Why Does Fat Increase Blood Glucose?

Has this ever happened to you?

— You eat a meal such as fettuccine alfredo with garlic bread and tiramisu for dessert.

— You take either the appropriate amount of insulin for the carbohydrate in the meal or your oral medications.

— You check 2 to 3 hours after eating and see a blood glucose reading that is in range.

So far, so good, right?

—Then you wake up the next morning with a very high number?

Ever wondered what causes this?

There are two reasons.

First, Fettuccine Alfredo, garlic bread and tiramisu are, for the most part, a mixture of carbohydrate and fat.  But it’s the fat in the meal that is contributing to the elevated readings.

Although carbohydrate is the nutrient that has the most immediate affect on blood glucose levels, fat is not glucose neutral. But only a small portion of the triglyceride (fat) molecule, called the glycerol backbone, can be used as glucose.  This very small addition to the glucose pool can’t be the source of your high blood glucose readings.  So if fat doesn’t directly raise blood glucose, what is it doing?

For many years scientists thought that fat was a metabolically inert substance.  Fat on the body was considered dead weight, just extra blubber people carted around. Well it turns out that fat has been masquerading as the quiet shy guy in the back row, all the while packing a considerable metabolic punch.

A high fat meal can increase the amount of free fatty acids (FFAs) in the blood.   Both repeatedly elevated levels of FFAs as found in chronic intake of high fat (especially high saturated fat) meals and obesity are associated with both skeletal muscle and liver insulin resistance.

That resistance means that it will take more insulin—either made by your pancreas or from an injection—to move the glucose in the blood stream into the cells.  There is also evidence that FFAs may have a direct role in reducing the amount of insulin secreted by the beta cells in the pancreas, although an exam mechanism for this role is unknown.

Second, fat also changes the timing of the rise in blood glucose after a meal.

Unlike carbohydrate, which is digested fairly quickly, fat takes a long time to move through the gastrointestinal tract. It can take 4 to 6 hours and sometimes even longer to be fully metabolized. This can be a problem for someone taking insulin.

Fast acting insulins such as Novolog, Humalog or Apridra are active in the body for 3 to 4 hours. When you eat a high fat meal, the insulin may start working before a significant amount of glucose reaches the blood.  The insulin often is finished working before the rest of the glucose makes an appearance.

That is why blood glucose numbers can look in range 2 hours post eating a high fat meal and look significantly above goal 5 hours later.

The Bottom Line:

  • An occasional fatty meal is fine, but eating large amounts, for example, a meal containing 40 or more grams of fat, especially if the fat is saturated (found in animal meats, etc.), can make it harder to control blood glucose levels.
  • You may have to alter the amount and timing of your insulin if you eat high fat meals.
  • For those people taking oral medications, doing some type of physical activity—for example, walking—after a high fat meal can help lower blood glucose.

For more information about diabetes and nutrition, check out our library of articles on the Joslin Diabetes Center web site (www.joslin.org).

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30 Responses to Why Does Fat Increase Blood Glucose?

  1. Dorothy Farrell says:

    What about the oil in salad dressing? Too many carbs? I am new to this carb counting and wanted to switch from 1000 island dressing to oil and vinegar. Any suggestions, please?

    • joslin reply says:

      Dear Ms. Farrell:
      Oil and vinegar is a good choice. As long as you are using reasonable amounts of salad dressing, a serving is 2 tablespoons, the amount of fat is not a problem. It is only very high fat meals that cause delays in gastric emptying and insulin resistance.

  2. Cristina (Spain) says:

    Hi,
    With regard to the second reason, I have never noticed that in my son. What I ‘ve noticed is that is the meal is really fatty and includes proteins as well, which doesn’t happen too often, the glucose level after the meal is low. I usually give him more carbs and then hyperglycemia happens. I think now I understand the reason:
    As not all the glucose from the meal has been absorbed by the small intestine by the time the insuline has finished working, hypoglycemia happens because we set a certain amount of Humalog for a higher number of carbs. Then after a while, the extra carbs I give him are added to the previous glucose from the food and as there are not more insuline in the blood, hyperglycemia happens.

    If I’m right (not sure), what can I do to avoid that?
    He injects insuline (humalog) with a pen, so timing is not so easy to change as is if he used an insuline pump and neither is the amount of insuline because it affects the hypo/hyperglycemia post meal. The only thing I can think about is avoid such unhealthy meals that luckily only occur in ceremonies and parties. But if you have any other suggestions, I’ de be grateful for them.

