“Fat Affects Your Blood Sugars”

I was reading a post, “Gary Taubes and the Cause of Obesity” on a site called ScienceBasedMedicine. The author, Harriet Hall, said:

“… serum insulin levels are driven by the carbohydrate content of the diet.”

Many people think this. As you know, it is inaccurate. Dietary carbohydrate is not the sole driver of serum insulin. But it persists in people’s minds. It is the reason people go on low-carb diets … thinking they can reduce their blood glucose and insulin by eating fewer carbs. Unfortunately, that means eating more fat and…

High-fat diets have been shown to increase insulin levels, because fat (especially saturated fat) can decrease insulin sensitivity. When cells become resistant to insulin, the pancreas produces more insulin to compensate, resulting in high serum insulin levels or hyperinsulinemia.

Also, when cells becomes resistant to insulin, glucose in the blood can’t enter cells so blood glucose levels rise.

Here’s an unfortunate result … high insulin levels can feed insulin resistance, which can feed high insulin, which can feed insulin resistance … in a self-propagating manner:

Insulin Resistance and Hyperinsulinemia, Is hyperinsulinemia the cart or the horse? Diabetes Care, 2008

That quote in my title, “Fat Affects Your Blood Sugars” came from this video out of the Joslin Diabetes Center. People think it’s just the carbs that affect blood sugar; it’s the fat too:

9 thoughts on ““Fat Affects Your Blood Sugars”

  1. Melinda

    Are carbs a driver at all? (You say they’re not the sole driver.) I ask b/c, as you may remember (at least you commented on it at the time), my doc told me a few years ago, when my fasting blood sugar was around 100-102, to “eat fewer carbohydrates.” I still eat a lot of healthy carbs, and the fasting blood sugars I test at home range from the high 70s to the mid 90s. But they’re always a bit higher at the blood lab. (Grrrr….).


    1. Bix Post author

      Sure, carbs affect blood glucose. But regulation of blood glucose is complex. As you know, Barnard reduced the blood glucose of a whole group of people with diabetes by having them eat more carbs.


  2. M

    Hmmm… my comment seems to have disappeared … No need to post twice, but in case it really did get dropped:

    I’m afraid the investigators have led you to a mistaken conclusion, based on their own non-parsimonious interpretation of their bizarre, pointlessly-constructed study.


    This study tells you nothing about any specific role of fat in insulin sensitivity or requirements. In the study, “Each dinner had identical carbohydrate and protein content, but different fat content (60 vs. 10 g). … The caloric content for breakfast and lunch was calculated to compensate for the 450-kcal difference in the two dinner meals and to keep total calories consumed during the 48-h admission equal to the subject’s estimated 48-h energy requirement.” It’s not surprising that if you load up a meal with an extra 450 Calories of ANYTHING at one meal, you’re going to increase insulin requirements (and that shifting your Calories around elsewhere in the day probably perturbs overall glycemic regulation too): this doesn’t tell you anything about the effect of fat per se. If they had wanted to test that, they should have fed subjects isocaloric meals with different macronutrient ratios.

    Additionally, because these were Type I diabetics and were dosing their insulin shots based on carb intake, it’s unlikely that even the results as they stand are of much relevance to people with normal, functioning beta-cells.

    The truth is, people have been looking for a specific effect of fat on insulin requirements or sensitivity for many years now, and haven’t come up with much. There *may* be a small effect specific to *saturated* fat, but that’s about it.



    1. Bix Post author

      Hi M,

      So, you said that it was not surprising that if you eat fat you increase insulin requirements. Gary Taubes says that dietary fat does not increase insulin.

      Type 1s dose their insulin to cover their carb, not their fat. This study found they should probably be dosing for the meal’s fat content too. Should they or shouldn’t they? You say you’re not surprised that fat or “anything” increases insulin requirements, yet in the end you say fat doesn’t increase insulin requirements. Not sure what you mean.

      The study was valid. The researchers had 3 macronutrients to vary – carb, protein, and fat. They held carb and protein constant and varied fat. If there was an effect, one can assume it was from fat. If you vary carb and protein in addition to fat to achieve isocaloric meals (as you suggest), you cannot narrow the cause of the effect.

      I appreciate your feedback. I do have difficulty posting comments that disparage. What if one of these researchers was known to me? If the study was indeed bizarre and pointlessly-constructed, how could it have withstood peer review and been published?


      1. M

        No, I didn’t say that it’s not surprising that if you eat fat you increase insulin requirements. I said that it’s not surprising that if you eat a lot more Calories in a meal that you increase insulin requirements, irrespective of the source. I am not a defender of much of Gary Taubes’ conclusions, but in ordinary terms he’s right when he says that dietary fat does not increase insulin. Insulin is secreted in response to carbohydrate, and secondarily protein depending on its structure and amino acid composition. You can swallow all the butter you want if there’s nothing else in the meal, and I would assume that there would also be no increase if you added more and more fat to meals that were keeping you in a ketogenic state, because you would still not have enough dietary carb to cover basic physiological requirements.

        Again, if one wants to know whether fat increases insulin requirements or decreases insulin sensitivity, one must hold Calories constant, or have nothing but fat in the experiment.

        I acknowledge the value of this study for helping Type I diabetics dose their insulin (and so retract, in that specific context, my characterization of the study as “pointless” in that context (and I note that the investigators are at Joslin, who are more specialized in Type II diabetics). But to say that fat increases insulin requirements or decreases insulin sensitivity in the context of healthy people or Type II diabetics is just misleading.

        I didn’t intend to disparage you, FWIW — just to correct the misimpression that you’ve taken away and that is then disseminated by this blog entry,particularly the title.


        1. M

          (Oops: I meant, of course, to say that Joslin is more focused on Type I, not Type II).

          You asked, “What if one of these researchers was known to me?” Er, well … what if one of them was?

          You also asked, “If the study was indeed bizarre and pointlessly-constructed, how could it have withstood peer review and been published?” I’m sure you’re aware that many, many studies are published that are poorly-designed, uninformative, or are misinterpreted by the authors. Peer review is one important check on GIGO in the scientific process, but it’s far from an impregnable wall.


        2. Bix Post author

          M, dietary fat does decrease insulin sensitivity … even when you hold calories constant, even when subjects are healthy.


          “During the oral-glucose-tolerance test, both glucose and insulin rose to higher concentrations after the high-fat diet than after the high-carb diet, showing lower glucose tolerance and insulin sensitivity with the high-fat diet.”

          I’ll post about it if I get some time.


        3. Bix Post author

          There are several types of diabetes. People with type 1 diabetes (the nomenclature was changed in the late 1990s from use of Roman numerals to Arabic to lessen confusion) can develop insulin resistance. And people with type 2 can have some late-in-life autoimmune involvement. That’s sometimes called type 1.5 diabetes or LADA. There are other types too. Any group that calls itself “the world’s largest diabetes research organization” as Joslin does cannot avoid studying the many types of the disease owing to their distinctions and overlap.



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