Vegan for Life (27 page)

One issue regarding isoflavones may be of particular interest to vegetarians and vegans. The way in which isoflavones are metabolized can differ significantly among individuals and this could impact their health effects. For example, one type of isoflavone is metabolized by intestinal bacteria to a compound called equol, which may be beneficial to health. But only around 25 percent of westerners have equol-producing bacteria in their intestines, compared to roughly 50 percent of Asians.
¹⁰
Interestingly, a small study found that vegetarians are more likely than meat-eaters to be equol producers.
¹¹
Therefore, vegetarians and vegans may stand to gain more from consuming soyfoods than those eating a more typical American diet.

Because soyfoods are low in saturated fat, using them in place of meat and dairy can reduce blood-cholesterol levels by as much as 3 to 6 percent.
¹²
But there is much more to the story about soy and heart health. The protein in soy has a direct effect on blood-cholesterol levels, and simply adding it to the diet has been shown to lower cholesterol levels.
¹³
Moreover, studies of people in the United Kingdom
¹⁴
and Asia
¹⁵
have linked higher soy protein consumption to lower cholesterol levels.

Most studies show that it takes as much as 25 grams of soy protein per day (the amount in about three servings of traditional soyfoods) to lower cholesterol, but lower amounts could be beneficial too. The effect is modest—about a 4-percent reduction in LDL-cholesterol—but that can be enough to reduce heart disease risk by as much as 10 percent over time.
^16
,
17

The impact of soy is much greater when it is teamed up with other heart-healthy components. The Portfolio Diet is an experimental approach that derives much of its protein from soy and includes plenty of fiber, nuts, and foods fortified with plant sterols (compounds with natural cholesterol-lowering properties). This approach lowers LDL-cholesterol by nearly 30 percent; it’s as effective for reducing cholesterol as some drug therapies.
¹⁸
Also, soy protein may impact LDL-cholesterol in ways that make it less harmful and less likely to cause clogged arteries.
¹⁹

And there is reason to believe that soy has other coronary benefits that have nothing to do with its protein or fat content or, for that matter, blood-cholesterol levels. Several Chinese and Japanese studies show that people who consume two servings of soyfoods per day are only half as likely to have heart disease compared to those who consume marginal amounts of soy.
²⁰
,
²¹
,
^22
–
23
That’s a dramatic difference—far more than could be due to the cholesterol-lowering effects of soyfoods. These additional protective effects may be related not to protein or fat but to isoflavones, which may directly improve the health of the arteries.
²⁴
Since many people who suffer heart attacks don’t have elevated cholesterol, these additional potential benefits of soyfoods could mean that they offer protection even for people with low cholesterol levels.

Since estrogen therapy reduces bone loss and fracture risk in post-menopausal women, there has been a great deal of interest in determining whether isoflavones have the same benefits.
²⁵
More than twenty-five clinical trials have looked at the effects of isoflavones on bone health, mostly in postmenopausal women.
²⁶
,
^27
–
28
Some have found that soyfoods, soy protein, and isoflavone supplements improve bonemineral density, but others haven’t shown any benefit.

It may be that soy isoflavones simply aren’t protective. Remember that isoflavones are SERMS, which means they don’t always act like estrogen, so they may not have estrogen-like effects on bones. Or possibly
the isoflavone supplements used in the studies are less effective than actual soyfoods because compounds sometimes act differently when they are isolated from a food. Here’s one other possible explanation: It could be that a protective effect requires lifelong soy consumption. The clinical studies involved postmenopausal women who typically consumed soy for no more than two years. In contrast, epidemiologic studies of 35,000 subjects in Singapore and 24,000 subjects in China found that women with the highest soyfood consumption—approximately two servings per day—were one-third less likely to fracture a bone.
^29
,
30
It is reasonable to presume that these women consumed soyfoods throughout their lives.

Since most soyfoods are rich in protein and many are good sources of calcium, they certainly promote bone health. But whether soy isoflavones add additional protection remains to be seen.

Although hot flashes are relatively common among western women as they go through menopause, women in Japan seldom report having them. One reason might be that they benefit from the estrogen-like effects of isoflavones. Nearly fifty studies have examined the effect of soyfoods and different types of isoflavone supplements on hot flash incidence and/or severity and, again, the results are inconsistent.
³¹
,
^32
–
33
In some studies, women had considerable relief from hot flashes; in others the relief was minor or nonexistent. The varied responses might be due in part to the fact that individuals metabolize isoflavones differently.
³⁴
Some women may have metabolic advantages that allow them to experience relief with isoflavones.

An alternative explanation is that some soy products may be more effective at minimizing the effects of hot flashes than others. In one comprehensive analysis of seventeen studies, the supplements that were most effective were those that had an isoflavone pattern similar to soybeans. For example, soyfoods are typically rich in an isoflavone
called genistein; supplements in which at least half of their total isoflavone content was in the form of genistein were very effective in studies. Consuming an amount of isoflavones that is equivalent to approximately two servings of soyfoods consistently reduced the frequency and severity of hot flashes by about 50 percent. For women who have as many as seven to ten hot flashes per day, a 50 percent reduction can provide significant relief.

In 1990, the National Cancer Institute began looking at soyfoods and isoflavones as a possible way to reduce the risk of cancer.
³⁵
While their interest was in all cancers, they placed a particular focus on breast cancer. Most breast tumors are stimulated by estrogen, and there was evidence early on that isoflavones could have anti-estrogenic effects on breast tissue.
^36
,
37
In addition, the historically low rates of breast cancer in Asia suggested that there was something about an Asian lifestyle that was protective.
³⁸

Twenty years later, the effect of soy consumption on cancer risk remains a question mark. Soy detractors have suggested that soy is not protective and may, in fact, be harmful for women at risk for breast cancer. However, the most recent evidence, as we’ll see below, suggests that soyfoods may actually be beneficial for breast cancer patients.

