Resistant Starch Glycemic Impact Study (n=10)

Does resistant starch impact blood glucose? This question is often asked, but no studies are available…so I tested it on some volunteers.

The Study

Purpose: This study was designed to test the glycemic impact of several common resistant starch supplements as compared to a high-glycemic food such as white bread (GI=75) or freshly cooked potato (GI=78). “Starch” is often thought of as a high-glycemic food ingredient. Diabetics and those on low-carb diets are instructed to avoid starchy foods.

Study subjects: 10 non-diabetic adults (n=10).

Study design: Each person was provided with a home blood glucose test kit (OWell Bayer Contour NEXT EZ), potato starch (Bob’s Red Mill), green banana flour (Zuvii), and Hi-Maize (Lifesource).

Each person was instructed to check their fasting blood glucose for one week to ensure there were no diabetics among the participants. The experiment was then divided into four phases:

  1. Post-prandial blood glucose monitoring after a meal of white bread (two slices) or potato (one medium), each meal providing about 35g of carbohydrate.
  2. Post-prandial blood glucose monitoring after a serving of potato starch (2TBS).
  3. Post-prandial blood glucose monitoring after a serving of green banana flour (2TBS).
  4. Post-prandial blood glucose monitoring after a serving of Hi-Maize (2TBS).

The glucose monitoring tests were done at least two days apart to rule out a subsequent-meal effect of resistant starch. The foods were eaten alone, on an empty stomach.

The quantities used represent a normal serving of each food or supplement, not dose-matched for carbohydrate content.

Conclusion: Resistant starch does not raise blood glucose of non-diabetics significantly when compared to a high-GI food. Further studies should be made on diabetics.

Discussion

Resistant starch is found in several commercial supplements (ie. Gut Garden) and also common cooking starches such as potato starch, green banana flour, and Hi-Maize. Resistant starch has been shown in numerous studies to lead to a long-term reduction in blood glucose and can be helpful in preventing Type 2 Diabetes. Unfortunately, the word “starch” strikes fear in the minds of dieters who are avoiding carbohydrates.

Resistant starch (RS) differs from readily digested starch (such as Argo corn starch) in that RS is not well-digested in the small intestine, instead being delivered to the large intestine where it serves as prebiotic fiber for gut bacteria. Other types of starch are digested by enzymes in the small intestine and converted to glucose which “spikes” blood glucose considerably.

The Glycemic Index was devised to show which foods cause increases in blood glucose. Diabetics and people on special diets are instructed to limit the intake of high-glycemic foods such as white bread and potatoes. Over the 5+ years I have been studying RS, the question of the glycemic impact of RS often arises. To my knowledge, there are no peer reviewed, published studies showing the glycemic impact of RS on non-diabetic volunteers. Perhaps this private experiment will prompt researchers to test this on diabetics to determine the safety of RS supplementation for the glucose-impaired.

Figures 1-10, Blood Glucose Curves

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Glycemic Index

The Glycemic Index was developed to examine the length of time a food raises blood glucose levels. This is known as the area under the curve (AUC).

The glycemic index of a food is defined as the incremental area under the two-hour blood glucose response curve (AUC) following a 12-hour fast and ingestion of a food with a certain quantity of available carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC of the standard (either glucose or white bread, giving two different definitions) and multiplied by 100. The average GI value is calculated from data collected in 10 human subjects. Both the standard and test food must contain an equal amount of available carbohydrate. The result gives a relative ranking for each tested food.

It’s clear to see from the glucose charts (figures 1-10) that resistant starch does not cause a glucose increase comparable to white bread or potatoes. However, in this experiment we did not dose-match the serving sizes as is done when testing the glycemic impact of foods.

A normal “serving” of supplemental RS is 1-2TBS taken 1-2 times per day for a daily dosage of 10-40g. I believe if we would have tested each RS at a dose of 50g (5TBS) the results would have been similar. But this is an inherent problem with the Glycemic Index as described:

A disadvantage with this system is that the reference food is not well-defined, because there is no universal standard for the carbohydrate content of white bread.

The following figures are from two test subjects who agreed to ingest 50g of Hi-Maize. [Note: This is a very high dosage, very likely to cause gastric upset in people who are not accustomed to such a large amount]:

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Blood Glucose

Blood glucose is in constant flux. The figures showing blood glucose response after ingesting RS are very similar to blood glucose over the same time as if nothing had been eaten. Here is a chart showing the blood glucose of one participant while fasting for two hours:

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Variations in blood glucose throughout the day are normal. Some factors that can cause blood glucose changes: stress, exercise, sleeping, anticipation of eating, or eating non-caloric foods (ie. sugar-free gum). Many studies have been conducted on “average” blood glucose responses. When monitored continuously, a chart like this will appear (wikipedia):

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Conclusion

Two tablespoons of resistant starch does not impact blood glucose significantly. This should put those concerned with blood glucose at ease when deciding if they should supplement with RS.

Every good study ends with some questions for other researchers to explore. Here are mine:

  • Would these results be reproducible in a diabetic cohort?
  • What effect does RS supplementation have on insulin levels?

I hope that an established researcher will duplicate this study and publish the results in a peer-reviewed journal for all to see. Until then, rest assured that supplementing with RS is not causing undue blood sugar spikes (if you are concerned with such things).

And a big “Thanks!” to all the folks who volunteered to do this for me. I hope your fingers have healed from all the pokes.

Tim Steele

8 Comments on “Resistant Starch Glycemic Impact Study (n=10)”

  1. Gemma May 11, 2017 at 10:02 pm #

    Very interesting.

    “Study subjects: 10 non-diabetic adults (n=10).”

    What do they normally eat, though?

    Like

    • Tim Steele May 12, 2017 at 8:43 am #

      I only requested that none of the volunteers were eating a VLC/keto type diet longterm. I think you’d find that most of them eat a very sane diet. My fear with low carb diets was more their reaction to the bread/potato meal, and if you look at the chart of “M,34” you’ll see signs of insulin resistance. This man has been dieting for some time and eats relatively low carbs. He’s a far cry from being diabetic, though.

      Like

  2. Steve May 13, 2017 at 5:54 pm #

    Hi Tim,
    I’m living in rural Thailand and have done the potato hack several times always with good results. Unfortunately, I’m finding it impossible to buy potato starch locally (even in Bangkok). But tapioca starch is abundant and cheap. How “bad” is tapioca starch compared with potato starch?

    Like

  3. sprague May 16, 2017 at 4:59 pm #

    Tim, are you able to publish the raw data from your study? It would be great if you put all the (anonymized) info on Github so other people can see your statistical calculations (and perhaps suggest ways to improve it).

    Like

    • Tim Steele May 16, 2017 at 5:33 pm #

      The charts all represent the raw data, no calculations. I’ve not used Github, any particular place to post such data?

      My goal was to see if a normal serving of RS caused a spike similar to a normal serving of bread or potatoes. I think it’s clear from the charts that it does not. If I had some captive test subjects, it would be fun to try larger amounts and see where it does cause a spike, and also test insulin and some other markers. I did this experiment on a lark since I had some funds leftover from the RS analysis project and have been hearing people complain that “RS raises their blood glucose.” N=10 seems to add legitimacy to experiments, the Glycemic Index was developed using groups of 10 to make their predictions.

      Thanks for reading!

      Like

  4. Jo tB May 25, 2017 at 11:34 am #

    Apparently you have a hacker on your patch!!

    https://www.facebook.com/groups/538939886178286/

    Like

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