The moment we’re diagnosed with diabetes, we’re taught that carbohydrates have the biggest impact on our blood sugar — and this is true — but dietary fat and protein affect your blood sugar, too.

We need fat in our diet, for flavor and health, but as people with diabetes, we need to be aware of when and why it affects our blood sugars, too.

In this article, we’ll discuss how dietary fat impacts your blood sugar levels and your insulin needs.

Dosing Insulin for Dietary Fat: How Fat Affects Your Blood Sugar

What is dietary fat?

Fat is a “macronutrient,” just like carbohydrates and protein. Macronutrients are the sole source of calories and fuel for the human body! 

Dietary fat contains 9 calories for every gram — more than twice as much as protein and carbohydrates, at 4 calories per gram. But that doesn’t mean fat is bad. Actually, dietary fat is vital for your long-term survival!

But dietary fat is not broken down into glucose like carbohydrates (and sometimes protein). Instead, it’s broken down into fatty acids. Without those fatty acids from your diet, your body would eventually fail to function.

You need fat in your diet to:

  • Absorb vitamins A, D, E, & K
  • Protects your organs
  • Maintain healthy growth of skin, hair, and nails

Dietary fat is also very satiating. For example, eating an apple by itself will leave you feeling hungry, but eating an apple along with substantial fat from peanut butter or cheese will give your body longer-lasting fuel because it takes longer to digest fat than carbohydrates.

How dietary fat affects your blood sugar

A meal containing 8 grams of fat won’t have a noticeable impact on your blood sugar levels, but eating more than 15 grams of fat in one sitting or short period of time will create some trouble if you’re not prepared for it.

Let’s take a look…

Dietary fat can delay digestion by a few hours

When you eat a large serving of dietary fat in one sitting, it will slow down the digestion of the carbohydrates in your meal, too. 

For those who don’t take insulin, this can sometimes actually help prevent post-meal spikes in your blood sugar.

For people who take insulin, this can be a tricky thing to manage, because it changes the timing of when your body needs insulin for that meal. If you take insulin when you start eating, like you are normally instructed to do, the insulin will become active in your bloodstream before the carbohydrates are digested and broken down into glucose. 

Instead, it’s ideal to spread your insulin dose out a bit. If you’re eating a high-fat meal containing carbohydrates, Gary Scheiner, MS, CDE author of Think Like a Pancreas suggests the following:

  • Split your calculated insulin dose in half.
  • Take the first half of your dose when you begin eating.
  • Take the second half of your dose 1 to 2 hours after you’ve finished eating. 
  • For those using an insulin pump, you can do this with an “extended bolus.”
  • For those taking multiple daily injections, you can simply take two separate injections. 

By spreading your meal dose out over several hours, you’re matching the rate of digestion of that high-fat meal.

Dietary fat can trigger your liver to release glucose, too!

Unfortunately, it’s not always as simple as splitting your insulin dose in half, because of really high-fat meals — like pizza, Chinese food, or rich ice cream.

When you eat a meal containing 30 or more grams of fat (which is easy to do with a few slices of pizza), that high-fat content can impact your blood sugar for up to 12 hours thanks to your liver.

Normally, your liver produces a very small amount of glucose all day long, 24 hours a day. This helps provide your body and your brain with the fuel it needs while you sleep, and between meals.

The sugar produced by your liver comes from its “glycogen stores,” which are essentially stored glucose. Your liver is designed to release this “back-up” glucose as a source of fuel for other situations, too, like during adrenaline-inducing roller coaster rides, soccer games, stressful arguments, dawn phenomenon, and “fight or flight” fearful moments.  

When you eat a very large amount of fat, your liver produces more glucose because of insulin resistance. Your liver senses heightened levels of insulin resistance due to the large fat quantity and tries to balance this by producing more glucose. 

This is normally suppressed by the presence of insulin — something non-diabetics manage automatically by producing more insulin. In people with diabetes, it means we need to take more insulin for sometimes up to 12 hours after a very fatty, heavy meal. 

Gary Scheiner recommends a few ways of managing this:

  • For pumps: set a temporary basal rate increase by 50 percent for 8 hours after eating. 
  • For injections: take an injection of NPH (lasts 8 to 10 hours) or Regular (lasts 4 to 6 hours) insulin after eating.
  • On top of splitting your meal dose, you can add another small injection of fast-acting insulin 4 hours after eating.

Work with your healthcare team to establish the best approach for you!

Why ketogenic diets can increase your insulin needs

This glucose production from the liver is also why some people see their insulin needs rise when they begin following a ketogenic diet. This can be very confusing and frustrating considering low-carb and ketogenic diets are promoted as reducing your insulin needs and lowering your blood sugar levels.

For many, that is the case! But for some, it’s not that simple.

The intensely high fat intake in a high-fat dieting approach at first means you need less insulin because there are so few carbohydrates, but it can increase your insulin needs through the insulin resistance created by the presence of dietary fat.

For some people, this becomes harder to manage than simply eating a diet containing more carbohydrates. It can mean you may benefit from a lower-carb, medium-fat, medium-protein diet, instead of a specifically high-fat diet.

For others, a high-fat, low-carb diet means they experience smoother blood sugar levels and lower insulin doses overall. 

Regardless, you’ll have to figure out what works best for your body and work closely with an experienced endocrinologist or CDE to fine-tune your insulin-dosing approach!