends then you're missing out an important ingredient for health that he's not telling you. Fact is that you can adhere to the strictest diet, watch everything you eat and get the exercise of amarathon runner and still come down with diabetic complications. Diet, exercise and standard drug treatments simply aren't enough to help keep your diabetes under control.