Is sweet potato really a low GI food?

My sister and I see a spike in our blood sugar when we eat sweet potatos even though it is classified as low (GI=48). Other supposedly low GI foods are as follows: Pumpernickel or rye bread Oats, bran cereals, Grape-nuts Most fruits Whole Grain Pasta Rice, barley, couscous Beans, peas, lentils Most... show more My sister and I see a spike in our blood sugar when we eat sweet potatos even though it is classified as low (GI=48). Other supposedly low GI foods are as follows:

Pumpernickel or rye bread
Oats, bran cereals, Grape-nuts
Most fruits
Whole Grain Pasta
Rice, barley, couscous
Beans, peas, lentils
Most vegetables

Are there genetic differences between people?
Update: Neither my sister nor I are diabetic, however, my sister monitors her blood sugar as a preventative measure. For us, sweet potatos are like sugar. Other foods do not have that impact.
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