Nutrition Myths
Do potatoes raise blood glucose level more than sugar?

SUMMARY

  • When comparing standard servings, cooked potatoes raise blood glucose levels more than table sugar.
  • However, comparing gram for gram, sugar raises blood glucose more than potatoes.
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Do potatoes raise blood glucose level more than sugar?

The impact of glucose and fructose on blood sugar

There are two ways to measure how carbohydrates raise blood glucose:

  • Glycemic Index (GI): measures how blood glucose rises after ingesting carbohydrate-containing foods (usually containing 50g of carbohydrates);
  • Glycemic Load (GL): is a more precise and realistic measurement since it also considers the common serving size.

The glucose component of dietary carbohydrate has the greatest effect on the blood glucose and is used as the benchmark for testing other foods. GI set to 100 represents the standard for pure glucose.

Fructose, on the other hand, has very little impact on blood glucose increase.

The following table shows the comparison of the Glycemic Index and Glycemic Load for both glucose and fructose.

CarbsGlycemic IndexServe (g)Glycemic Load
Glucose (1)1001010
Fructose (2)23102

What carbohydrates are in potatoes and table sugar?

Table sugar consists 100% of a disaccharide called sucrose, which is half glucose and half fructose. The breakdown process of sucrose into glucose and fructose and the subsequent absorption of these monosaccharides into the bloodstream is very rapid.

Potatoes, on the other hand, consist mainly of starch which is glucose molecules joined together in chains of hundreds or thousands (hence they don’t taste sweet but don’t let it fool you – if we had taste buds in the intestine we would taste sugar after eating potato).

Besides starch and a very small amount of free glucose, there is also a negligent amount of free fructose in potatoes (see table below).

Starch from cooked potatoes is very rapidly digested and then broken down to disaccharides and subsequently to glucose molecules which is very quickly absorbed into the bloodstream (just like glucose and fructose from table sugar).

The following table shows the glucose vs fructose composition of boiled potatoes and table sugar:

FoodGlucoseFructose
Boiled, mashed potatoes (1)98.10% (94.78% from starch and 3.32% from free glucose)1.90%
Table sugar (sucrose) (2)50%50%

Higher glucose contents of potatoes explains why the spike in blood sugar is quicker after eating potatoes, as is shown by the GI test in the table below.

FoodGlycemic IndexServe (g)Glycemic Load
Boiled, mashed potato (1)83150 (medium - large potato)17
Table sugar (sucrose) (2)6010 (2.5 teaspoons)6

Glycemic Load result shows a more realistic impact on blood sugar since it takes into consideration serving size of each food.  

Therefore, considering the normal serving size of each product, 150g of potatoes have a GL of 17, while 2.5 teaspoons of sugar only have a GL of 6.

Please note that, if we used the same number of grams and compare 150g of boiled potatoes to 150g of table sugar, the GL of table sugar would be 90 (6*15).

Potatoes vs sugar – blood sugar level is not everything!

The fact that a standard serving of potatoes raises blood glucose levels more than a serving of table sugar doesn’t mean that sugar is healthier.

Sugar contains 50% of fructose, which in concentrated amounts (usually in the form of sweeteners), rather than as part of a whole fruit, acts as a toxin and leads to all sorts of damage in the body. (read more..)

While consuming sugar is, in general, harmful, eating a medium potato as part of a balanced meal containing some fiber, good amounts of vitamin C, vitamin B6 and copper, and some amounts of other micro-nutrients (read more..) is a better choice.

The impact on blood sugar depends on serving size

A can of coke contains 39g of sugar (10 teaspoons). The GL of this amount is 24.

A small potato, weighing 50g (for instance as a part of a meal full of vegetables and protein), has a GL of 6, which is within the healthy range and certainly much less than a can of coke.

Therefore, although potatoes have a higher GL than sugar when using the standard serving size, we need to be aware of the individual serving size.

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