  3. Emm says:

    I was accidentally led to this link from a Paleo website (I eat a Paleo diet). WHY ON EARTH if you have diabetes would you EVER EAT PASTA? Seriously? Bread, pasta, grains, any starches are just KILLING YOU. Please go on over to the Paleo sites and see how it can cure diabetes. I can’t believe an organization or blog that claims to support diabetics would ever say, “it’s okay, honey, eat your carbs and sugar and just take your insulin.” That’s not doing anyone any favors. There is a cure, there is a solution. Search for ‘paleo’.

    • joslin reply says:

      Dear Emm,
      Whole grains and many other starches such as legumes when eaten in appropriate amounts are part of a healthful diet and can be included in the diet of those with diabetes.

      • Doris J Dickson says:

        Come on guys … you know darn well pasta is a big pain in the butt and is virtually impossible to safely dose for. There is NO insulin that matches up well to pasta. Fat or no fat, pasta takes a good five hours to completely digest and usually results in initial lows followed by highs later.. If you want a normalized blood sugar (85), pasta is not defined as healthful! It’s “easier” to eat a bowl of icecream!

    • Douglas D says:

      Emm- Those of us with type 1 diabetes cannot be cured. I have tried the Paleo diet for over a year and I still require insulin. Please do your research before you make comments like this. Thank you.

    • Doris J Dickson says:

      You lost your credibility when you used the word “cure” relative to type 1 diabetes!. Paleo is low carb and thus makes type 1 much easier to manage but it’s hardly a cure of an autoimmune-based disease.

    • Catherine says:

      It’s truly kind that you care & are sincere in your post; however, there is no cure for diabetes & posting such a statement is false – & inadvertently cruel. You should understand the context & subject matter before making such a post. .Ignorant statements can give newly diagnosed & those unfamiliar with diabetes false hope.

      • Dreemuk says:

        There is a cure for type II diabetes. The Paleo diet or raw food vegan diet has been known to completely cure Type II diabetes. I understand Type I cannot be cured. I think the poster giving advice on Paleo was referring to Type II i.e. the adult onset type.

        • Anna says:

          There is no cure for Type 1 diabetes. And there is no cure for Type 2 diabetes. That’s all.

          • Jojo says:

            There might not be a “cure” for Type II in the sense that you can eat a “SAD” diet and your body will respond the same as a non diabetic person, but a lot of people have basically managed all of their Type II symptoms by simply changing their diet, namely by putting an end to things like unnecessary grains in their diet. I notice people have more problem with the term “cure” than with the poster’s pointing out the problem of telling diabetics that things like pasta meals are OK.

  4. Olive oil (Extra virgin cold pressed) is the healthiest oil around and consuming most of the fat in your diet in the form of mono-unsaturated fatty acids (MUFAs) can improve insultin sensitivity, unlike saturated fat which increases insulin resistance. However, do mix the oil with vinegar and keep the total amount of fat in a meal and per day controlled, like Maryke also said. Also get familiar with the GI (glycemic index). Low GI carbs are digested much slower than high GI carbs and therefore low GI carbs require less insulin, which is also better for weight loss, blood pressure, etc.

  5. joslin reply says:

    Dear Cristina:
    Splitting the insulin into two injections: one before the meal and one after the meal for this type of situation is used for adults. You may want to discuss this with your pediatriician or endocrinologist.

    • Doris J Dickson says:

      Some of us having been using that technique for years. It’s still a russian roulette game. You can also use Regular insulin in addition to the rapid acting but pasta is still a big pain in the unrepeatable butt!

      Consider mentioning the alternative many type 1s are using – spaghetti squash.

  6. Cristina (Spain) says:

    Thank you very much. I’ll ask her for sure. I think it’s really important to try new things when the ones you know don’t work. I find this blog very useful with very accurate and detailed information (something I’m always looking for) and it’s very kind of you to answer all our questions. After some months reading your blog, I understand why my pediatrician who is specialized in child diabetes is always encouraging my son to go to a summer camp at Joslin!

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  9. Jane says:

    A real problem for me after occasionally eating out in the evening. Fine after the meal and then in the morning my blood sugars are high. If I injected humalog before bed, I would be worried I might go low at night. I don’t know how to correct for this. It makes me dread going out to eat and results in me avoiding certain events.

