Science: Muscle fibre may be to blame if you can't slim

日期:2019-02-26 07:02:10 作者:欧阳倒铈 阅读:

By STEPHANIE TAYLOR THE REASON that some people grow fat while others have no difficulty in staying lean may be because of differences in their muscle fibres, according to a British study. Obesity may be caused in part by the relative proportions of different types of fibre in a person’s skeletal muscles. According to Andrew Wade and his colleagues at the London Hospital Medical College, this is the first study to link obesity and muscle fibres. The findings could change our ideas about how useful it is to exercise in order to lose weight. Skeletal muscle makes up the body’s largest mass of tissue. Like all human muscle it contains a mixture of fibres. These fibres differ in the way that they generate energy in order to contract. ‘Slow’, or type 1, fibres use fatty acids, the constituents of fat, as an important source of fuel. There are stores of fat within the muscle fibres, so slow fibres are relatively resistant to fatigue. Meanwhile, ‘fast’, or type 2, fibres readily use glucose as a fuel, especially those fibres that are not adapted by regular exercise. According to Wade, the relative proportions of the two different fibres may vary widely from one individual to another. Slow fibres can compose up to 96 per cent of the mass of the thigh muscle, the quadriceps, or as little as 13 per cent. Wade says that the quadriceps is a good indicator of the composition of the major skeletal muscles of an individual. The researchers studied 11 healthy men, none of whom was particularly athletic. They took samples of tissue from the quadriceps muscles and stained them. The two types of fibre stained differently, allowing the researchers to measure their relative proportions. Next, Wade and his colleagues compared these proportions with estimates of the total body fat in each man. They found that the leaner the man the greater the proportion of his slow fibres in his quadriceps (The Lancet, vol 335, p 805). It is possible to calculate the dominant type of fuel a muscle is using by measuring how much oxygen it uses up and how much carbon dioxide it produces during activity. Wade and his colleagues set out to do this. They used 50 healthy, but mostly sedentary, men, including the 11 from the first experiment. They made the men do equal amounts of work. Wade and his colleagues found that fatter men – and those with a higher proportion of fat muscle fibres – burnt less fat than lean men, who had a greater proportion of slow fibres. This, they say, suggests that the rate at which the muscle burns fat is related both to how fat the individual is and to the relative properties of slow and fast fibres in the muscle. If, as the researchers have found, obese people have a greater proportion of fast fibres in their muscles, this could explain why they may find it difficult to start a programme of exercise, claims Wade. Not only do their muscle fibres tire more easily, but they tend to burn less fat and more glucose when they are working hard. He believes that these are important factors in explaining why doctors are often unsuccessful when they try to persuade people to take more exercise. A person’s composition of muscle fibre cannot change, but every fibre can be trained by exercise. After at least six weeks, Wade found that the trained fast fibres behave much more like slow fibres. Wade feels that if fat people understood the role of their muscle fibre types,