Muscle and glucose

Muscle And Glucose: Why doesn't muscle use glucose straight from the bloodstream as required?

The reason why working muscle does not use glucose directly from the blood as it goes along is primarily to maintain enough glucose in the blood for other body tissues, especially the brain.

The brain is particularly susceptible to low blood glucose (which is partly why one gets irritable when one's blood glucose drops - hence the saying 'a hungry man is an angry man'). This susceptibility is partly because glucose is virtually the only fuel the brain uses (ie, it doesn't use fat, as does muscle) and the brain doesn't even store glucose (as muscle does in the form of glycogen). So the brain uses only glucose which it doesn't store; hence it is totally dependent on blood vessels of glucose for its energy supply. In other words, the brain has a vested interest in a reasonably normal blood glucose level.

The main supplier of glucose into the blood is the liver - either directly from its own glucose store (as glycogen, like muscle) or because it can turn fat and protein into glucose (called 'gluconeogenesis' - the new formation of glucose). However, the liver weighs only about 2kg, compared to the body muscle mass which may weigh 10-20 times this, and which could totally deplete the blood of glucose in a few minutes of fairly hard work. Simply, the liver can nowhere near supply glucose at a rate that the muscle can utilise.

Thus, to protect the blood levels of glucose, and to protect the glucose supply to the brain, muscle has somehow to be prevented from exhausting the blood glucose. And the method whereby this is done is threefold: first, muscle is given a good store of glucose as glycogen; second, muscle is prevented from using much glucose directly from the blood during muscular work; and third, muscle is switched over to the massive fat energy store as soon as the glycogen runs low.

The only problems with this strategy are that the rate of energy extraction from fat is much lower than that from glucose/glycogen, and that more oxygen is required to gain the same amount of energy from fat as from glucose/glycogen (and hence more blood may need to be supplied, which may in turn require a higher heart rate). Fat may be regarded as a lower-octane fuel, giving a lower rate of performance.

The 'two-legged' experiment, quoted in the article, utilised subjects with one glycogen-depleted leg and the other glycogen-normal. On exercise, the depleted legs did not use much more glucose from the blood than the normal legs, and the body's overall strategy here was to maintain a normal supply of blood glucose to the brain, and other tissues (such as the retina).

Many of the reasons for the physiological limitations to sport and exercise lie in the need of the body as a whole to survive - or at least to make sure its less athletic tissues are democratically catered for, and the above is a good example!

Craig Sharp
Professor of Sports Science University of Limerick

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