As animals and humans age there is a slow deterioration in the efficiency of particular physiological systems. One such decline results in a decrease in the sensitivity of skeletal muscle for insulin. Insulin is the primary hormone in the body and is the primary driver of amino acid and glucose uptake to skeletal muscle. Insulin also causes significant increases in blood flow postprandially in order to be able to drive the absorbed nutrients into the muscle cells. As age related insulin sensitivity declines, the anabolic effects of insulin are lessened and this contributes somewhat to the reduction in muscle mass seen with ageing. Protein synthesis rates therefore decline and the nitrogen balance of muscle is reduced, and this leads to a chronic proteolytic effect that over years can lead to significant muscle wasting, termed sarcopenia. Factors such as oxidative stress and inflammation increase with age, and both are contributory factor in a reduction in insulin resistance, and both may accelerate sarcopenia.
Dietary factors are pivotal in controlling insulin resistance. In particular, consumption of refined crystalline fructose has been shown to cause significant insulin resistance in animal models and human studies. Studies show that fructose consumption, through the wider use of sucrose and high fructose corn syrup mainly in soft drinks, has increased significantly in recent decades, and this may be contributing to the obesity epidemic currently being experienced by Western nations. Refined crystalline fructose is able cause insulin resistance because it leads to an oversupply of energy to the liver which results in the synthesis of fatty acids as well as the generation of oxidative stress and inflammatory conditions. This perfect storm of conditions leads to a desensitisation of the insulin receptor and may also decrease the efficiency of the β-cells of the pancreas. Feeding rats high intakes of fructose or sucrose has been shown to cause the development of insulin resistance within a few weeks.
Based on the evidence that fructose causes a reduction in insulin sensitivity, it might be expected that fructose also accelerates the age-related decline in muscle mass seen in animals. In fact experimental studies using rats has shown that this might be so. For example, in one study1, researchers fed rats either a high starch diet (13 % sucrose and 49 % wheat starch) or a sucrose diet (62 % sucrose and 0 % wheat starch) for 5 months. As they aged, the sucrose fed rats experienced significantly more body mass loss compared to the starch fed rats (-8.1 versus -5.4 %, respectively). The sucrose fed rats also retained more body fat compared to the starch fed rats (-0.2 versus -33 %, respectively). The researchers also measured the insulin sensitivity of the rats and showed that it was significantly reduced in the sucrose fed group. In addition, protein synthesis rates also declined significantly in the sucrose fed rats compared to the starch fed rats. Sucrose, in the diet may therefore accelerate age-related reductions in muscle mass.
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