Insulin to Carb Ratio
Last updated on August 12th, 2016
For patients with type 1 diabetes or insulin-dependent type 2 diabetes who take rapid-acting insulin when eating, it is important to understand the insulin to carb ratio, which can be used in planning the diabetic’s meals. It is relatively easy to do without having to worry about using math to calculate this ratio.
When using the insulin to carb ratio, it is assumed that the carbohydrates we take in are responsible for increasing the blood glucose levels. Carbohydrates include simple sugars, fiber, complex carbohydrates, and starch. Fiber is not usually counted as part of the insulin to carb ratio because it doesn’t break down well in the gastrointestinal tract and therefore does not increase the blood glucose levels as much as other types of carbohydrates. Fats and protein content are not counted in the insulin to carb ratio because they, too, do not increase the blood glucose level as much as carbohydrates.
When diabetics eat a meal, they often use rapid-acting insulin, such as Novolog (aspart), Humalog (lispro), and Apidra (Glulisine). The diabetic may also use regular insulin, such as Humulin R and Novulin R, but these are slower in activity so they are less effective at decreasing the blood sugar accumulated during the meal.
Regardless of the type of insulin used, the important thing to do is to match the dose of the insulin to the amount of carbohydrate taken in the meal. This is why the insulin to carb ratio is used. The insulin to carb ratio tells the diabetic patient how many grams of carbohydrates they can eat that will be covered by each unit of insulin.
As an example, a 1 unit per 10 gram of carb ratio (1:10 ratio) means that it takes 1 unit of insulin to cover for eating 10 grams of carbohydrates. Similarly, a 1:20 ratio means that you can eat 20 grams of carbohydrate for every 1 unit of rapid acting insulin you take. When you know your insulin to carbohydrate ratio, you divide your carbohydrate intake by your ratio to determine how much rapid-acting insulin to take.
If your insulin to carbohydrate ratio is 1:20, you only need two units of insulin to cover for a meal that contains 40 grams of carbohydrate the meal. The higher the insulin to carb ratio, the more carbohydrate you can take in for each unit of insulin necessary to cover for the carbs you eat. As the second number in the insulin to carb ratio increases, the amount of insulin you need to take in drops proportionately.
The good thing about calculating the insulin to carb ratio is that it allows the diabetic to eat varying amounts of carbohydrates as long as the insulin dose covers for the carbohydrates eaten. This keeps the blood sugar in tight control, which should be the goal at all times.
Commonly, the insulin to carb ratio is different depending on the time of day. This is because hormone levels change during the day and this can affect insulin sensitivity. The amount of physical activity you do also affect the insulin to carb ratio as exercise can improve insulin sensitivity. Finally, the amount of long-acting insulin you take each day affects the insulin to carb ratio so the more long-acting insulin you take in, the less rapid acting insulin you need to take.
For most diabetics, the insulin sensitivity is somewhat lower during the morning hours when compared to later in the day. It is not uncommon to have an insulin to carb ratio of about 1:10 in the morning and an insulin to carbohydrate level of 1:15 at night. Exercise after the evening meal can greatly affect the insulin to carb ratio, dropping it as much as 1:25.
Calculating the Insulin to Carb Ratio
There are two techniques used for selecting the starting point of the insulin to carb ratio. It is better to start conservatively with lower insulin doses when just starting out using the ratio, as using too much insulin can result in low blood sugar levels after meals. It is better to add more insulin than it is to try and recover from having the blood sugar be too low.
The 500 Rule
This is based on the idea that the average diabetic consumes about 500 grams of carbohydrate through meals and snacks, and also produces carbohydrates in the liver (which adds to the total carb intake). By dividing 500 by the average total number of insulin units you take on a daily basis, you can get a good measure of what the insulin to carb ratio is. This includes both the long-acting and mealtime insulin doses.
As an example, if you take about 25 units of insulin each day, each unit of insulin should cover for about 20 grams of carbohydrate as 500 divided by 25 is 20. If you take 60 units of insulin per day, your insulin ratio should be about 1 unit for every 8 grams of carbohydrate you eat (this is calculated by taking 500 and dividing it by 60 to get a 1:8 ratio).
