October 30, 2021
Insulin is a polypeptide hormone made up of 51 amino acids. It is a key player in intermediary metabolism of carbohydrates, proteins and fats. Insulin is called a hormone of abundance. It is secreted after meal when there is abundant of nutrients, especially carbohydrates. It organizes the use of fuels for either storage or oxidation. It promotes the storage of glucose, amino acid, and fatty acids and facilitates glucose utilization by tissues for energy. It also inhibits glucose production by the liver. if insulin is deficient or there is tissue resistance to insulin, all its actions get reverse and glucose, fatty acids and amino acids levels increase in blood. Amino acids are used to make glucose. Glucose production by liver also increases but body can not use it and use fatty acids for energy and glucose is secreted in the urine.
Insulin was discovered in 1921 and it transformed the management of patients with T1D which was a fatal disorder till then. Insulin was made by extraction and purification from the pancreas of animals. In 1980 human insulin was produced by recombinant DNA technology. Human Insulin (regular or soluble insulin) aggregates in subcutaneous tissue which must dissociate prior to absorption resulting in delayed onset and longer duration of action. A number of analogs were created in 1990 by modifying the regular insulin molecule with novel pharmacokinetics. By altering the amino acids sequence or protein structure of human insulin, its behavior following injection is either accelerated or prolonged.
There are three main groups of insulin
- Animal insulin
- Human insulin, conventional or Standard insulin
- Analogs of insulin
Action can be prolonged for example in insulin glargine by addition and substitution of amino acids, action is prolonged to 24 hours. Duration of action can also be extended by adding chemicals to regular insulin, For example, protamine and zinc at neutral PH ( isophane or NPH insulin) or by adding excess zinc ( Lente insulin). In insulin detemir and degludec, the duration is extended by adding fatty acids. Following injection, these bind to albumin in the blood and then slowly dissociate.
There is wide variability in the kinetics of insulin action among different people and even with repeated doses in the same person. The time to peak hypoglycemic effect and insulin levels can vary by 50% due in part to large variation in the rate of absorption from injection site.
Standard insulin concentration is 100U/ml. New concentrated insulins 200U (Degludec and lispro insulin), 300U (Toujeo -glargine insulin) or U500 (regular insulin) are also available. Which are respectively two, three or five time more concentrated than regular insulin. Concentrated insulin are given to patients who are resistant to insulin and require higher dose. Recently, an oral insulin has also became available.
Types of Insulin
Preparation of insulin are classified according to their duration of action.
Rapid Acting Analogs
Lispro , aspart and glulisine are rapid-acting Analogs of insulin. They are absorbed rapidly from the injection site. The onset of action is within 15 minutes so can be injected just before, during, or even after meals but have a shorter duration of action less than 4 hours. The time action profile of these analogs is similar. They instantaneously dissociate into monomers increasing the rate of absorption. These insulin are best used to control glucose spikes after a meal. They have lower rates of hypoglycemia and better A1c levels compared to regular insulin.
Short Acting Insulin
Short-acting insulin, like regular insulin, needs to be injected 30-60 min before meal. Regular insulin forms hexamers in subcutaneous tissue which break down slowly to monomers delaying their absorption from the injection site. They have a prolonged peak action of 2-5 hours and action can last up to 8 hours. Because of this they cause a mismatch between the need and availability of insulin. Delayed onset of action cause hyperglycemia after food and long duration of peak effect cause late hypoglycemia.
Intermediate Acting Insulin
NPH (Neutral Protamine Hagedorn) or isophane insulin is an intermediate-acting insulin. It is a suspension of regular insulin complexed with zinc and protamine. This produces a cloudy or whitish solution. NPH insulin dissolves more gradually thus its action is prolonged. It is given once or twice a day in combination with short-acting. Isophane and Lente insulin have to be resuspended before giving injection by mixing it well. The more modern long-acting Analogs insulins (Glargine, detemir, and degludec) are clear.
Long Acting Analogs
Insulin glargine, detemir, and degludec are long acting insulins. Insulin glargine (Lantus) is a clear solution with a pH of 4.0. When injected into neutral PH of subcutaneous space aggregation occurs, resulting in prolonged absorption from the injection site. Owing to its acidic PH it can not be mixed with short-acting insulin which has neutral PH. Glargine, unlike NPH, has no discernible peak of action and provides a steady basal effect over 24 hours. It causes less hypoglycemia especially nocturnal hypoglycemia comparing NPH insulin.
Insulin detemir and glargine has similar action profile but detemir has a shorter duration of action so has to be administered twice daily. Insulin degludec has an even longer duration of action over 40 hours. It is dosed daily. Degludec causes less severe hypoglycemia than glargine. Basal insulin is required to suppress hepatic glucose production overnight and between meals.
