Thursday, 5 September 2013

Boiled potatoes & Area Under the Curve (AUC): some thoughts.

Here are three "curves"... a 4 x 1 rectangle, a 2 x 2 square and a 1 x 4 rectangle.
The AUC for all three "curves" = 4.
Imagine that the three curves are for blood glucose level increase above baseline vs time.

a) "X" grams of a high-Glycaemic Index (GI) carb e.g. glucose, maltodextrin or amylopectin will result in a large glucose response that goes away rapidly, as the carbs leave the gut rapidly, pass into the blood rapidly and are cleared from the blood rapidly due to the large insulin response.

b) "X" grams of a 50:50 mixture of high & low-GI carbs will result in a lower but longer sustained glucose & insulin response, as some carbs leave the gut rapidly but some carbs leave the gut slowly, pass into the blood slowly and are cleared from the blood slowly due to the small insulin response.

c) "X" grams of a low-GI carb e.g. amylose or resistant starch will result in an even lower glucose & insulin response that is sustained for even longer, as the carbs leave the gut very slowly, pass into the blood very slowly and are cleared from the blood very slowly due to the very small insulin response.

Will a), b) & c) produce the same satiety? I think not. I think that a) results in lower satiation than b) and b) results in lower satiation than c). Whether returning hunger is caused by a sudden drop in blood glucose level or by a sudden drop in the amount of food in the gut, I don't know.

The reason for this post is A satiety index of common foods (scanned image of full study here) and the related study An insulin index of foods: the insulin demand generated by 1000-kJ portions of common foods.

In the first study, boiled potatoes produced the highest satiety, yet in the second study, boiled potatoes produced one of the highest glucose & insulin AUCs. How can this be? Consider the preparation method for the Russet potatoes:-
"Peeled, boiled for 20 min, and stored at 4 °C overnight; reheated in a microwave oven for 2 min immediately before serving."

Potato starch when refrigerated, produces resistant starch RS3, which gives it a low GI (see item 605 in International table of glycemic index and glycemic load values: 2002). Therefore, refrigerated potatoes contain a mixture of high & low-GI starches. This, I believe, is why boiled, refrigerated & reheated potatoes produced the highest satiety. The combination of water, fibre & resistant starch kept hunger pangs away the longest. I suspect that boiled potatoes that are eaten without being refrigerated won't produce quite as much satiation, as they contain no resistant starch.

EDIT: From https://en.wikipedia.org/wiki/Resistant_starch#Definition_and_categorization :-
"RS3 Resistant starch that is formed when starch-containing foods are cooked and cooled, such as pasta. Occurs due to retrogradation, which refers to the collective processes of dissolved starch becoming less soluble after being heated and dissolved in water and then cooled."
RS3 forms a gel in the stomach, which delays stomach emptying. This is most likely the reason for the increased satiation.

28 comments:

ProudDaddy said...

I seem to remember reading that reheating restores resistant starch to normal? (I'd research it myself, but the baby is screaming and Needs me.)

Nigel Kinbrum said...

The potatoes in item 605 were refrigerated & reheated, but the GI was 23 (rel. to glucose), compared to the other Indian potatoes which weren't refrigerated & reheated and had a GI of 76.

I can believe that recooking RS turns it into high-GI starch.

Sanjeev Sharma said...

Is there good reason to believe that resistant starch is actually digested?


If many of the calories are not digested then GI comparisons are invalid


Also, uncooked potato starch exists in an undigestable crystal structure, is resistant starch anything special? Just eat the potato raw, if you can stand the taste

Nigel Kinbrum said...

I don't know if the starch produced by refrigerating cooked potatoes has the same structure as uncooked potato starch.

I thought that refrigeration made the short starch molecules link together to form long starch molecules (which are slow to be enzymatically "snipped" from the ends inwards). I could be completely wrong.

I tried eating raw potato once. I hated it.

Sanjeev Sharma said...

I took the taste reduction as reduction of the non-starch molecules that elicit the raw taste - somewhat like onion & garlic - the starch becomes more bioavailable but the more sentitive chemical entities degrade non-reversibly

Nigel Kinbrum said...

I don't know how accurate the information is, but according to http://en.wikipedia.org/wiki/Resistant_starch , raw potato contains RS2 whereas cooled cooked potato contains RS3.

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H Ford said...

According to Montignac

http://www.montignac.com/en/the-factors-that-modify-glycemic-indexes/

'It is handy to know that retrograded starches lose some of their gelatinization potential. Approximately a 10% portion of the retrograded starch becomes thermo-resistant, which indicates that reheating carbs after cold storage contributes to lowering their GI'

Nigel Kinbrum said...

Thanks for that.

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That's good news

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Nancy Jorden said...

This is great. Thank you so much for your nutrition.

Suzanne said...

Great post Nigel, I heard that protein is the best macronutrient for satiety, is this true?

Suzanne

TedHutchinson said...

Re:(full study not available)
It looks like this is the full text of the study
Eur J Clin Nutr. 1995 Sep;49(9):675-90. A satiety index of common foods. Holt SH, Miller JC, Petocz P, Farmakalidis E.

Source Department of Biochemistry, University of Sydney, Australia.

Nigel Kinbrum said...

Thanks, Ted.
As I suspected, the potatoes are the same type and are prepared using the same method as those in the Insulin Index study.


Post edited to include your link.

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Interesting!

insaneampd said...

That's Right boiled Potatoes created the biggest satisfied, yet in the second study, boiled Potatoes created one of the biggest glucose & insulin AUCs.

Nick said...

I'm curious if it matters if the potato is cooked before it is frozen. I have in mind hash browns that are sliced raw, frozen and shipped prior to cooking. Would this produce a RS like the low GI carb in the graph?

Nigel Kinbrum said...

As the cooking process produces high-GI starches, I suspect that chilling/freezing before cooking won't make a difference to the GI after cooking.
Reheating involves heating to a lower temperature than cooking, so doesn't produce high-GI starches. Also, see H Ford's comment below.

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Sport Nutrition said...

Hash brown colours that are chopped raw, freezing and delivered prior to food preparation.

Richard J D'Souza said...

This is pretty amazing research. I wonder what the GI is of triple cooked chips.

xcelweightloss said...

The deficiency of excessive gas from un-digested carbs claims that proof starchy foods is consumed and is not available for fermentation

polypodium leucotomos Extract said...

In the first research, boiled apples created the biggest satiety, yet in the second research, boiled apples created one of the biggest glucose

Insane Amp'd said...

The mixture of water, fiber & resistant starchy foods kept the urge to eat away the lengthiest.

Nigel Kinbrum said...

That's the point of this post. AUC is just a number and doesn't convey any information as to the shape of the curve.

A higher glucose & insulin AUC following the shape of the blue "curve" produces higher satiety than a lower glucose & insulin AUC following the shape of the red "curve". See http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4204795/

Insane Amp'd said...

It is useful to know that retrograded starchy foods reduce some of their gelatinization prospective.