Insulin sensitivity refers to the ability of cells to respond to the hormone insulin. Cells possess insulin receptors on their cell membranes and these receptors bind insulin that circulates in the blood following release from the pancreas. Once activated by insulin, the insulin receptors initiates cellular effects that cause the appearance of glucose transporters in the cell membranes, and this facilitates the transport of glucose from the blood into the cell. In this way insulin lowers blood glucose levels by transporting it into cells. As well as glucose, insulin is also facilitates the transport of certain amino acids into the cells. Insulin is the primary anabolic hormone in the body and its ability to facilitate the transport of glucose and amino acids into skeletal muscle cells provides the energy and nitrogen necessary to cause skeletal muscle growth. As insulin sensitivity declines this anabolic effect is diminished somewhat, and this can result in a reduced anabolic drive for the maintenance or creation of new muscle.
The reduction in insulin sensitivity seen with ageing may therefore explain some of the reduction in muscle mass seen in the elderly. From a health perspective, muscle mass is important because skeletal muscle is inversely associated with mortality. Further, reductions in skeletal muscle decrease the resting metabolic rate, and this increases the risk of weight gain. Optimising insulin sensitivity, particularly into old age is therefore pivotal to health, and one way to achieve this is to perform regular whole body exercise such as resistance training. Many nutritional factors are known to improve insulin sensitivity, and in particular dietary antioxidants such alpha lipoic acid, certain herbs such as cinnamon, as well as the mineral chromium, and the mineral vanadium, have been evidence to have particular insulin sensitising effects. Antioxidants may be beneficial to insulin sensitivity because oxidative stress impairs the insulin cascade in the cells. Chromium and vanadium may be required for activity of the insulin receptor.
Fish oils also have insulin sensitising effects, and this may in some respect explain their ability to cause weight loss. Fish oils contain long chain fatty acids of the omega-3 variety including eicosapentaenoic acid (EPA, C20:5 (n-3)) and docosahexaenoic acid (DHA, C22:6 (n-3)). These fatty acids accumulate in the membranes of cells where they increase the fluidity of the cell membrane, and this may have direct effects on the insulin receptor, giving it a higher affinity to bind to the hormone insulin. One study investigating the effects of fish oils in the elderly showed that 1.8 grams EPA and 1.5 grams DHA per day for 6 months increased thigh muscle volume by 3.6 %, increased hand grip strength by 2.3 kg, increased 1 rep max strength bu 4 % and increased isokinetic power by 5.6 % in 60 to 85 year old men, compared to a corn oil control. Fish oil may be effective in supplemental form or in the form of whole fish. Consuming more cold water fish or taking fish oil supplements is therefore recommended to maximise insulin sensitivity.
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