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The Fat of the Matter: How Dietary Fat Effects Blood Glucose

By Gary Scheiner MS, CDE

Posted On: 22 October 2010

Pizza. A hot fudge sundae. Movie theater popcorn. Cheesesteaks wit (hey, I’m a Philly guy).

What do they all have in common? More than being mighty tasty, they’re also loaded with fat. Fat doesn’t usually receive a whole lot of attention from those on intensive insulin programs; carbs seem to get all the glory. And deservedly so: Carbs cause a rapid rise in blood glucose, while fat seems to have little effect. Or does it?

You may have noticed that your blood glucose level rises overnight after a restaurant meal. Or perhaps it climbs excessively in the evening after having a big piece of birthday cake during the day. The culprit is most likely the fat content of these types of meals and snacks, not the carbohydrates.

It has long been known that adding fat to a meal will slow down the digestion/absorption of carbohydrates. This is due to a slowdown in gastric emptying – the rate at which food passes from the stomach into the intestines, where the nutrients (such as glucose) are absorbed into the bloodstream. This is why the carbohydrates in high-fat meals tend to take longer to raise the blood glucose level. But the difference is generally an hour or two: Whereas a low-fat meal will raise the blood glucose level quickly (usually within an hour), a high-fat meal may take two to four hours to produce a blood sugar peak.

So what about after the carbohydrates are finished doing their thing? That’s when the fat itself begins to exert its effects. The process goes something like this:

  1. You eat a high-fat meal or snack (this is the fun part).
  2. In a few hours, the fat begins to digest; this continues for several hours.
  3. The level of fat in the bloodstream (triglycerides) rises.
  4. High triglycerides in the bloodstream cause the liver to become resistant to insulin.
  5. When the liver is insulin resistant, it produces and secretes more glucose than usual.
  6. The blood glucose rises steadily as the liver’s glucose output goes up.

This is what causes the gradual, delayed blood glucose rise after consumption of large amounts of fat. The response seems to be “dose-dependent” – the more fat you consume, the more insulin resistant the liver becomes, and the more glucose it produces. The type of fat also appears to play a role. Saturated fats (the type found in dairy and animal products) seem to cause more insulin resistance than monounsaturated and polyunsaturated fats (the type found in vegetable products).

So what can be done about it? The obvious answer is to cut back on portions of fatty foods, and choose foods containing healthier types of fats. But when confronted with your favorite culinary indulgence from time to time, you can still partake and manage your blood glucose level reasonably well.

First, to properly cover the slower-digesting carbs, consider delivering your mealtime rapid-acting insulin a bit later than usual, or split the dose into two parts (part right before the meal, the other part a little while after the meal). Use of Regular insulin, with its slower onset and delayed peak, may also be an option. For those using an insulin pump, this is a great time to employ the prolonged bolus feature (extended/square/dual/combo, etc…). I usually find that a dual/combination bolus works well, with 30% of the bolus delivered up front with the meal and the other 70% delivered over the next two hours.

As for the long-term delayed rise caused by the fat itself, there are a couple of strategies that might work well. For those using injections, a small dose of NPH insulin given right after the meal usually provides just enough extra insulin to offset the delayed glucose rise for the next 4-10 hours. For those on a pump, an increase in the basal rate will accomplish the same thing. Try setting a temporary basal rate increase of approximately 50% for the next 6-10 hours, starting after the meal.

As with any insulin dosage adjustment, it’s a good idea to run these ideas by your healthcare team before trying them, and be sure to monitor frequently to determine whether some fine-tuning is necessary.

Now if you’ll excuse me, there’s a nice Philly cheesesteak here with my name on it.


Gary Scheiner MS, CDE is a diabetes educator with a private practice (Integrated Diabetes Services) specializing in intensive insulin therapy.  He is author of “Think Like A Pancreas:  A Practical Guide to Managing Diabetes With Insulin” and “The Ultimate Guide to Accurate CarbCounting.”  He has had Type-1 diabetes for 25 years.  He and his team offer diabetes management consultations via phone and internet to insulin-users throughout the world.  For more information, call (877) 735-3648 or e-mail gary@integrateddiabetes.com.


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