Premixed formulations containing short-acting or rapid-acting insulin and Isophane Insulin in various proportions (30:70 or 50:50) are available. It provides convenience by reducing the number of daily injections and the need for directly mixing them. The drawback of premixed formulations is that the individual components can not be adjusted. These may be used in patients who are unable to inject more often or live a very fixed lifestyle and are more commonly used by type 2 diabetic patients.
Indications of Insulin in Type-2 Diabetes
Insulin is required for all patients with type-1 diabetes (T1D) and is also helpful for the management of many patients with type-2 diabetes (T2D). Many oral antihyperglycemics medicine are available for type-2 diabetes but sometimes insulin is needed to control high blood sugar. Following are indications to use insulin in type 2 diabetes:
- Marked hyperglycemia such as fasting plasma glucose >250 mg/dl or Random blood glucose >350mg/dl
- Diabetic ketoacidosis or hyperosmolar state
- Ketones in urine 2+ or greater
- Newly diagnosed Diabetic with elevated Hb A1c 9 % or more
- Significant weight loss
- Pregnancy (preferably before pregnancy start).
- Acute illnesses requiring hospitalizations
- Before surgical procedures or during intensive care when metabolic decompensation occur.
- Post myocardial infarctions
- High dose glucocorticoid therapy
- Inability to tolerate or contraindication to use oral hypoglycemic medicines (hepatic or renal diseases)
- Patient no longer achieving goals on other diabetic medications.
Advantages of Insulin
- Insulin is the oldest of the currently available medicines. It is a natural body hormone. Safe and effective. It decreases glucose toxicity and lipotoxicity. It cause no serious drug interactions. There is no contraindications to use insulin.
- Most effective of diabetes medications in lowering glucose. Can decrease any level of elevated glucose.
- Easy to titrate. No maximum or minimum level beyond which therapeutic action can not occur.
- Beneficial effects on blood lipids
- Reduce risk of developing long term complications
- Improve health and wellbeing
Side Effects of Insulin
- The most common side Effect is hypoglycemia
- Local and generalized allergic reactions
- Local fat atrophy or hypertrophy
- Circulating anti- insulin antibodies.
- Weight gain
Hypoglycemia is the major risk that must be compared against the benefits of efforts to normalize glucose control. It may result from a mismatch between insulin and carbohydrates intake, exercise, or alcohol consumption. Too high a bedtime insulin can cause nocturnal hypoglycemia and stimulate a counter-regulatory response, leading to morning hyperglycemia (Somogyi phenomenon).
A more common cause of unexplained morning hyperglycemia, however, is a rise in early morning growth hormone and cortisol (Dawn Phenomenon). In this case, the evening insulin dose should be increased, changed to a longer-acting preparation, or injected later. Exercise-induced hypoglycemia can occur because of improved blood flow at the injection site. Both insulin and muscles contractions increase glucose uptake causing a fall in blood glucose level. It occurs mostly with prolonged aerobic exercise.
Weight gain is a common side effect of insulin therapy. It is because of the anabolic action of insulin, increase appetite, and decrease loss of glucose in urine. This can be avoided by eating healthy food with and increasing physical activity. Counting calories in food taken and being physically active helps to control weight. There are some diabetic medications if taken along with insulin helps in weight loss and decrease insulin requirement also.
Decrease in blood potassium level may be caused by insulin but it is uncommon. Hypokalemia more commonly occurs in when IV insulin is used.
Local allergic reactions at the site of insulin injections are rare, especially with the use of human insulins. They can cause immediate pain or burning followed by redness, itching, and induration sometimes persisting for days. Most reactions spontaneously disappear after weeks of continued injection and require no specific treatment, although antihistamines may provide symptomatic relief.
Generalized allergic reaction is extremely rare with human insulins but can occur when insulin is restarted after a lapse in treatment. Symptoms develop 30 minutes to 2 hours after injection and include urticaria, angioedema, pruritus, bronchospasm, and anaphylaxis. Antihistamines are given, but epinephrine and IV glucocorticoids may be needed. If insulin treatment is needed after a generalized allergic reaction, skin testing with purified insulin preparations and desensitization is carried out.
Local Fat Hypertrophy or Hypotrophy
Local fat atrophy at injection sites is relatively rare and is thought to result from an immune reaction to a component of the insulin preparation. Lipohypertrophy (enlargement of subcutaneous fat depot) is due to lipogenic action of high local concentration of insulin. It can be prevented by shifting of injection sites.
Insulin obtained from animals sources is antigenic. Circulating anti-insulin antibodies are a very rare cause of insulin resistance. Some people still prefer to use animal insulin. This type of insulin resistance can sometimes be treated by changing insulin preparations (eg, from animal to human insulin) and by administering corticosteroids if needed. Most patients use human insulin or insulin analogs as allergic reactions are less common with human insulin. Allergic reactions to recombinant human insulin may sometimes occur because of a small amount of aggregated or denatured insulin in preparations, minor contaminants, or because of sensitivity to added components (protamine or zinc, etc. )