    • joslin reply says:

      Dear Jane,
      The issue you describe is often associated with high fat meals. Some adults handle this by splitting the dose of insulin they take in two if using injections (some insulin before the meal and the rest anywhere from an hour to three hours after the meal) or using a form of extended bolus if they are on a pump. If you are worried about low blood glucose reactions during the night setting a higher overnight goal glucose level or having a looser correction factor may be appropriate. I would suggest discussing this with your health care provider.

  10. Barbara says:

    My child has T1. We try very hard to manage food intake and food quality. The input from nutritionists has been helpful in theory but not in practice. We read food labels like bibles but even this makes BG mgmt seem nearly impossible. There are 4-5 food groups (glycemic indices aside for the moment): Fruit/Veg = Carb; Grains = Carb; Dairy = protein, fat, carb; Meat = protein, fat; Nuts = fat. For many reasons it is clear and obvious that whole grains, low index, and low fat, (and low saturated fat) diets are the best way to go. And yet, this sort of diet is ever more expensive, hard to access if one does not have transportation, and entirely impossible in all school systems.
    My child is actually thin and tries hard to gain weight. Carbs are kept low because chasing BGs gets so very tiresome, and attempts at increase protein consumption are more complicated than counting carbs. So far not much has worked, for BG control or weight gain.
    It is great to see this post with at least some level of biochemistry in the message so that greater understanding is possible but this knowledge does not easily transfer into day to day life on a small budget.
    QUESTION: how to gain weight while controlling BGs and eating health foods?
    Anyone ???

    • Mike Mango says:

      The mixture of carb counting and humolog injections is the best formula to eat what you want and get to normal blood sugars. If he is evaluated to inject one unit of humolog for 10 grams of carbs and he has a blood sugar of 110 then he would need 8 units of humolog to offset a 80 gram bowl of pasta. The other part of this table is the correction scale which would be if his blood sugar was high and he wanted the same bowl of pasta. I inject 1 humolog for every 50 blood sugar reading starting at 50 so if I was 165 I would be adding 3 units to this inject on top of what I was going to eat. This formula gives you the freedom to do what you want and eat what you want as long as you offset it with an insulin injection of humolog.

      Mike Mango
      Cranston, RI

  11. Mike Mango says:

    After 40 years of dealing with this I have accidently fallen onto the fix that has me getting to normal blood sugars. When my doctor changed me from NPH to Levomir 18 months ago after I have been using it since 1970 I sat down this summer where I pour at least a 1/2 cup of extra virgin oil over my salad thinking. Can I get the long term benefits of Levomir mixed with the spike of NPH to work together to off set this “oil” problem with blood sugar in the morning? I experimented with a 70%(normal long range insulin commitment) dose of evening Levomir and a 50%(of my normal long range insulin commitment) dose of NPH. The Humolog was measured as it would for any meal. NPH seamed to act in this case as a “long range” Humolog and when the NPH fizzled out as I was sleeping the Levomir held it’s ground and continued to support long range control. It is my own experiment that works perfect for me and I am not sure the medical community would approve on the mixture of two long range insulins but this is my way of dealing with this to attain 90 blood sugar readings in the morning when overdoing the extra virgin oil exploitation amounts.
    Mike Mango
    Cranston, RI

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  14. Nicolas says:

    Hello,
    At the end of your article, you say that saturated fats may be worse than other types. Do you have more precise datas on that? What are the effects of medium-chain saturated fats (coconut), mono or poly-unsaturated fats?
    Thanks for this article.

    • Nora Saul, Nutrition Manager says:

      Dear Nicolas:
      There are a few articles on the subject that you may find interesting. One in Diabetologia 2001 Mar 44(3) :312-9 and the second from the British Journal of Nutrition 2008 Sep: 100(3)471-9. The first looked at insulin sensitivity and found that saturated fat impaired insulin sensitivity but the monounsaturated diet produced no change. The second article reviewed a series of studies and found overall no difference among various types of fats but indicated that the studies except one were significantly flawed in their designs. However, one well-designed study came to the same conclusion that saturated fat decreased insulin sensitivity while a monounsaturated fat diet neutral.

  15. Sharon Peterson says:

    The fettucine, garlic bread and tiramisu were the culprits here. The fat merely slowed down the spike so it did not show up until the morning. The sugar in the tiramisu also creates irregular spikes. Exogenous insulin cannot match the timing of a healthy pancreas. When we try to force it to do so, we end up riding a glucose roller-coaster.

    The message is to cut the carbs.

  16. Dave says:

    Time to stop propagating this antiquated stuff. You guys are setting yourself up for a class action lawsuit .

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