Diabetes is a group of metabolic disorders where malfunctioning insulin metabolism causes high blood sugar levels that, if left untreated, can do serious long-term damage. Within this group, there are types of diabetes: Type 1, Type 2, and gestational diabetes. Insulin is a hormone which is produced in the pancreas and manages how glucose is absorbed by the body. If there’s not enough insulin, the body can no longer effectively absorb glucose from the bloodstream.
Type 2 is by far the most common, accounting for 90% of global diabetes cases. It develops when the body’s cells can no longer receive and respond to insulin, a condition called “insulin resistance.” Type 2 diabetes cannot usually be cured, but it can be managed with a healthy lifestyle (a balanced, low-sugar diet and regular physical exercise) and with medications like insulin sensitizers.
Type 1 diabetes is much less common, making up about 8-10% of diabetes, globally. It is an incurable and lifelong condition. The disease typically develops in childhood, due to an autoimmune disorder, and results in the pancreas not producing sufficient insulin. Type 1 diabetes can be effectively managed with insulin injections or pumps that make up for the insulin no longer produced by the pancreas. Instead of scheduled injections, it’s becoming increasingly popular to continuously monitor glucose levels and have insulin delivered automatically using a pump.
Almost 10% of the world’s population suffers from diabetes or pre-diabetes. In particular, the disease is becoming a problem in more affluent countries like the United States, where obesity and a sedentary lifestyle are widespread. Currently, around 13% of American adults are diabetic, including 8 million who have not yet been diagnosed. As per an estimate by the US Center for Disease Control. Every year, around 300,000 Americans die from complications of diabetes.
The 500 rule (sometimes also called the 450 rule) is a widely used method for type 1 diabetes patients to estimate how many carbohydrates will be covered by one unit of fast-acting, or bolus insulin. This is called the insulin-to-carb ratio or simply, the carb factor. Knowing this ratio allows you to plan ahead for how much insulin you need to cover the carbohydrates in your meals, and keep your blood sugar levels under control.
Bolus insulin is specifically designed for fast action and thus ideal to be used after meals or during times when blood sugar levels are unusually high and require an immediate reduction. Fast-acting insulin takes effect within 15 minutes, peaks after one hour and remains effective for 2-4 hours afterwards.
The volume of bolus insulin used needs to correspond to the number of carbs a person eats during a meal. So it is essential that a person with diabetes knows how many carbohydrates they are taking in during a meal. For accurate calculation you also need to take into account the basal insulin you use — this is the set amount of long-acting insulin you inject or pump every day.
The 500 rule is based on the simple assumption that the average adult consumes 500 grams of carbs every day, including any carbs produced in the liver. So, to get your normal insulin-to-carb ratio (ICR) you divide 500 by the units of insulin you take on a typical day (both basal and bolus) with stable blood sugar levels. For example, let’s say that normally you take 20 units of insulin, then your ICR is 1:25 (500/20=25).
Here’s a table of common ICRs:
Based on your ICR, you can then roughly calculate how much bolus insulin you’ll need to cover the carbs of any meal. Of course, you’ll need to have an idea of how many carbs actually are in the meal. It takes some practice at first, but over time you’ll get a good sense of how many carbs are in your favourite foods.
The 500 rule is simple to use, but it comes at the expense of accuracy. After all, not all people eat exactly 500 grams of carb every day. Depending on body weight, age, sex, dietary preferences and many other factors, carb intake can vary widely.
With the 500 rule, overweight people tend to lowball their mealtime insulin needs, while leaner people will use too much. Therefore, more and more people are using a calculation that takes bodyweight into account. This approach also requires some trial & error and fine-tuning.
To learn more about the 500 rule and alternative ways of estimating mealtime insulin needs, talk to your primary doctor or an endocrinologist (a diabetes specialist).
Reviewed by Dr Roy Kedem, MD
Information last reviewed 12/06/21