Alkaline diet has no proven health benefits

DEAR DOCTOR K: I've heard that an alkaline diet can help prevent cancer. Is this true?

DEAR READER: I wish it were, but it's not. So-called alkaline diets do not fend off cancer. That's because it's nearly impossible to change your body's pH by changing what you eat.

Let me back up for a minute. Every day our bodies perform any number of intricate balancing acts. One of them is to make sure the body's fluids, tissues and cells don't get too acidic or, at the other extreme, too alkaline.

As you may remember from high school chemistry, acidity and alkalinity are measured as pH. A substance with a pH of less than 7.0 is acidic. The lower the pH, the more acidic the substance is. A substance with a pH higher than 7.0 is alkaline. The pH of water is about 7.0.

Our blood normally is slightly alkaline; the pH ranges between 7.35 and 7.45. That's no accident. The body has powerful mechanisms for keeping the pH in that range. The kidneys and the lungs are perhaps the most important organs in maintaining the normal range.

If you eat a meal that temporarily adds more acid to your blood as the meal is digested, the body responds. The kidneys remove acid from the blood and dump it into the urine. And breathing more rapidly and deeply eliminates acid from the lungs.

Just below or above the very narrow pH range of human blood (7.35 to 7.45), people start to develop bothersome symptoms.

If the pH gets below 7.0, or above 7.7, it is life-threatening because the cells of our body are built to function only within a fairly narrow range of pH.

Proponents of alkaline diets claim that when the body's pH is too acidic, your risk for many conditions, including cancer, increases. They also claim that by avoiding acidic or acid-producing foods, you can make your pH "alkaline enough" to prevent cancer.

There are only two problems with this theory. First, to repeat, changes in your diet have only a brief and minimal impact on your body's pH. Second, there's no evidence that a brief and minimal increase in pH (to make the blood slightly more alkaline) does anything to prevent cancer. Otherwise, the theory is a thing of beauty.

It is true that laboratory studies have shown that some cancer cells grow faster in an acidic environment. But it's a big leap from a test tube to the human body.

We talk frequently in this column about what's in a healthy diet because over the past 40 years nutrition science has come up with quite convincing evidence. Even more important, studies have shown how powerfully our diet (and exercise) choices can affect our health. An alkaline diet has no proven health benefits.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.