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 50 g 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 and, in adolescents, puberty hormones [3]. The insulin requirement at lunch is generally 30-50% lower than in the morning. The correct ratio is the one that brings your blood glucose back to the starting level two hours after the meal, without hypoglycemia in the hours that follow [4]. The starting ratio and every change to it are decided together with your medical team.

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 60 g 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, a correction dose is added, calculated with your insulin sensitivity factor (how much one unit of insulin lowers your blood glucose). If it is below target, the meal dose is reduced using that same factor, not by an amount picked by eye; the sensitivity factor and your blood glucose target are set together with your medical team. Bear in mind other factors too, such as active insulin, planned physical activity, the fat and protein in the meal (which may call for an extended bolus — that is, a dose of insulin the pump delivers gradually, over a longer period), alcohol consumed, or another acute condition [5]. Over time you will quickly estimate doses for meals, even without extremely detailed calculations, as the figure below shows.

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, Carbohydrates&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. The rule of subtracting half the fiber once it exceeds 5 g applies to American labels and food tables; on the European Union label the "Carbohydrate" line does not include fiber, so there is nothing to subtract (more about fiber and polyols). For restaurants, many chains publish nutritional information online. Learn useful equivalences. For example, one slice of bread generally has 15 g of carbohydrates, a cup of milk 12 g, and a medium apple 20-25g. Accept that the estimate will not be perfect [7]. Aim for an accuracy of ±10 g 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 ratio 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 blood glucose at two hours stays more than 30 mg/dL (1.7 mmol/L) above the pre-meal value, the ratio is too high: a larger number means less insulin for the same amount of carbohydrates. If you have hypoglycemia, the ratio is too low, meaning it leads to too much insulin. The adjustment is usually of 1-2 points, with retesting for a few days, according to the plan agreed with the medical team.

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 more than 30 mg/dL (1.7 mmol/L) above the pre-meal value, the ratio is usually decreased by one point, according to the plan agreed with the medical team. If you tend to have hypoglycemia at two hours, the ratio is increased. 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 [11];
• age — puberty decreases sensitivity;
• duration of diabetes — the progressive loss of residual beta cells;
• concomitant medications;
• hormonal changes.

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. Carbohydrates&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. Some diabetes diary apps integrate carbohydrate counting with an insulin bolus calculator.

For restaurants, the apps of fast-food chains publish detailed nutrition values for the standard recipe; the portion you are served may differ, so treat the figure as a starting estimate and check your blood glucose after the meal. 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.

Not all of them. 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. You don't need perfect precision, and chasing it can become exhausting; if carbohydrate counting is taking up more and more of your thoughts, or is changing the way you eat, tell your medical team. An error of ±10 g 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. When you are not sure of the amount of carbohydrates, a partial bolus is given and topped up after you see your blood glucose at one hour.

An overestimated dose is harder to correct than a passing hyperglycemia, because hypoglycemia sets in quickly. Hypoglycemia means a blood glucose below 70 mg/dL (3.9 mmol/L): you take 15 g of fast-acting carbohydrate and recheck after 15 minutes. The complete steps are on the pages about what hypoglycemia is and about how to treat it. 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 10 g 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.