Last Updated on September 9, 2025
Managing blood sugar isn’t only about counting carbs—protein and fat play a big role too. Carbohydrates raise glucose quickly and require precise insulin coverage. Protein has a slower, smaller effect, while fat can cause delayed rises and increase insulin resistance. Understanding how each macronutrient affects your glucose helps you fine-tune your insulin doses, improve time in range, and avoid frustrating highs after meals.
How do carbohydrates affect blood sugar?
Of all macronutrients, carbohydrates have the most direct effect on blood glucose. When you subtract fiber, carbs break down almost entirely into glucose. People using insulin pumps or injections rely on their CarbF (insulin-to-carb ratio) to match insulin to carb intake.
Example: If your CarbF is 1 unit per 10 grams of carbs and you eat 100 grams, you’ll need a 10-unit bolus. With correct basal insulin and accurate carb counting, this covers the meal effectively. Because carbs digest faster than insulin starts working, most people benefit from bolusing 15–30 minutes before meals.
What about protein and blood sugar?
Protein has a delayed, less dramatic effect. About half of a meal’s protein may eventually convert to glucose, raising blood sugar gradually over about 6 hours. For adults, meals with 20+ grams of protein (or at least 10 grams of carbs from protein) may require additional insulin.
How do fats impact blood sugar?
The type and quantity of fat in your meal can influence both blood sugar and inflammation. Fat slows digestion and can make glucose rise later, while also triggering insulin resistance through inflammatory processes.
- Saturated fats (meats, eggs, cheese, sausage, bacon, pizza) are inflammatory and often cause a delayed rise about 5 hours later.
- Monounsaturated fats (olive oil, avocado, nuts, natural peanut butter) usually don’t increase insulin needs.
- Polyunsaturated fats (many vegetable oils) can be either pro-inflammatory or anti-inflammatory depending on the type.
- Omega-3 fats (fish, seafood) are anti-inflammatory and may reduce insulin requirements—watch for added sauces.
Cooking methods also matter: heating fats can oxidize them, and cooked meats form advanced glycation end products (AGEs), both of which increase inflammation and insulin resistance.
Do high-fat meals require more insulin?
Yes. High-fat meals, such as pizza, often require larger or differently timed bolus doses. Because responses vary, there’s no single formula like CarbF for fat. A continuous glucose monitor (CGM) can help track how your body responds to repeated meals.
Many people use combo or dual-wave boluses—giving insulin up front for carbs, then spreading additional insulin over time for fat and protein. Keeping records helps you fine-tune doses.
The 25 Rule for High-Fat/Protein Meals
As a guide, try a bolus 25% larger than the carb bolus alone:
- Take 75% of the total bolus immediately to cover carbs.
- Deliver the remaining 25% as an extended bolus over 2.5 hours for fat and protein.
Example: If you need 10 units for carbs, multiply by 1.25 for a total of 12.5 units. Deliver 7.5 units immediately, then 2.5 units spread over 2.5 hours.
How can you improve post-meal glucose control?
Experiment with timing and dosing until you find the right balance for your preferred meals. Use your CGM to track patterns and avoid foods that consistently cause unexpected spikes. Over time, you’ll learn how carbs, protein, and fat interact with your insulin needs.
Final thoughts
Glucose management may feel complicated at first, but success comes from testing, learning, and making gradual adjustments. With the right tools and strategies, everyone can improve their readings and time in range.
Related Resources
External References
- American Diabetes Association
- PubMed Research Articles
- National Institute of Diabetes and Digestive and Kidney Diseases
Where did this equation come from? Do you have a source or article for it?
Thank you for the question, Christina. The link above for “Higher fat meals” leads to an excellent article by Ewa Pankowska, MD, PhD, who has done much work in this area.
I derived the 25 Rule from my clinical experience with patients. You can find it on page 122 in the 6th edition of Pumping Insulin. It offers a relatively conservative approach for handling higher fat and protein meals that, from experience, raise the glucose more than expected.
Love all your books Thank you!!! and her method for bolusing is something I have looked into. I have reached out to her years ago and she was so informative
Thank you!
What about if your meal is high fat and/or protein but no carbs? Would, say, a large plain steak or fish by itself require any bolus?
Hi Mike,
Even with no or few carbs, certain meals may need some insulin. Proteins break down gradually with up to half of the grams of protein gradually converting to glucose over a few hours. A bolus extended over 3 hours might help for a steak, for example. The animal fat in a steak will generate some insulin resistance and this often necessitates an increase in the bolus delivered over time or a delayed bolus or injection.
Vegetable fats, like an avocado, do not cause insulin resistance nor need a bolus.
John
Hi John, How would you recommend blousing for a bag of Quest Protein Chips with 20g protein, 4.5 g Total Fat (0.5g saturated), and 4g Carb? My ICF is 10. I don’t know the formula for figuring out protein plus fats. I’m T1D x 49 years, almost age 60. Pumping insulin. Thank you.
Hi Chris, your CGM is always the best guide for boluses. As a guide, the 4 grams of carb get covered by 0.4 U, of course. For protein, less than half of the protein is slowly converted to glucose over 3 to 6 hours. A good trial dose for one serving would be 0.8 U. The Quest Chips contain healthy fats that should not generate any insulin resistance. However, there are concerns about large quantities of whey protein, but a few occasional servings should not be a problem.
I advise people to avoid high protein diets and hope you are not doing this because they significantly shorten lifespans. Best of health and regards, John