Do potatoes raise blood glucose level more than sugar

Do potatoes raise blood glucose level more than sugar?

Pawel Malczewski
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Short Summary

Potatoes contain mostly glucose while table sugar (sucrose) contains half glucose and half fructose. Glucose derived from diet has the highest impact on the rise of blood glucose, while fructose has only a negligible effect. Therefore, considering “standard” servings, potatoes have a higher impact on blood glucose than table sugar.

Note, however, that the extent a given carbohydrate raises our blood glucose level depends on what is considered as “standard” serving size on an individual basis. For a quick answer click here.

Explanation

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

  • Glycemic Index (GI): measures how blood glucose rises after ingesting carbohydrate-containing foods;
  • 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)The University of Sydney. Glycemic Index of Glucose. Available here.1001010
Fructose (2)The University of Sydney. Glycemic Index of Fructose. Available here.23102

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 (3)USDA. Nutritional composition of baked potato. Available here.98.10% (94.78% from starch and 3.32% from free glucose)1.90%
Table sugar (sucrose) (4)USDA. Nutritional composition of table sugar. Available here.
50%50%
Therefore, the spike in blood glucose is quicker, as is shown by the GI test in the table below.

However, the GL is a more important measurement since it shows a more realistic impact of the serving size. (read more..)

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). 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.

FoodGlycemic IndexServe (g)Glycemic Load
Boiled, mashed potato (5)The University of Sydney. Glycemic Index of potatoes. Available here.83150 (medium - large potato)17
Table sugar (sucrose) (6)The University of Sydney. Glycemic Index of sugar. Available here.6010 (2.5 teaspoons)6

What if we change the amounts?

A can of coke contains 39g of sugar (10 teaspoons). The GL of this amount is 24.
On the other hand, 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.

Conclusion

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When considering standard servings, cooked potatoes raise blood glucose levels more than table sugar. However, bear in mind that by changing the serving sizes, these effects can be altered.

This 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.

References   [ + ]

1. The University of Sydney. Glycemic Index of Glucose. Available here.
2. The University of Sydney. Glycemic Index of Fructose. Available here.
3. USDA. Nutritional composition of baked potato. Available here.
4. USDA. Nutritional composition of table sugar. Available here.
5. The University of Sydney. Glycemic Index of potatoes. Available here.
6. The University of Sydney. Glycemic Index of sugar. Available here.