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Does Soy Cause Alzheimer's?

THERE IS ONE MORE ACCUSATION against soy that deserves attention. You may have seen headlines proclaiming that “tofu shrinks brains,” or heard that soy consumption causes Alzheimer's disease. These sensational allegations began appearing after a study was published in April 2000 in The Journal of the American College of Nutrition.

The study, conducted in Hawaii by Dr. Lon White and his associates, was part of the Honolulu Heart Study. Looking at the diets and the risk of dementia in Japanese men residing in Hawaii, the study found that men who ate the most tofu during their mid-forties to mid-sixties were more likely to develop dementia and Alzheimer's as they grew older.

The correlation between tofu and cognitive decline was strong, and could not be explained by confounding factors like age, education, and obesity. In this study, men who had eaten two or more servings of tofu per week in midlife were 2.4 times more likely than men who rarely or never ate tofu to become senile or forgetful by old age. Even the wives of men who ate tofu showed more signs of dementia.

This was startling and completely unexpected. Soy has been shown repeatedly to lower cholesterol levels, and high cholesterol levels have long been intimately linked to increased risk for Alzheimer's.

And for soy eaters, it was frightening. If that's all you know, it looks pretty bad for soy.

But that's not all we know. We also know, for example, that dementia rates are lower in Asian countries, where soy intake is high, than in Western countries. We know that the Japanese lifestyle, with its high soy intake, has long been associated with longer life span and better cognition in old age. And we know that Seventh Day Adventists, many of whom consume soy foods their whole lives, have less dementia in old age than the general population.

If tofu consumption increases Alzheimer's incidence, then there would be more Alzheimer's in Japan than in Hawaii, because more tofu is eaten in Japan. But, in fact, the reverse is true.

What, then, could have been the cause of these mysterious findings?

People with Alzheimer's disease characteristically have higher levels of aluminum in their brains. Many studies have shown a link between increased levels of aluminum consumption and the risk of Alzheimer's disease. Higher levels of aluminum in drinking water, for example, typically produce higher rates of the disease. When a physician practicing in Hawaii, Bill Harris, subsequently had soy products made in Hawaii and those from the mainland tested and compared for their aluminum levels, the levels of aluminum in the Hawaii products were found to be significantly higher. Could it be that the aluminum used in the refining of some soy products in Hawaii is the actual culprit? To my knowledge, no other study has found a link between soy consumption and Alzheimer's, but many studies have supported the link between aluminum and the disease.

Moreover, the Honolulu Heart Study has some very real limitations. There are many lifestyle factors for which it did not control. The researchers who conducted the study were the first to acknowledge that tofu consumption may be a marker for some other factor that negatively affects cognitive function. This would make tofu an innocent bystander. Results of many other studies suggest that this is the case.

A number of clinical studies have shown that soy (and isoflavones from soy) is actually beneficial for cognition. In one study, published in the journal Psychopharmacology in 2001, young adult men and women who ate a high-soy diet experienced substantial improvements in short-term and long-term memory and in mental flexibility. Other studies have found that isoflavone supplements from soy improve cognitive function in postmenopausal women.

While the Honolulu Heart Study is concerning, it appears to be an aberration. More than ten years have passed since the Honolulu Heart Study was published, and there have been no further long-term studies showing any relationship between soy and dementia in humans. There have, however, been a number of clinical trials that have found soy to improve memory and other forms of cognitive function.

Meanwhile, an ever-growing number of studies are pointing to the practical and proven steps you can take to lower your risk of Alzheimer's and help assure that you will retain the ability to think clearly throughout the length of your days.

What are these steps? Let's find out.

EXERCISE: EVEN MORE IMPORTANT
THAN YOU MAY THINK

You may be surprised to learn that many studies have found that regular physical exercise plays an essential role in preventing Alzheimer's disease. For example, a five-year study published in Archives of Neurology in March 2001 found that people with the highest activity levels were only half as likely as inactive people to develop Alzheimer's disease, and were also substantially less likely to suffer any other form of dementia or mental impairment. Even those who engaged in light or moderate exercise had significant reductions in their risk for Alzheimer's and other forms of mental decline. The study concluded that the more people exercise, the healthier their brains remain as they grow older.

Three years later, in September 2004, The Journal of the American Medical Association published a series of studies further confirming that regular exercise helps preserve clear thinking even at advanced ages. One study found that women aged seventy and older who had higher levels of physical activity scored better on cognitive performance tests and showed less cognitive decline than women who were less active. Even walking for only two hours a week at an easy pace made a marked difference, although the most marked benefit was found in women who walked six hours a week. Another study found that older men who walked two miles a day had only half the rate of dementia found among men who walked less than a quarter-mile a day.

