Soy can arguably be described as one of the oldest functional foods, with a history of over 5,000 years of lowering cholesterol levels in humans – or perhaps not? Despite extensive clinical data documenting health benefits for soy-based food products, some scientists say recent research casts doubt on widely-used, FDA-approved health claims, and may even indicate a certain level of risk associated with high levels of soy consumption. The ongoing debate is worth listening in on.
Soy, a member of the pea family (Fabaceae) native to Southeast Asia, has been part of the human diet since at least 3000 BC. During the Chou dynasty in China (1134-246 BC), fermentation methods were developed that enabled the preparation of more easily digestible forms of soy – including miso, tempeh, and tamari soy sauce. A long-running dispute between Korea and China as to which country invented tofu seems to have been decided in the latter’s favour, as it is now accepted that this popular form of soy was already consumed in China in the 2nd century. Only in the 18th century was soy first introduced to Europe. The United States first imported soy in the 19th century, but only started with large-scale soybean production in the mid-20th century. Today, American farmers generate around half of the world’s soybean supply, and over 30 million hectares of farmland, primarily in the Midwest, are dedicated to soybean cultivation.
The force driving these significant levels of production is the strong sales of soy-based food products. Between 1992 and 2003, for example, sales increased from $300 million to almost $4 billion, equivalent to a compound annual growth rate of 15% – remarkable by any standard. Soy milk has witnessed some of the largest gains, with US sales increasing from $2 million in 1980 to $300 million last year. This expansion in sales of soy-based foods and drinks can be directly attributed to the successful repositioning of soy in the marketplace, thanks to an emphasis on marketing its nutritional benefits.
Following a landmark 1995 study published in the New England Journal of Medicine (a meta-analysis of clinical trials which concluded that there was an appreciable cholesterol-lowering effect of regular soy consumption), the US Food and Drug Administration (FDA) approved health claims for products “low in saturated fat and cholesterol” containing at least 6.25 grams of soy protein per 100 gram serving. From that point onwards, various foods (including especially breakfast cereals, baked goods and meat substitutes) could be marketed as having benefits to
cardiovascular health.
“The road to FDA approval,” according to Marwin Zreik of the California-based company Naturade, “was long and demanding, consisting of a detailed review of human clinical data collected from more than 40 scientific studies conducted over the last 20 years. Soy protein was found to be one of the rare foods that had sufficient scientific evidence not only to qualify for an FDA health claim proposal but to ultimately pass the rigorous approval process.”
The current popularity of soy-based products is due in large measure to the results of these clinical studies, many of which indicated that including soy in the diet results in a moderate decrease in blood levels of total cholesterol and low-density lipoprotein (also referred to as “bad cholesterol”). Importantly, high-density lipoprotein (“good cholesterol’) values are not significantly affected, and some lowering of triglyceride levels also takes place. Also, people with elevated cholesterol levels appear to derive the greatest benefit from this effect, which continues as long as the soy-based diet is maintained. Complete replacement of animal-derived dietary protein with soy protein improves this benefit even further.
So what is it about soy that causes these health benefits? This is where things start to get somewhat controversial. The constituents of soy – particularly isoflavones such as genistein and daidzein – have also been the subject of extensive scientific investigation for decades. While several scientists have suggested that isoflavones are responsible for soy’s cholesterol-lowering effect, so far there is not sufficient research data to clearly back up this claim. For example, it is not yet known if food products containing only soy isoflavones (and no other soy ingredients) would have the same effects as sustained dietary intake of whole soy, which includes proteins unique to soy as well as numerous other components. In addition, another widely cited and more recent study, published in the journal Circulation in 2006, casts some doubt on the cholestrol-lowering claims of earlier reports, but does conclude that other components of soy may still have some benefits to cardiovascular health.
Complicating things further, soy isoflavones are also referred to as phytoestrogens because of their oestrogen-like effects on the human body. Interestingly, these compounds are actually able to act both as agonists and antagonists of the oestrogen pathway – leading to conflicting laboratory results and contributing to the ongoing controversy as to the relative advantages and risks of a high-soy diet. In any case, as most naturally occurring oestrogenic substances are weak in activity compared to human hormones, most researchers consider the regular consumption of foods that contain phytoestrogens not to pose a significant health risk. Nevertheless, soy clearly contains some oestrogenic activity. On the one hand, this is potentially useful for men who want to lower their risk for prostate cancer (but at the same time possibly decreasing their libido), on the other hand, it may slightly increase the risk of breast cancer in women.
Putting things in perspective somewhat, Christine Lewis, of the FDA Office of Nutritional Products, Labelling and Dietary Supplements, remarked “Soy by itself is not a magic food, but rather an example of the different kinds of foods that together in a complete diet can have a positive effect on health.”
One aspect of soy that is not in dispute is that it is a good source of dietary protein. Soy milk – a stable emulsion of protein, fat, and water produced by soaking dry soybeans and grinding them in water – contains similar proteins levels as cow’s milk, namely around 3.5%. Other soy products are even higher in protein content. Tofu, for example, contains over 10% protein (this figure refers to firm tofu; soft “silken” tofu has about half this amount).
So while it may be tempting to exploit both the proven health benefits of soy and its long history of safe consumption (theoretically enabling advertising slogans such as “soy - lowering cholesterol in humans for over 5,000 years”), manufacturers now need to balance these advantages with more recent research findings which indicate a more nuanced perspective, and possibly some level of risk as well. In any case, providing consumers with a balanced view of this ongoing debate could be just what the doctor ordered for this intriguing functional food.
The author
Camila Alexander,
Nutraventures, a Belgium-based consulting group focusing on functional foods and nutraceuticals.