Certain types of studies in mice have raised questions about soy consumption for women who have estrogen-positive breast tumors (the kind that are stimulated by estrogen).
³⁹
But humans and mice have very different physiologic responses to isoflavones, which means the relevance of these findings to humans is uncertain. Furthermore, even in the animal studies, whole soyfoods didn’t have adverse effects.
⁴⁰
More importantly, studies of women show that neither whole soyfoods nor isoflavone supplements have harmful effects on indicators of breast cancer risk, such as breast cell proliferation and breast tissue density.
⁴¹
The position of the American Cancer Society is that
breast cancer patients can safely consume up to three servings of traditional soyfoods daily.
⁴²

In fact, new epidemiologic studies show that women who consume soyfoods after a diagnosis of breast cancer have an improved prognosis. A Chinese study involving more than 5,000 women with breast cancer found that those who consumed the most soy after their diagnosis—about two servings per day—were about one-third less likely to have a cancer recurrence or to die from their disease compared with women who consumed little soy.
⁴³
Eating soy was found to be as protective as taking the breast cancer drug tamoxifen.

A much smaller study, also from China, showed similar findings in postmenopausal breast cancer patients.
⁴⁴
This study also found that soyfood consumption enhanced the efficacy of one of the most commonly used types of breast cancer drugs. Because these were epidemiologic studies, and because they involved Chinese women who almost certainly had consumed soy throughout their lives, we have to use care in interpreting the results for western women. But the evidence increasingly suggests that soy is safe and even potentially beneficial for women with breast cancer.

This, of course, raises an important question: Could adding soy to the diet help prevent breast cancer? Unfortunately the clinical studies suggest that adults who add soy to their diet don’t lower their risk. In contrast, there is impressive evidence that modest soy intake (as little as one serving of soyfoods per day) during childhood and/or the teen years reduces the risk for breast cancer later in life by as much as 50 percent.
^45
,
46
Since girls in Asian countries grow up eating soyfoods, this may in part explain the lower breast cancer rates in these populations.

At present, we can say that in healthy women, soyfoods don’t raise breast cancer risk, and, while it remains speculative, they may offer benefits for women who have had breast cancer. The most promising findings, though, are that young girls who consume soy could have a lower lifetime risk of getting breast cancer.

There is reason to believe that soy lowers the risk for prostate cancer. First, the rates of prostate cancer are low in soyfood-consuming countries compared to western populations.
⁴⁷
Asian men who consume higher amounts of soy are about 30 percent less likely to develop prostate cancer than those who consume little soy.
⁴⁸
Recent clinical research involving prostate cancer patients suggests that isoflavones might inhibit the spread of prostate cancer. This suggests that soyfoods may turn out to be useful for both the treatment and prevention of prostate cancer.
⁴⁹
Since prostate tumors are slow growing and are typically diagnosed late in life, anything that delays tumor onset or growth can profoundly impact prostate cancer mortality. As we mentioned in Chapter 12, there is evidence that a diet high in dairy products could raise the risk for prostate cancer. So it may be that men who adopt a vegan diet and replace milk with soymilk will have added protection against prostate cancer.

Since estrogen appears to help maintain cognitive function in older women, there is speculation that soy isoflavones could have a similar effect. However, results from a study of Japanese men living in Hawaii—the Honolulu Heart Study—showed that those who ate the most tofu exhibited more signs of mental decline in their seventies through their nineties.
⁵⁰
The study wasn’t designed to look specifically at cognitive function, and the researchers measured the intake of only a small number of foods—two factors that limit interpretation of the findings. Also, the way in which soy intake was assessed changed over the course of the study. This too may have influenced conclusions about how much soy the men ate. In another study, which looked at women in Indonesia, tofu consumption was linked to memory loss, but tempeh—a fermented soy product that is a staple of Indonesian diets—had the opposite effect. The reason for the difference could be that in Indonesia, tofu has been
typically preserved with formaldehyde, a toxic compound that can harm brain function. Recently, there have been successful efforts to prevent the use of formaldehyde in tofu production in Indonesia.
⁵¹

Another study in Hong Kong found no effect—positive or negative—of soyfood consumption on cognitive function.
⁵²
More importantly, most clinical studies have shown improvements in some aspects of cognitive function with soy consumption.
⁵³

Many foods, including soyfoods, millet, cruciferous vegetables, and some herbs, contain goitrogens. These compounds interfere with thyroid function (and in extreme cases can cause an enlarged thyroid, which is called a goiter). Generally, they cause problems only in parts of the world where iodine intake is low, since iodine is an essential nutrient needed for thyroid function. The effects of iodine deficiency can be made worse if the diet is high in goitrogens. For western vegans, this shouldn’t be a problem as long as your diet includes sufficient iodine—an easy task if you use small amounts of iodized table salt every day or take an iodine supplement.

Concerns about the effects of isoflavones on thyroid function derive mostly from studies in animals.
⁵⁴
In humans, the evidence clearly shows that neither soyfoods nor isoflavones adversely affect thyroid function in healthy people.
⁵⁵
This research includes two recently published longer-term studies, which took place over three years and found no adverse effects of high doses of isoflavones on thyroid function. One of these studies assessed extremely sensitive indicators of thyroid function, much more sensitive than simply measuring thyroid hormones, and still it found no adverse effects.
⁵⁶
In most studies, isoflavone intake was quite a bit higher than what Japanese people typically consume, and there was no ill effect on thyroid function.