Two years later, a study published in The Annals of Internal Medicine found that older adults who exercise three or more times a week have a 30 to 40 percent lower risk of developing dementia than their more sedentary counterparts.

UNEXPECTED BENEFITS OF A
PLANT-STRONG DIET

Meanwhile, a plethora of studies are telling us that there is something even more important than exercise in preventing Alzheimer's disease. And that—drum roll please—is a healthy plant-strong diet.

Why? There are many reasons. One is that plant-strong diets are high in antioxidants. Antioxidants are substances that keep you young and healthy by increasing immune function, decreasing the risk of infection and cancer, and, most important, protecting against free-radical damage. Free radicals are cellular desperadoes that play a pivotal role in the aging process. Their damage takes a toll on virtually every organ and system in the aging human body. This, in turn, sets the stage for all sorts of degenerative diseases, including Alzheimer's. Antioxidants help to prevent this damage by neutralizing free radicals.

Antioxidants are found in fresh vegetables, whole grains, fresh fruits, and legumes like soy. If your diet is high in antioxidants, your risk of many age-associated diseases—including dementia, cancer, heart disease, macular degeneration, and cataracts—decreases.

Another reason plant-strong diets help prevent Alzheimer's is that they help you to stay slim. How important is that? In 2004, Dr. Miia Kivipelto of the Karolinska Institute in Sweden told an international conference on Alzheimer's disease in Philadelphia of his twenty-one-year study, which found that people who were obese in middle age were twice as likely to develop dementia when they got old as those who were of normal weight. For those who also had high cholesterol and high blood pressure in middle age, the risk of dementia was six times higher.

Plant-strong diets also help you stay mentally clear as you grow older by keeping your homocysteine levels low. Homocysteine is a toxic amino acid, a breakdown product of protein metabolism that has been strongly linked to Alzheimer's and also to heart attacks, strokes, depression, and a type of blindness. Even small elevations in homocysteine can significantly increase the risk for these conditions.

Blood levels of homocysteine are typically higher in people whose diets are high in meat and low in leafy vegetables, whole grains, legumes, and fruits. Plant-strong diets provide folic acid and other B vitamins that help the body get rid of homocysteine.

One study found that the incidence of Alzheimer's was a staggering 3.3 times greater among people whose blood folic acid levels were in the lowest one-third range, and 4.3 times greater for those with the lowest levels of vitamin B12.

In 2001, the journal Neurology published the results of a three-year Swedish study of 370 healthy elderly adults. The study found that those with even slightly lower levels of vitamin B12 and folic acid had twice the risk of developing Alzheimer's disease compared with those with normal levels.

Folic acid and the B vitamins, remember, are crucial because they help to maintain low homocysteine levels. On October 18, 1998, Dr. David Smith and his colleagues from Oxford University presented their findings to the American Medical Association's annual Science Reporters’ Conference. Their study, published in Archives of Neurology the following month, found that the risk of getting Alzheimer's disease was 4.5 times greater when blood homocysteine levels were in the highest one-third.

A study of 3,000 Chicago residents aged sixty-five and older published in The Journal of Neurology, Neurosurgery and Psychiatry in 2004 found that those with the lowest intake of dietary niacin (vitamin B3) were 70 percent more likely to develop Alzheimer's than those with a higher intake, and their rate of cognitive decline was twice as fast. The take-home message is simple: Eat greens. The best dietary sources of niacin are dark-green leafy vegetables.

Vegans in particular need to understand that adequate levels of vitamin B12 are necessary for folic acid to carry out its functions effectively. The most common sources of B12 for vegans include supplements (a pleasant-tasting supplement typically costs a few pennies per week), and fortified products including soy milk and nutritional yeast.

But it is meat-eaters who are most at risk for high homocysteine levels, because animal foods (and meat in particular) tend to contribute to the production of homocysteine. One study found that subjects who ate meat as their main source of protein were nearly three times as likely to develop dementia as their vegetarian counterparts. A survey of the medical literature on diet and Alzheimer's noted how frequently a meat-centered diet raises homocysteine levels. The report was aptly entitled “Losing Your Mind for the Sake of a Burger.”

In our society, we often take for granted that aging will bring restricted short-term memory and diminished mental faculties. A visit to most nursing homes demonstrates how commonly and how markedly people in our society experience cognitive decline as they age.

But there is good science to show that we can experience clear thinking well into our later years. It turns out that it is not soy consumption, but the standard American high-saturated-fat diet—low in vegetables and fruits and whole grains, and thus low in brain-preserving antioxidants—that is primarily responsible for the unhealthy outcomes we so often see in our elderly.

If you eat consciously, exercise regularly, and seek to enjoy every precious moment of your life, you won't be another one of the people who end up lamenting: “If I had known I was going to live this long, I would have taken better care of myself.”