The biological activity of insulin, like any protein, is affected if stored at extremes of temperature. The older, acidic form of insulin required refrigeration at 2 to 8°C, but the newer neutral formulation is much more stable at room temperature (15-30°C).1,2 Manufacturers recommend storing insulin under cool, shaded conditions, neither of which is available in the desert, where temperatures frequently exceed 40°C, thus constituting a major problem for diabetic patients. The possibility of adverse side effects due to heat-induced breakdown of products must also be considered. The purpose of our study was to demonstrate the possible effect on the bioactivity of insulin, following storage in a zeer, a. semiporous clay pot containing water, in desert conditions. The small amounts of water that slowly seep through the walls of the zeer evaporate, using up heat, thereby cooling the remaining water. This principle is widely used in the Middle East to cool drinking water in such containers.
Materials and MethodsEight physicians, aged 25-60 years of age (median age 45 years), volunteered and gave informed consent to participate in the experiment. All were healthy individuals free of any systemic diseases, and not on medications. They were allocated to the trial using a block design, which allowed partial double-blinding. Block 1 consisted of the four males and block 2 of the four females. For block 1, pre-storage insulin was used as the standard (reference) at the start of the experiment, thereby precluding blinding of both subjects and experimentors. For block 2, the reference insulin was that stored in a refrigerator at 4°C. For block 2, both the reference and zeer insulins were available at the same time, and their order of use was randomized using a coin toss. Both subjects and experimentors were blinded to the allocation in block 2.
Insulin Storage ConditionNeutral human insulin was used and stored in a refrigerator at 4°C or in a zeer. This is a clay pot that is unglazed, enabling water to slowly seep through the wall and evaporate, thus lowering the temperature of the remaining water. The zeer used to store the insulin had an internal diameter of 30 centimeters and a height of 30 centimeters. A glass jar was partially filled with desert sand and put in the center of the insulin zeer. A water bath was created by adding water outside the jar to the same level as that of the sand in the glass jar. Water was added to the mark (topped up) daily from a larger water storage zeer. The zeer was closed with a clay lid and kept in the shade. An exact replica, but without water (a dry zeer), was also used to store insulin and record temperatures. Additional bottles of insulin were kept outside in the shade, in the bathroom of an air-conditioned house and in a refrigerator. A small amount of insulin (10 units) was removed twice daily from each insulin vial to simulate actual usage conditions. Temperature readings were taken on 12 separate occasions during the six-week storage period (Table 1).
Insulin Sensitivity TestThe pr...