The primary weakness of using this approach to determining the insulin to carb ratio is that there is the assumption that all diabetics eat about the same amount of carbohydrates per day, producing the same amount of glucose per day in the liver. People who are overweight and those who eat a greater number of carbs per day will have an underestimation of the insulin dosing necessary each day. In the same way, diabetics who are thin, eat few carbs, or are more physically active will overestimate the amount of insulin needed to cover for each meal or snack.
The Weight Method
This method of calculating the insulin to carb ratio is based on the idea that insulin sensitivity goes down as a person’s weight increases. This means that each unit of insulin taken in with every meal will cover for fewer carbs in a person who weighs more when compared to a person who doesn’t weigh as much.
When using this method, the approximate insulin to carbohydrate ratios can be calculated as this:
- Weight less than 60 pounds yields an insulin to carb ratio of 1:30
- Weight between 60 and 80 pounds yields an insulin to carb ratio of 1:25
- Weight between 81 and 100 pounds yields an insulin to carb ratio of 1:20
- Weight between 101 and 120 pounds yields an insulin to carb ratio of 1:18
- Weight between 121 and 140 pounds yields an insulin to carb ratio of 1:15
- Weight between 141 and 170 pounds yields an insulin to carb ratio of 1:12
- Weight between 171 and 200 pounds yields an insulin to carb ratio of 1:10
- Weight between 201 and 230 pounds yields an insulin to carb ratio of 1:8
- Weight between 231 and 270 pounds yields an insulin to carb ratio of 1:6
- Weight greater than 270 pounds yields an insulin to carb ratio of 1:5
One of the downsides of using this method to calculate the insulin to carb ratio is that it doesn’t take into account the person’s body composition. For example, a person weighing 250 pounds but who has a lot of muscle mass is more sensitive to insulin when compared to a person weighing 250 pounds who has more fat than muscle in their body.
Calibrating the Insulin to Carb Ratio
Before trying to fine tune the insulin to carb ratio used at any given meal, it is best to determine your basal insulin levels first. Any changes to the total amount of insulin you take in per day will mean you have to calculate your insulin to carbohydrate ratio all over again. This is done through basic trial and error. Make sure you take into account factors such as the time of day you are eating and your total body weight while deciding on the right insulin to carbohydrate ratio at any given meal.
It is important to keep a detailed record of your eating habits, the amount of carbohydrates you eat per day, the amount of insulin you are using, and the amount of exercise you do per day. Write down your blood sugar level prior to eating and then again at 3 to 4 hours after your meal. Don’t eat anything else or exercise during this time and don’t take extra insulin before measuring the second blood sugar level.
The best thing to do is to eliminate those readings you get that are impacted by other factors that affect insulin sensitivity, such as strenuous physical activity. You should also discount meals you eat at a restaurant as they are often very high in fat and you may not be able to determine how many carbohydrates are in the meal. This can throw off your insulin requirements and won’t be helpful in determining your average insulin to carbohydrate level.
Because every day is different, you need to take your blood sugar readings for about 2 weeks along with the other factors influencing insulin sensitivity before calculating your insulin to carb ratio. Try to eat meals with differing carbohydrate contents, adjusting the rapid acting insulin you take in based on the blood sugar level you got before the meal.
Find out which insulin to carb ratios are ineffective in keeping your blood sugars low enough after the meal as well as which insulin to carb ratios cause the blood sugar to become too low. Which insulin to carb ratios keep the blood sugar in the normal range after eating? The idea is to choose an insulin to carb ratio that keeps your blood sugar in the normal range after eating your meals and snacks. Even if the blood glucose level is a bit too high or a bit too low, if it remained roughly the same before and after the meal, this is probably the insulin to carbohydrate level you should stick with.
- Insulin-to-Carb Ratios Made Easy. http://www.mendosa.com/insulin_carb_ratios.htm
- How to calculate the Insulin to Carb Ratio. http://www.bd.com/us/diabetes/page.aspx?cat=7001&id=7303