Insulin-to-carbohydrate ratio (ICR)

The insulin-to-carbohydrate ratio (ICR) answers the question of how many grams of carbohydrates are covered by one unit of rapid-acting insulin [1]. For example, a ratio of 10 means that one unit of insulin covers ten grams of carbohydrates. This ratio lets you calculate more accurately how much insulin you need for a meal, depending on its size. So, for 50g of carbohydrates with an ICR of 10, you need 5 units of insulin (without correction).

The ratio varies greatly between people and, in the same person, between different times of the day [2]. Most patients need more insulin in the morning because of counter-regulatory hormones (or puberty hormones, if applicable) [3]. The insulin requirement at lunch is generally 30-50% lower than in the morning. The correct ICR is the one that brings your blood glucose back to the starting level two hours after the meal, without any risk of hypoglycemia [4].

Count the total carbohydrates in the meal (by reading labels, weighing foods, using nutritional tables or apps), then divide by your insulin-to-carbohydrate ratio [1]. For example, for 60g of carbohydrates with an ICR of 12, you calculate 60 / 12 = 5 units. Round to the nearest half unit if you use a pen, or to 0.1 units if you have an insulin pump.

Also take into account your pre-meal blood glucose. If it is above your target, add a correction. If it is below target, reduce the dose slightly. Also consider factors such as insulin on board, planned physical activity, the fat and protein in the meal (which may require an extended bolus on a pump), alcohol consumed, or another acute condition [5]. Over time you will quickly estimate doses for meals, even without extremely detailed calculations.

Start by reading labels. Look for "total carbohydrates", not "of which sugars" [6]. For foods without a label, weigh the portion and use nutritional tables or apps (MyFitnessPal, Foodvisor, Carbs&Cals). For homemade dishes, calculate the carbohydrates from each ingredient and divide by the number of servings. Later, learn to visually estimate the portions of foods you use frequently.

Also pay attention to fiber. If it is more than 5g, subtract half of it from the total carbohydrates. For restaurants, many chains publish nutritional information online. Learn useful equivalences. For example, one slice of bread generally has 15g of carbohydrates, a cup of milk 12g, and a medium apple 20-25g. Accept that the estimate will not be perfect [7]. Aim for an accuracy of ±10g for your usual meals [8].

Yes, it is normal and common for the ratio to differ throughout the day [2]. Most people need up to 50% more insulin per gram of carbohydrate at breakfast (the dawn phenomenon, counter-regulatory hormones) [3]. The ICR might, for example, be 8 in the morning, 12 at lunch, and 10 in the evening. If meals are larger, some people need different ratios for simple versus complex carbohydrates [9].

To identify your own ratios, systematically test how your blood glucose changes after a meal. Eat a meal with known carbohydrates when your starting blood glucose is on target and without other variables such as exercise or stress. Measure your blood glucose one hour and two hours after the meal for several days [4]. If the values are above target, the ratio is too high. If you have hypoglycemia, it is too low. Adjust by 1-2 points and retest for a few days.

The ratio is correct if your blood glucose two hours after the meal returns to within ±30 mg/dl (1.7 mmol/L) of the starting value, without hypoglycemia at three hours [4]. For more precise testing, start with your blood glucose on target, eat a meal with well-calculated carbohydrates, without excessive fat or protein, avoid physical activity for the next four hours, and don't make any corrections during this interval [5].

Test each time of day separately, repeating the test at least three times for confirmation [10]. If your blood glucose at two hours is consistently above 150 mg/dl (8.3 mmol/L), decrease the ratio by one point. If you tend to have hypoglycemia at two hours, increase the ratio. Document everything in a journal to spot patterns. Don't test on atypical days (illness, major stress, menstruation).

The ratio can change over time for several reasons, such as weight changes (weight loss improves insulin sensitivity), level of physical activity (regular exercise increases sensitivity), age (puberty decreases sensitivity), duration of diabetes (the progressive loss of residual beta cells), concomitant medications, and hormonal changes [11].

The season can also have an influence [12]. Some patients need less insulin in summer, when they are more active. Chronic stress, illness, or infections temporarily decrease insulin sensitivity. Pregnancy dramatically changes the insulin requirement. That is why you should reassess your ratios every three months, or whenever you notice consistent patterns of postprandial hyper- or hypoglycemia. Adjustments are a normal part of diabetes management.

MyFitnessPal has the largest food database, including many traditional dishes. Carbs&Cals offers photos of portions for visual estimation. Foodvisor uses AI for photo recognition [13]. You photograph the food and the app estimates the carbohydrates. mySugr and Diabetes:M integrate carbohydrate counting with a diabetes journal and an insulin bolus calculator.

For restaurants, the apps of fast-food chains have accurate nutritional information. Yuka scans barcodes and provides instant information. FatSecret has recipes with automatic per-serving calculation. Most apps let you create custom meals and recipes that you use frequently [14]. Choose the app that suits your style. Some prefer simplicity, others want advanced features.

Ideally, you should weigh important foods for accuracy, especially at the beginning when you are learning portions [7]. After a few months, you will be able to visually estimate the portions you use frequently, with an acceptable error. Always weigh carbohydrate-dense foods such as bread, potatoes, pasta, rice, and cereals [8]. For low-carbohydrate vegetables (under 5%), visual estimation is sufficient.

Invest in a digital kitchen scale. When you eat out, use the experience you have built up to estimate the carbohydrates. Some people prefer to eat standard portions that they have memorized. Don't become obsessed with achieving perfect precision. An error of ±10g of carbohydrates is acceptable for most meals [7].

Use your experience at home with similar portions as a reference [8]. Most restaurants serve standard portions. A slice of pizza has 30-40g, a burger 30-40g, a small portion of fries 25-30g. Ask the restaurant for nutritional information, as many restaurants already have it available. For complex dishes, mentally break them down into ingredients and estimate each one.

Apply the "better more than less" rule. Under-dosing insulin leads to prolonged hyperglycemia, while overdosing is corrected more easily, with some fast-acting carbohydrates. Photograph the food for future reference. Bear in mind that restaurant food often has extra sugars, hidden in sauces and dressings [7]. Measure your blood glucose at two hours and adjust your estimate for the next time you eat something similar [4].

For complex dishes, break them down into the main ingredients and estimate the carbohydrates from each [6]. For lasagna, you might consider, for example, pasta sheets + bechamel sauce + tomato sauce + cheese. Ignore ingredients with negligible carbohydrates (cheese, meat). For sauces, you can assume 10g of carbohydrates per serving if they taste sweet. For cream soups, consider the vegetables, the flour, and the milk/cream. The fat and protein in complex dishes may sometimes require additional insulin [5].

Create a personal library of "recipes" in your app, in the frequent-meals section [14]. When you cook at home, calculate once for the whole tray and divide by the number of servings. For pies or cakes, calculate the total carbohydrates from the flour, sugar, and fruit, and divide by the number of slices. Over time, you will develop your intuition for frequent combinations and won't have to calculate from scratch every time.