Healthnotes Newswire (July 24, 2008)—Potatoes, often thought of as just a starch, are actually an important source of potassium; provide modest amounts of calcium, magnesium, phosphorus, and sulfur; and have small amounts of zinc, boron, copper, manganese, and iron. Cooking them reduces their mineral content, however, and a new study found that many of potatoes’ nutritious minerals are lost when they are cut into small pieces and then boiled. More nutritional value can be preserved by cooking them whole.

Heat depletes

In the study, published in the Journal of Food Science, six varieties of potato were washed and peeled, then cut into small cubes or shredded into strips. The potato cubes and strips were either placed in cold water overnight or boiled for ten minutes. Soaking resulted in minimal potassium loss, but boiling the potato cubes cut their potassium content by about 50%. Boiling shredded potatoes cost even more potassium—about 75% was lost. When potatoes in either form were soaked and boiled, the results were the same as when they were only boiled.

Other minerals showed similar trends: about 50% of the shredded potatoes and about 30% of the cubed potatoes lost their original sulfur, magnesium, zinc, and manganese during boiling. The amounts of phosphorus and iron were also significantly lower after boiling. Calcium, copper, and boron content were not consistently affected.

This degree of potassium loss was more dramatic than that seen in previous studies using whole peeled potatoes. Other research has shown that zinc and iron are preserved when the potato is boiled whole with its skin intact.

Why potassium matters

Potassium is necessary for maintaining normal fluid balance, acidity, and heart rhythm, and a healthy body keeps potassium carefully balanced with sodium. Although potassium is abundant in fruits and vegetables, many people don’t get enough from their diets, especially relative to sodium, an imbalance that is linked to high blood pressure.

For people with kidney disease, too much potassium can be more of a concern than too little. Soaking potatoes in cold water before cooking them, known as leaching, is a common practice among people with kidney disease on potassium-restricted diets, but this study’s results suggest that leaching is unnecessary. Simply cutting the potatoes into small pieces and boiling them has the same potassium-depleting effect.

Most of us, however, can and should eat lots of potassium-rich foods, including tomatoes, sweet potatoes, winter squash, avocadoes, oranges, bananas, and, of course, potatoes. “Although cutting potatoes into small pieces is appealing because it reduces cooking time, it is appears that this is not the most nutritious way to prepare them,” said study co-author Dr. Shelley Jansky of the US Department of Agriculture. “People who want to get the most out of their potatoes should boil them whole and unpeeled, or consider other cooking methods like baking or roasting.”

(J Food Sci 2008;75:H80–5)