Insulin absorption, and thus the liming of its action, is affected by many variables.
• The insulin formulation. The protamine in NPH and the excess zinc in Lente and Ultralente promote crystallization, which causes the cloudy appearance and slower absorption compared with Regular insulin or the rapid-acting analogs ("clear insulins").
• Raising skin temperature, such as by taking a shower or hot tub after an injection, increases absorption. Similarly, massaging the injection site increases local circulation and speeds absorption. Both are to be discouraged.
• Exercise increases blood flow, and thus absorption of locally injected insulin. It is recommended that patients be taught not to inject into a limb that recently has been exercised, or will be exercised within the next hour. Instead, the abdomen is the most neutral site in terms of this effect, and best used when physical activity is planned.
• Intramuscular injections increase the absorption rate, as already discussed.
• Subcutaneous injection sites vary in absorption rates (from fastest to slowest: abdomen, arms, legs, and buttocks). It's useful to match sites to a desired effect, e.g., use of the stomach before meals, and giving the evening shot in the buttocks or leg so it lasts through the night. However, it is important to realize that this effect does not occur with the rapid-acting analogs lyspro and aspart or the long-acting analog glargine; all are considered to have absorption rates that are unaffected by where they are injected.
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